Assessment of fluid responsiveness in a mechanically ventilated patient

Abstract

The present disclosure relates to a method for assessing fluid responsiveness of a patient (3) connected to a breathing apparatus (2) providing mechanical ventilation to the patient (3). The method comprises the steps of monitoring a respiratory pressure of the patient (3), controlling a pneumatic unit (17) of the breathing apparatus (3) to provide baseline ventilation of the patient (3) with alternating inspiration phases and expiration phases during a period of baseline ventilation, monitoring at least one hemodynamic parameter related to fluid responsiveness of the patient (3), aborting baseline ventilation and initiating an end-expiratory fluid responsiveness assessment [EEFRA] period at an end of an expiration phase, and determining a degree of fluid responsiveness of the patient (3) based on a change in the at least one monitored hemodynamic parameter between the period of baseline ventilation and the EEFRA period. The method further comprises the step of dynamically controlling the pneumatic unit (17) during the EEFRA period based on the monitored respiratory pressure to counteract a change in an intrathoracic pressure of the patient (3) during spontaneous respiratory efforts by the patient (3).

Description

[0001]Assessment of Fluid Responsiveness in a Mechanically Ventilated Patient TECHNICAL FIELD The present disclosure relates to a method, computer program and breathingapparatus for assessing fluid responsiveness of mechanically ventilated patients.BACKGROUND ART Hemodynamic monitoring, i.e. the continuous display and recording of hemodynamic parameters to inform about the status of the cardiovascular system, is an essential component in the initial and continued management of all critically ill patients treated in the emergency, operating room and in the intensive care unit (ICU). One of the most important determinants of the status of the cardiovascular system is its effective volume of blood, or its volemia. There is a certain amount of blood within thecardiovascular system in a healthy state, called normo- or euvolemia, which is related to normalcardiovascular function and hence to an adequate delivery of oxygenated blood to all body tissues. In contrast, any decrease in intravascular volume or hypovolemia due to for example bleeding, dehydration, microcirculatory failure, third space fluid sequestration or excessive vasodilation may cause a deficit in oxygen delivery to tissues and if severe and / or prolonged enough may lead to organ failure. A concept inherent to volemia is preload. Preload is defined as the volume of blood within the ventricles at the end of diastole that stretch the myocardial fibers to a certain sarcomere length necessary for a normal effective and efficient heart muscle contraction during systole. Any decrease in cardiac preload will result in a decrease of the heart´s efficiency and in systemic hypo-perfusion of different degrees, which can frequently coexist with normal standard hemodynamic parameters such as mean arterial or central venous pressure. Standard hemodynamic monitoring systems can easily detect severe hypovolemic states but often fail in diagnosing moderate to mild hypovolemia. This is an important limitation in the monitoring of critical care patients because when such occult hypovolemia remains for many hours it can be associated with several complications such as acute renal failure, heart ischemia, cerebral stroke or wound infection among others. This occult hypovolemia is manifested as a “preload dependency” which in medical terms means that the cardiovascular system operatesat the steep portion of the Frank-Starling relationship. According to this relationship, theventricle will respond with an increase in cardiac output (more often expressed as stroke volume, SV, or the volume ejected by the ventricle with each systole) in response to theadministration of intravascular volume. In other words, the patient is “fluid responsive”.Intravenous fluid administration is considered the first line intervention in hemodynamically unstable patients to restore euvolemia (or in other words, to optimize preload). However, only about 50% of hemodynamically unstable patients are fluid responsive and it has been clearly established that an excess of intravenous fluids (i.e. over-resuscitation) is associated with an increased morbi-mortality as it can precipitate lung edema, worsen corpulmonale or induce left heart failure. Therefore, assessment of fluid responsiveness (i.e. theprospective identification of patients in whom intravenous administration of fluids increasecardiac output) is becoming an essential component in optimizing the intravascular volumestatus and avoiding the deleterious consequences of fluid overload. Many tests and indices for assessing fluid responsiveness have been developed through the years. In some of these tests, the relationship between cardiac output and cardiac preload is assessed through the hemodynamic effects of mechanical ventilation. This is the case for the well-established end-expiratory occlusion test (EEOT). The EEOT involves interruption of ventilation by occluding the airways of the patient atend-expiration for 15-30 seconds, and observing the resulting changes in cardiac output. Duringpositive pressure ventilation, insufflation raises intrathoracic pressure, which in turn increases right atrial pressure, impeding venous return. Consequently, the right cardiac preload decreases. When ventilation is halted at end-expiration, at the level of positive end-expiratory pressure (PEEP), the cyclic impediment to venous return is removed, allowing the right cardiac preload to reach its maximum. If the EEOT lasts long enough, the increased right cardiac preload is transmitted to the left side, potentially increasing stroke volume and cardiac output. Anincrease in these parameters may indicate preload responsiveness in both ventricles. The study“Predicting volume responsiveness by using the end-expiratory occlusion in mechanically ventilated intensive care unit patients”, by Monnet et. al, Crit Care Med. 2009;37:951-6,demonstrated that a cardiac output increase of ≥5% during a 15-second EEOT reliably predicteda positive response to a 500 mL saline infusion. There are however some limitations with the EEOT. In particular, it has been found that the assessment of fluid responsiveness during EEOT has poor reliability in presence ofspontaneous breathing activity by the patient. This is because breathing efforts during the end-expiratory occlusion period will induce changes in the pressure in the pleura and in theintrathoracic space, thus affecting the change in cardiac output during the EEOT. The change incardiac output might be either increased or decreased, depending on the timing of the patient’sbreathing efforts, compared to a stable situation without the breathing-induced pressureswings (for the same fluid status). Consequently, spontaneous breathing efforts during EEOTcan lead to false negative or false positive results in the assessment of fluid responsiveness.Requiring absence of spontaneous breathing activity is a limitation that leads to a significant reduction in number of applicable patients in the ICU where this method can be used. Thus, there is a desire to find a more robust and precise alternative to EEOT for assessingfluid responsiveness of a mechanically ventilated patient. SUMMARY It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least the above mentioned problem. In particular, it is an object of the present disclosure to present a technique for assessing fluid responsiveness of a mechanically ventilated patient, which technique is robust and at least relatively reliable also in case of spontaneous breathing activity by the patient. According to a first aspect of the present disclosure, there is provided a method forassessing fluid responsiveness of a patient connected to a breathing apparatus providing mechanical ventilation to the patient. The method comprises the steps of monitoring a respiratory pressure of the patient; controlling a pneumatic unit of the breathing apparatus toprovide baseline ventilation of the patient with alternating inspiration phases and expirationphases during a period of baseline ventilation; monitoring at least one hemodynamic parameter related to fluid responsiveness of the patient; aborting baseline ventilation and initiating an end-expiratory fluid responsiveness assessment [EEFRA] period at an end of anexpiration phase, and; determining a degree of fluid responsiveness of the patient based on achange in the at least one monitored hemodynamic parameter between the period of baselineventilation and the EEFRA period. The method further comprises the step of dynamicallycontrolling the pneumatic unit during the EEFRA period based on the monitored respiratorypressure to counteract a change in an intrathoracic pressure of the patient duringspontaneous respiratory efforts by the patient.By aborting baseline ventilation and initiating the EEFRA period at the end ofexpiration, the method mimics an end-expiratory occlusion without actually occluding theairways of the patient during the EEFRA period. The intrathoracic pressure at the end ofexpiration is lower than a mean intrathoracic pressure of the patient during baselineventilation, and the relatively lower intrathoracic pressure of the patient during the EEFRAperiod will cause a change in the monitored hemodynamic parameter for a fluid responsivepatient. In accordance with the principles of a conventional end-expiratory occlusion test, thischange may be used to determine a degree of fluid responsiveness of the patient.At the same time, by dynamically controlling the pneumatic unit to counteract changesin the intrathoracic pressure of the patient during spontaneous respiratory efforts during theEEFRA period, pressure swings in the intrathoracic pressure can be avoided, thereby avoiding or at least mitigating fluctuations in the monitored hemodynamic parameter during the EEFRA period. The reduction in pressure swings in the intrathoracic pressure thus provides for more reliable determination of the hemodynamic parameter during the EEFRA period, and hencefor a more reliable assessment of fluid responsiveness of the ventilated patient.Negative (sub-zero) pressure swings in intrathoracic pressure caused by stronginspiratory efforts have a particularly strong impact on the hemodynamic system. Therefore,to be able to effectively counteract a reduction in the intrathoracic pressure in response tospontaneous inspiratory efforts during the EEFRA period, the dynamic control may comprise astep of controlling the pneumatic unit to deliver a flow of breathing gas into an inspiratory lineof the breathing apparatus at least during spontaneous inspiratory efforts by the patient.Depending on the selected control strategy for the dynamic pressure control, the flow of breathing gas delivered to the inspiratory line of the breathing apparatus during the EEFRAperiod may be either continuously delivered during the entire EEFRA period, or delivered onlyin response to an inspiratory effort by the patient during the EEFRA period.The overarching goal of the dynamic pressure control is hence to avoid pressure swings in the intrathoracic pressure during the EEFRA period, i.e. to keep the intrathoracic pressure substantially constant during the EEFRA period. The intrathoracic pressure should in thiscontext be interpreted as encompassing all pressures within the thoracic cavity affecting thehemodynamic system of a mechanically ventilated patient and hence the monitored hemodynamic parameter. Non-limiting examples of such intrathoracic pressures include the pleural pressure, the oesophageal pressure and the transpulmonary pressure of the patient. The goal of keeping the intrathoracic pressure of the patient substantially constantduring the EEFRA period may be achieved by using various respiratory pressures as controlparameters in the dynamic control of the pneumatic unit. Typically, the dynamic controlinvolves the step of regulating the monitored respiratory pressure to keep the respiratorypressure substantially constant at a target pressure level, at least during respiratory efforts bythe patient, which target pressure level may be selected to minimize variations in the monitored hemodynamic parameter during the EEFRA period. In some embodiments, the intrathoracic pressure itself may be the respiratory pressure that is monitored and used as control parameter in the dynamic control of thepneumatic unit. In this case, the dynamic control may be applied continuously throughout theduration of the EEFRA period in order to keep the intrathoracic pressure substantially constant at a target pressure. The target pressure for the intrathoracic pressure may, in some examples, correspond to the intrathoracic pressure at the end of expiration when the EEFRAperiod is initiated and the airway pressure of the patient substantially corresponds to a setpositive end-expiratory pressure (PEEP). For example, an oesophageal pressure of the patientmay be monitored during ventilation of the patient, e.g. by means of an oesophageal catheter, whereby the pneumatic unit may be dynamically controlled to keep the oesophageal pressure substantially constant during the EEFRA period. However, intrathoracic pressures are rarely readily available in clinical practice, while the airway pressure of the patient is typically directly available from conventional sensors of the breathing apparatus. Therefore, in some embodiments, the respiratory pressure that is monitored and used in the dynamic control of the pneumatic unit is the airway pressure of the patient. The control level of the airway pressure, i.e. the target pressure towards which theairway pressure is regulated, may, in some examples, correspond to a set PEEP of the baselineventilation. In this case, the control period during which the pneumatic unit is dynamicallycontrolled to keep the airway pressure substantially constant at the target pressure may correspond to the duration of the EEFRA period, meaning that the dynamic control is applied continuously during the entire EEFRA period, in order to keep the airway pressure at the PEEP level. According to other embodiments in which the airway pressure of the patient ismonitored and used in the control of the pneumatic unit, the target pressure for the airwaypressure is above the set PEEP of the baseline ventilation. As mentioned above, a negativeintrathoracic pressure may have a significant impact on the hemodynamic system of thepatient and, therefore, the control strategy is preferably particularly adapted to avoid negativeintrathoracic pressures during inspiratory efforts by the patient. If the target pressure for theairway pressure is set to PEEP, an inspiratory effort by the patient causes an airway pressurebelow PEEP and hence a flow of breathing gas flowing into the patient, which flow will cause areduction in the intrathoracic pressure. By setting the target pressure above the set PEEP, negative pressure swings in the intrathoracic pressure caused by spontaneous inspiratory efforts can be effectively eliminated or at least mitigated during the EEFRA period. Furthermore, using a target pressure that is higher than the set PEEP reduces the risk of lung collapse and may further reduce the risk of stimulating new inspiratory efforts during the EEFRA period. In embodiments where the airway pressure is regulated towards a target pressureabove PEEP, the dynamic control may be applied continuously during the entire EEFRA period,or it may be applied only during periods of spontaneous inspiratory efforts by the patient. Forexample, the method may involve a step of detecting both a start and an end of an inspiratoryeffort of the patient based on at least one monitored breathing-related parameter, such as a respiratory pressure or flow, and to dynamically control the pneumatic unit to keep the airway pressure substantially constant at the target pressure above PEEP only during the duration of the breathing effort. An advantage of applying the dynamic control during the entire EEFRA period is that there is no need for trigger condition or cycle-off criterion detection. Anadvantage of applying the dynamic control only during inspiratory efforts, on the other hand,is that the elevated airway pressure above the set PEEP level is maintained only duringrelatively short periods of time, thereby minimizing a deviation between a mean expiratorypressure and a set desired PEEP for the patient. In embodiments where the airway pressure isregulated towards an elevated target pressure (i.e., a target pressure above PEEP) only duringspontaneous inspiratory efforts by the patient, the pneumatic unit may be controlled to keepthe airway pressure of the patient substantially constant at the PEEP level for periods withoutspontaneous inspiratory efforts. According to some embodiments, the target pressure above PEEP for the airwaypressure may be a predefined fixed pressure, e.g. a pressure corresponding to PEEP + 2cmH2O. Typically, the predefined fixed target pressure is selected to be in the range of PEEP +1cmH2O to PEEP + 4 cmH2O.According to some embodiments, the target pressure above PEEP for the airwaypressure may be set based on a pressure difference between the monitored airway pressureand a monitored intrathoracic pressure of the patient during an inspiratory effort by thepatient. For example, the target pressure above PEEP for a coming control period may be setbased on a difference between the monitored airway pressure and a monitored oesophagealpressure during a previous inspiratory effort by the patient.The pneumatic unit may comprise an inspiratory valve for controlling the flow of breathing gas flowing into the inspiratory line of the breathing apparatus, and an expiratory valve for controlling a flow of gas flowing out of an expiratory line of the breathing apparatus. Depending on the selected control strategy, the dynamic control of the pneumatic unit may comprise dynamic control of any or both of the inspiratory valve and the expiratory valve of the pneumatic unit. Various control strategies may be employed to counteract changes in the intrathoracicpressure caused by spontaneous respiratory efforts by the patient during the EEFRA period.According to a first exemplary control strategy, the dynamic control comprises dynamically controlling both the inspiratory valve and the expiratory valve of the pneumatic unit duringthe EEFRA period to counteract the change in intrathoracic pressure. Coordinated andsimultaneous control of both the inspiratory valve and the expiratory valve is advantageous inthat it provides for short response times and hence allows an undesired reduction inintrathoracic pressure caused by an inspiratory effort by the patient to be effectivelycounteracted by delivering (or increasing delivery of) a flow of breathing gas through theinspiratory valve while simultaneously reducing or interrupting the flow of exhalation gas through the expiratory valve. According to a second exemplary control strategy, the dynamic control compriseskeeping the expiratory valve of the pneumatic unit open in a fixed position or by applying aconstant control current to the expiratory valve, and dynamically controlling the inspiratoryvalve of the pneumatic unit to counteract the change in intrathoracic pressure.According to a third exemplary control strategy, the dynamic control comprises delivering a fixed inspiratory flow through the inspiratory valve of the pneumatic unit, and dynamically controlling the expiratory valve of the pneumatic unit to counteract the change in intrathoracic pressure. According to a fourth exemplary control strategy, the dynamic control comprises, when no inspiratory effort is made by the patient, dynamically controlling the expiratory valve of the pneumatic unit to keep an airway pressure of the patient substantially constant at a set PEEPlevel, and, when a spontaneous inspiratory effort by the patient is detected, closing theexpiratory valve and dynamically controlling the inspiratory valve of the pneumatic unit to counteract a decrease in the intrathoracic pressure during the inspiratory effort. The first exemplary control strategy may hence be advantageous in that it provides thefastest response time to counteract pressure. Another advantage with the first exemplarycontrol strategy is that it avoids undesired waste of breathing gas. All exemplary controlstrategies can be applied in both controlled modes (e.g. volume-controlled or pressure-controlled modes of ventilation) and support modes (e.g. pressure-support or volume-supportmodes of ventilation) of mechanical ventilation.According to some embodiments, the hemodynamic parameter is selected from the group consisting of cardiac output, pulse pressure (PP), stroke volume (SV), mean arterial pressure (MAP), central venous pressure (CVP), arterial pressure, heart rate (HR), or any parameter derived therefrom. While cardiac output is the gold standard parameter for fluid responsiveness assessment, any of these hemodynamic parameters may, taken alone or in combination with other parameters, be used to derive information that is relevant in the assessment of fluid responsiveness. According to a second aspect of the present disclosure, there is provided a computerprogram for assessing fluid responsiveness of a patient connected to a breathing apparatusproviding mechanical ventilation to the patient. The computer program comprises computer- readable instructions which, when executed by a processor of the breathing apparatus, causethe breathing apparatus to perform the above-describe method.According to a third aspect of the present disclosure, there is provided a breathing apparatus for assessing fluid responsiveness of a patient connected to the breathing apparatus. The breathing apparatus comprises at least one pressure sensor for measuring a respiratory pressure of the patient, a pneumatic unit for regulating a flow of respiratory gas toand / or from the patient, and at least one processor which typically but not necessarily formspart of a control computer of the breathing apparatus. The at least one processor isconfigured to: monitor the respiratory pressure of the patient; control the pneumatic unit toprovide baseline ventilation to the patient with alternating inspiration phases and expiration phases during a period of baseline ventilation; monitor at least one hemodynamic parameter related to fluid responsiveness of the patient; abort the baseline ventilation and initiate anEEFRA period at an end of an expiration phase, and; determine a degree of fluidresponsiveness of the patient based on a change in the at least one monitored hemodynamic parameter between the period of baseline ventilation and the EEFRA period. The at least oneprocessor is further configured to dynamically control the pneumatic unit during the EEFRAperiod based on the monitored respiratory pressure to counteract a change in an intrathoracicpressure of the patient during spontaneous respiratory efforts by the patient. According to some embodiments, the at least one processor is configured to control the pneumatic unit to deliver a flow of breathing gas into an inspiratory line of the breathingapparatus at least during spontaneous inspiratory efforts by the patient.According to some embodiments, the at least one processor is configured to controlthe pneumatic unit to keep the monitored respiratory pressure substantially constant at atarget pressure at least during the spontaneous respiratory efforts by the patient.According to some embodiments, the monitored respiratory pressure is an airwaypressure of the patient, the at least one processor being configured to dynamically control the pneumatic unit to keep the airway pressure substantially constant at a target pressure above a set PEEP of the baseline ventilation. According to some embodiments, the target pressure is set to a predefined fixed valueabove the set PEEP, or set based on a difference between the monitored airway pressure anda monitored intrathoracic pressure of the patient during a previous inspiratory effort by the patient. According to some embodiments, the at least one processor is configured to dynamically control any or both of an inspiratory valve and an expiratory valve of thepneumatic unit to counteract a the change in intrathoracic pressure.According to some embodiments, the at least one processor is configured to apply any of the following control strategies: -a first control strategy involving the steps of dynamically controlling both theinspiratory valve and the expiratory valve of the pneumatic unit to counteract the change in intrathoracic pressure;- a second control strategy involving the steps of keeping the expiratory valve of thepneumatic unit open in a fixed position or by applying a constant control current to the expiratory valve while dynamically controlling the inspiratory valve of thepneumatic unit to counteract the change in intrathoracic pressure;- a third control strategy involving the steps of delivering a fixed inspiratory flowthrough the inspiratory valve of the pneumatic unit while dynamically controlling the expiratory valve of the pneumatic unit to counteract the change in intrathoracicpressure, or -a fourth control strategy involving the steps of dynamically controlling theexpiratory valve of the pneumatic unit to keep an airway pressure of the patient substantially constant at a set PEEP level when no spontaneous inspiratory effort is made by the patient, and, when a spontaneous inspiratory effort by the patient is detected, closing the expiratory valve and dynamically controlling the inspiratory valve of the pneumatic unit to counteract a decrease in the intrathoracic pressureduring the spontaneous inspiratory effort.Effects and features of the second and third aspects of the disclosure are to a largeextent analogous to those described above in connection with the first aspect of thedisclosure. Other effects and advantages of the method, computer program and breathing apparatus of the present disclosure will become apparent from the detailed description following hereinafter. BRIEF DESCRIPTIONS OF THE DRAWINGS The above objects, as well as additional objects, features and advantages of the present disclosure, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments of the present disclosure, when taken in conjunction with the accompanying drawings, of which: Figure 1 illustrates an exemplary embodiment of a mechanical ventilation systemcomprising a breathing apparatus configured to perform an automated assessment of fluidresponsiveness. Figure 2 illustrates schematically an example of how the airway pressure of amechanically ventilated patient may vary during an end-expiratory occlusion test (EEOT)according to prior art. Figures 3-5 illustrate schematically how the airway pressure of a mechanically ventilated patient may vary during an end-expiratory fluid responsiveness assessment (EEFRA) period during which the airway pressure of the patient is dynamically controlled in accordance with some exemplary embodiments of the present disclosure. Figure 6 illustrate a simulation of how the airway pressure and the oesophageal pressure of a mechanically ventilated patient may vary during an occlusion period and an EEFRA period during which the airway pressure of the patient is dynamically controlled in accordance with an exemplary embodiment of the present disclosure. Figure 7 is a flowchart illustrating a method for assessing fluid responsiveness of a patient connected to a breathing apparatus providing mechanical ventilation to the patient,according to an exemplary embodiment of the present disclosureDETAILED DESCRIPTION The proposed method, computer program and breathing apparatus for assessing fluid responsiveness of a mechanically ventilated patient will now be described with reference tothe accompanying drawings, in which preferred exemplary embodiments of the disclosure areshown. It should be understood, however, that the method, computer program and breathingapparatus may be embodied also in other forms and the disclosure should not be construed aslimited to the exemplary embodiments disclosed herein.It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps. The present disclosure relates in general to an automated method or manoeuvre for assessing fluid responsiveness of a mechanically ventilated patient. The method may be automatically performed by a breathing apparatus providing mechanical ventilation to the patient. The method may be fully implemented in software, thus allowing a conventionally equipped breathing apparatus to perform the manoeuvre with no modification of existing hardware. Fig. 1 illustrates an exemplary embodiment of a mechanical ventilation system 1comprising a breathing apparatus 2 configured to perform an automated assessment of fluidresponsiveness of a patient 3 in accordance with the principles disclosed herein. The breathingapparatus 2 may be any type of apparatus capable of providing mechanical ventilation to the patient 3 through the supply of pressurised breathing gas to the airways of the patient 3. Ventilators and anaesthesia machines are non-limiting examples of such breathing apparatuses. The breathing apparatus 2 is connected to the patient 3 via a patient circuit comprising an inspiratory line 5 for supplying breathing gas to the patient 3, and an expiratory line 7 for conveying expiration gas away from the patient 3. The inspiratory line 5 and the expiratory line 7 are connected to the patient 3 via a patient connector 13, such as an endotracheal tube. The inspiratory line 5 and the expiratory line 7 may be connected to the patient connector either directly (if using double lumen tubing) or via a Y-piece. In the illustrated example, the inspiratory line 5 and the expiratory line 7 are connected to a common line 9 via a Y-piece 11, which common line is connected to the patient 3 via the patient connector 13. The breathing apparatus 2 comprises a control unit or control computer 15 for controlling the ventilation of the patient 3 based on pre-set parameters and / or measurements obtained by various sensors of the breathing apparatus. The control computer 15 controls the ventilation of the patient 3 by controlling a pneumatic unit 17 of the breathing apparatus 2, which pneumatic unit 17 may comprise various gas regulating means for regulating a flow of respiratory gas to and / or from the patient 3 based on control signals from the controlcomputer 15. The pneumatic unit 17 is typically connected to one or more gas sources 19, 21for receiving one or more breathing gases, and to the inspiratory line 5 for regulating a flow ofinspiration gas to be delivered to the patient 3. The pneumatic unit 17 is typically alsoconnected to the expiratory line 7 for regulating a flow of expiration gas from the patient 3.The pneumatic unit 17 may comprise any type of gas mixing and flow regulating means knownin the art of ventilation, including but not limited to gas mixing chambers, controllable gasmixing valves, turbines, controllable inspiration and / or expiration valves, etc. In the context ofthe present disclosure, “controlling the pneumatic unit” means to control any or both of the flow of gas flowing into the inspiratory line 5 and the flow of gas flowing out of the expiratory line 7. The ventilation system 1 further comprises one or more flow sensors 23, 23’, 23’’ for measuring respiratory flow, and one or more pressure sensors 25, 25’, 25’’ for measuring respiratory pressure. The flow sensor 23 may be a proximal flow sensor located close to the patient 3 (e.g. in or close to the Y-piece 11) and configured to measure both an inspiratory flow of breathing gas delivered to the airways of the patient 3 during inspiration, and an expiratory flow of gas exhaled by the patient 3 during expiration. Likewise, the pressure sensor 25 may be a proximal pressure sensor located close to the patient 3 (e.g. in or close to the Y- piece 11) and configured to measure, during both inspiration and expiration, a proximate patient pressure substantially corresponding to an airway pressure of the patient 3. Alternatively or in addition to the flow sensor 23 and the pressure sensor 25 disposed in the Y-piece 11 of the patient circuit, the breathing apparatus 2 may comprise one or more internal flow sensors for measuring respiratory gas flow, and / or one or more internal pressure sensors for measuring respiratory gas pressure. For example, the breathing apparatus 2 maycomprise a flow sensor 23’ for measuring a flow of breathing gas in the inspiratory line 5 of thebreathing apparatus 2, and / or a pressure sensor 25’ for measuring a gas pressure in theinspiratory line 5 of the breathing apparatus. In addition, the breathing apparatus 2 maycomprise a flow sensor 23’’ for measuring a flow of expiration gas in the expiratory line 7 ofthe breathing apparatus 2, and / or a pressure sensor 25’’ for measuring a gas pressure in theexpiratory line 7 of the breathing apparatus.The breathing apparatus 2 may further comprise one or more pressure sensors 26 formeasuring an intrathoracic pressure of the patient 3. In the illustrated example, the pressure sensor 26 is an oesophageal balloon catheter for measuring an oesophageal pressure of the patient, substantially corresponding to a pleural pressure of the patient. The measurement signals obtained by the one or more flow sensors 23, 23’, 23’’ andthe one or more pressure sensors 25, 25’, 25’’, 26 are transmitted to the control computer 15,whereby the control computer 15 can control the flow and volume of breathing gas deliveredto the patient 3, as well as the airway pressure of the patient 3, by controlling the pneumaticunit 17 based on the measurement signals. In this exemplary embodiment, the pneumatic unit 17 comprises a controllable inspiratory valve 27 for regulating inspiratory flow and pressure,and a controllable expiratory valve 29 for controlling an expiratory pressure applied to thepatient 3 during expiration. The expiratory valve 29 is typically controlled to achieve a set positive end-expiratory pressure (PEEP) at the end of expiration, and is therefore sometimesreferred to as a PEEP valve. Thus, in this exemplary embodiment, the flow of gas flowing intothe inspiratory line 5 is controlled by controlling the inspiratory valve 27 of the pneumatic unit15, and the flow of gas flowing out of the expiratory line 7 is controlled by controlling the expiratory valve 29 of the pneumatic unit 17. Typically, the operation of the breathing apparatus 2 is at least partly controlled based on an airway pressure of the ventilated patient 3. When herein referring to an airway pressure of the patient 3, the airway pressure may be the pressure measured by a sensor disposed close to the airways of the patient, such as the proximal pressure sensor 25, or it may be a pressure substantially corresponding to an airway pressure of the patient, derived from other pressure and / or flow measurements, such as pressure and / or flow measurements obtained byany of the pressure sensors 25’, 25’’ and 26 and / or any of the flow sensors 23, 23’ and 23’’.The control computer 15 comprises a processor 30 and a memory 31. The processor 30 may be constituted by e.g. a microprocessor, and the memory 31 may be constituted by e.g. anon-transitory memory hardware device. The memory 31 stores one or more computerprograms for controlling the operation of the breathing apparatus 2, including a computerprogram for fluid responsiveness assessment comprising instructions for causing the breathingapparatus 2 to perform an automated fluid responsiveness assessment in accordance with the principles described herein. Unless stated otherwise, actions and method steps describedherein are performed by, or caused by, the control computer 15 of the breathing apparatus 2upon execution by the processor 30 of different code segments of the computer program forfluid responsiveness assessment, stored in the memory 31. The principles of the proposed method or manoeuvre for fluid responsiveness assessment of a mechanically ventilated patient will now be described with reference to Figs.2-7, with simultaneous reference made to the ventilation system 1 of Fig. 1. In general, the principles of the proposed method are similar to the principles of theconventional end-expiratory occlusion test (EEOT) in that both methods suggest that the fluidresponsiveness of the ventilated patient is assessed based on a difference in a monitored hemodynamic parameter, for example cardiac output, between two different pressure levelsaffecting the hemodynamic system of the patient in different ways. To facilitate understandingof the proposed manoeuvre for fluid responsiveness assessment, it will now be described incomparison with a conventional EEOT. Fig. 2 illustrates schematically an example of how the airway pressure, Paw, of amechanically ventilated patient may vary during a conventional EEOT according to prior art,where the airways of the patient are occluded during an end-expiratory occlusion (EEO)period. First, the patient is ventilated during a period of baseline ventilation comprisingalternating phases of inspiration and expiration. At a point in time, t1, at the end of an expiration when the airway pressure of the patient is at a set positive end-expiratory pressure(PEEP) level, the EEO period is initiated by occluding the airways of the patient. This may beachieved, e.g., by closing an inspiratory valve and an expiratory valve of the breathingapparatus. In order to assess the fluid responsiveness of the patient, the cardiac output of the patient is monitored and a difference in cardiac output between the period of baseline ventilation and the EEO period is determined. A significant increase in cardiac output during the EEO period indicates that the patient is fluid responsive. As discussed in the background portion, a spontaneous respiratory effort made by thepatient during the EEO period will cause a change in the intrathoracic pressure of the patient,which change affects the cardiac output of the patient. The change in intrathoracic pressuremay cause either an increase or decrease in cardiac output depending on the timing of therespiratory effort, and spontaneous respiratory efforts during the EEO period may hencecause any of a false negative or false positive result in EEOT-based assessment of fluid responsiveness. In Fig.2, the arrows denoted by reference letters A and B indicate start points of two distinct spontaneous inspiratory efforts made by the patient during the EEO period, as manifested by the negative pressure swings in the airway pressure following the inspiratory efforts.To prevent pressure swings in intrathoracic pressures affecting the hemodynamics ofthe patient, and hence affecting the cardiac output determination and the assessment of fluidresponsiveness of the patient, it is suggested according to the principles of the presentdisclosure that the EEO period of the EEOT, with its passive occlusion of the airways of thepatient, is exchanged for a period of dynamic and active pressure control during which amonitored respiratory pressure indicative of the intrathoracic pressure of the patient isdynamically regulated to counteract changes in the intrathoracic pressure.Thus, in accordance with the proposed principles and with reference again made to theventilation system 1 in Fig.1, the breathing apparatus 2 may comprise at least one pressuresensor 25, 25’, 25’’, 26 for measuring a respiratory pressure indicative of an intrathoracicpressure of the patient 3, a pneumatic unit 17 for regulating a flow of respiratory gas toand / or from the patient 3, and at least one processor 30. The processor 30 is configured tomonitor the measured respiratory pressure; control the pneumatic unit 17 to provide baselineventilation to the patient with alternating inspiration phases and expiration phases during a period of baseline ventilation; monitor at least one hemodynamic parameter related to fluidresponsiveness of the patient; abort the baseline ventilation and initiate an end-expiratoryfluid responsiveness assessment (EEFRA period) at an end of an expiration phase, and;determine a degree of fluid responsiveness of the patient based on a change in the at least one monitored hemodynamic parameter between the period of baseline ventilation and theEEFRA period. The at least one processor 30 is further configured to dynamically control thepneumatic unit 17 during the EEFRA period based on the monitored respiratory pressure tocounteract a change in the intrathoracic pressure of the patient 3 during spontaneousrespiratory efforts by the patient. That the pneumatic unit 17 is dynamically controlled meansthat it is controlled in real-time or near real-time based on the monitored respiratory pressure. The proposed pressure control serves to counteract a potential increase in theintrathoracic pressure caused e.g. by inspiratory efforts by the patient, and a potentialdecrease in the intrathoracic pressure caused e.g. by expiratory efforts or coughing by thepatient, and so serves to keep the intrathoracic pressure of the patient substantially constant during the EEFRA period.To counteract a reduction in intrathoracic pressure during spontaneous inspiratoryefforts by the patient 3, the processor 30 may be configured to control the pneumatic unit 17to deliver a flow of breathing gas into the inspiratory line 5 of the breathing apparatus at leastduring inspiratory efforts by the patient. As will be further discussed below, the breathing gas may be delivered only during inspiratory efforts by the patient 3 or be delivered continuously throughout the entire EEFRA period. The flow of breathing gas serves to compensate for theflow of breathing gas that is drawn into the airways of the patient as a result of thespontaneous inspiratory effort and thus counteracts a reduction in the airway pressure andthe intrathoracic pressure of the patient during inspiratory efforts. The control of thepneumatic unit 17 should be quick enough and the flow rate with which the breathing gas isdelivered into the inspiratory line 5 should be high enough to prevent the intrathoracicpressure from dropping below zero. In a healthy adult, spontaneous inspiratory flow rates typically range fromapproximately 20 to 60 litres per minute (lpm). For paediatric patients, the spontaneousinspiratory flow rate may be substantially lower. The flow rate of the breathing gas delivered into the inspiratory line 5 of the breathing apparatus 2 may thus vary in dependence of the patient and the strength of the inspiratory efforts but should be at least 10 litres per minute (lpm) in order for the proposed dynamic control to have any significant effect. Preferably, the flow rate of the breathing gas should be at least 20 lpm, more preferably at least 40 lpm, even more preferably at least 60 lpm, and most preferably at least 80 lpm. Typically, the dynamic control involves controlling the pneumatic unit 17 to keep themonitored respiratory pressure substantially constant at a target pressure or control level, atleast during respiratory efforts by the patient. The respiratory pressure that is monitored and used as control parameter in thedynamic control of the pneumatic unit 17 may be the intrathoracic pressure itself, or anyother respiratory pressure that is indicative of the intrathoracic pressure and, therefore, could be used to indirectly control the intrathoracic pressure. Non-limiting examples of intrathoracic pressures that may be used as control parameters in the control of the pneumatic unit 17include the pleural pressure, the oesophageal pressure and the transpulmonary pressure of the patient, whereas the airway pressure constitutes an example of a respiratory pressure that is indicative of the intrathoracic pressure of the patient. In some embodiments employing an intrathoracic pressure as control parameter, theintrathoracic pressure is directly measured and used as control parameter in the control of thepneumatic unit 17 to keep the intrathoracic pressure substantially constant at a set targetpressure for the duration of the EEFRA period. According to one example, the intrathoracicpressure may be an oesophageal pressure (Pes) measured by the oesophageal balloon catheter26. The oesophageal pressure is often used as a surrogate measure for pleural pressure inclinical practice. In this scenario, the processor 30 may be configured to dynamically controlthe pneumatic unit 17 to keep Pes substantially constant at a target pressure during the EEFRAperiod, e.g. a target pressure corresponding to the Pes level at initiation of the EEFRA period.According to another example, the intrathoracic pressure is a transpulmonary pressure of thepatient 3. The transpulmonary pressure of the patient 3 may for example be determined asthe difference between a measured airway pressure and a measured oesophageal pressure ofthe patient 3, or estimated based on a compliance of the respiratory system. The processor 30may hence be configured to monitor the transpulmonary pressure of the patient 3 and dynamically control the pneumatic unit 17 to keep the transpulmonary pressure substantially constant at a target pressure during the EEFRA period, e.g. a target pressure corresponding to the transpulmonary pressure level at initiation of the EEFRA period. However, oesophageal balloon catheters and other means for direct measurements of intrathoracic pressures are not readily available in clinical practice, while the airway pressureof the patient is typically directly available from pressure measurements obtained byconventional sensors of a breathing apparatus, such as the proximal pressure sensors 25 ofthe breathing apparatus 2 in Fig. 1. Therefore, the respiratory pressure that is used as controlparameter in the dynamic control of the pneumatic unit 17 is preferably the airway pressureof the patient 3. No matter which pressure is used as control parameter, various control strategies maybe employed to counteract changes in the intrathoracic pressure caused by spontaneousrespiratory efforts by the patient 3. For example, the processor 30 of the breathing apparatus2 may be configured to apply any of the following control strategies during the EEFRA period:1) dynamically controlling both the inspiratory valve 27 and the expiratory 29 valve ofthe pneumatic unit 17; 2) keeping the expiratory valve 29 of the pneumatic unit 17 open in a fixed position orby applying a constant control current to the expiratory valve while dynamicallycontrolling the inspiratory valve 27 of pneumatic unit 17; 3) delivering a fixed inspiratory flow through the inspiratory valve 27 of the pneumaticunit 17 while dynamically controlling the expiratory valve 29 of the pneumatic unit 27, or 4) dynamically controlling the expiratory valve 29 of the pneumatic unit 17 when nospontaneous inspiratory effort is made by the patient 3, and, when a spontaneous inspiratory effort by the patient 3 is detected, closing the expiratory valve 29 and dynamically controlling the inspiratory valve 27 of the pneumatic unit 17. In the above-mentioned examples of control strategies, “dynamically controlling theinspiratory valve” means to actively control the inspiratory valve 27 of the pneumatic unit 17to regulate a flow of breathing gas flowing into the inspiratory line 5 of the breathingapparatus 2. “Dynamically controlling the expiratory valve” means to actively control the expiratory valve 29 of the pneumatic unit 17 to regulate a flow of gas flowing out of theexpiratory valve 29. “Closing the expiratory valve 29” means, in the context of control strategynumber 4, to fully interrupt the flow of exhalation gas by closing and maintaining the expiratory valve 29 in a closed position. In the following, for the sake of simplicity, the principles of the proposed manoeuvrefor fluid responsiveness assessment will be described in the context of exemplary embodiments where the respiratory pressure that is monitored and used as control parameterin the control of the pneumatic unit 17 is the airway pressure of the patient 3. In theseembodiments, the dynamic control involves regulating the airway pressure towards a target pressure or control level, at least during respiratory efforts by the patient 3. In a first example, the control level for the airway pressure may be selected tocorrespond to a set positive end-expiratory pressure (PEEP) used during the period of baseline ventilation preceding the EEFRA period. An example of a scenario where PEEP is set as control level for the airway pressure of the patient 3 is illustrated in Fig.3.As schematically illustrated by Fig. 3, when studied in comparison with Fig. 2, thereduction in airway pressure, Paw, caused by the inspiratory efforts made by the patient 3 atthe points in time A and B may be substantially mitigated by employing dynamic pressurecontrol during the EEFRA period instead of using a conventional occlusion. An advantage ofusing PEEP as target pressure, PT, is that any of the control strategies 1-3 described above canbe used continuously throughout the entire EEFRA period, without any need for triggerdetection, i.e. without the need for detection of respiratory efforts by the patient. Thus, in thisexample, the processor 30 of the control computer 15 is configured to monitor the airway pressure, Paw, of the patient 3, and to dynamically control the pneumatic unit 17 based on Paw continuously during the EEFRA period in order to keep Pawsubstantially constant at a targetpressure level corresponding to the set PEEP level, thereby preventing substantial changes inthe airway pressure and the intrathoracic pressure of the patient 3 otherwise caused byrespiratory efforts during the EEFRA period. In this scenario, no trigger event or cycle-off event detection is required. Any of the control strategies 1-4 mentioned above may be applied in order to keep Paw substantially constant at the PEEP level for the duration of the EEFRA period. During end of expiration when the airway pressure of the patient is maintained at a set PEEP level, the intrathoracic pleural pressure of the patient is typically positive relative to atmospheric pressure and slightly less than PEEP. For example, if PEEP is set to 5 cmH2O, the pleural pressure of the patient at the end of expiration is typically around 2-4 cmH2O. During strong inspiratory efforts by the patient, the airway pressure of the patient 3 may drop under the set PEEP level, causing a flow of breathing gas into the airways of the patient and a drop inintrathoracic pressure to negative, sub-atmospheric levels. To prevent negative intrathoracicpressures, the control level for Paw should preferably be selected to be somewhat higher thanthe set PEEP level in order to more effectively avoid negative pressure swings in theintrathoracic pressure. Fig.4 illustrates an example of a scenario where the target pressure, PT, for Paw is set above PEEP. As illustrated in the drawing, in this exemplary embodiment, the processor 30 is configured to increase Paw from the PEEP level to a target pressure, PT, above PEEP at initiation of the EEFRA period, and to dynamically control the pneumatic unit 17 continuously during theEFFRA period to keep Paw substantially constant and equal to PT. In this scenario too, no triggerevent or cycle-off event detection is required and any of the control strategies 1-4 mentioned above may be applied in order to keep Pawsubstantially constant at the target pressure PTfor the duration of the EEFRA period. Fig.5 illustrates another example of a scenario where the target pressure, PT, for Pawis set above PEEP. As illustrated in the drawing, in this exemplary embodiment, the processor 30 is configured to increase Pawfrom the PEEP level to PTand dynamically control the pneumatic unit 17 to keep Pawsubstantially constant and equal to PTonly during inspiratory efforts by thepatient 3. In this scenario, trigger and cycle-off event detection is required, meaning that thecontrol computer 15 of the breathing apparatus 2 must be capable of detecting a trigger eventindicating a start of an inspiratory breathing effort by the patient 3, and a cycle-off eventindicating an end of the inspiratory breathing effort. When the trigger event is detected, thecontrol computer 15 starts controlling the pneumatic unit 17 according to any of the controlstrategies 1-4 mentioned above in order to keep Paw substantially constant to the targetpressure PT during the inspiratory effort. When the cycle-off event is detected, the processor30 aborts the applied control strategy and controls the pneumatic unit 17 to keep Pawsubstantially constant at the set PEEP level during parts of the EEFRA period where noinspiratory efforts are made. The effect of the pressure control applied in the scenario illustrated by Fig. 5 will nowbe illustrated with reference to Fig.6. Fig. 6 illustrates the airway pressure, Paw, and the oesophageal pressure, Pes, of anartificial patient during a simulation of two different manoeuvres: a low-assist manoeuvre(LAM) in accordance with the exemplary embodiment illustrated in Fig. 5, and an occlusion(Occ) in accordance with the principles of an EEOT according to prior art. The graph in Fig.6 illustrates pressure variations in Paw and Pes for the LAM manoeuvre and the occlusion during an inspiratory effort by the patient. The occlusion was applied continuously throughout the simulation period whereas the LAM manoeuvre, in accordance with the control period used inthe scenario illustrated in Fig. 5, was applied only during a period starting with the detectionof a trigger event at a point time Time=0 and ending with a detection of a cycle-off event at apoint in time Time=1,5s. As shown in the graph, the simulation was made for a scenario wherePEEP was set to 10 cmH2O and the target pressure or control level for the airway pressureduring inspiratory efforts by the patient was set to 12 cmH2O.As also shown in the graph, the simulated occlusion resulted in a substantial reductionin the airway pressure, PawOcc, and a substantial reduction in oesophageal pressure, PesOcc,causing the oesophageal pressure to drop below zero. As discussed above, this negativepressure swing in intrathoracic pressure affects the hemodynamic system of the patient and may introduce undesired fluctuations in the monitored hemodynamic parameter used in the assessment of fluid responsiveness. When applying the LAM manoeuvre, on the other hand, the dynamic pressure control is seen to keep the airway pressure, PawLAM, substantiallyconstant at a target pressure above PEEP during the inspiratory effort, which effectivelycounteracts any substantial reduction in the oesophageal pressure, Pes LAM, and prevents theintrathoracic pressure from dropping below zero.As discussed above, the target pressure above PEEP may e.g. be predetermined as afixed target pressure above PEEP, e.g. as PEEP + 2cmH2O in accordance with the example illustrated in Fig.6. In other embodiments, the target pressure above PEEP may be determinedbased on the pulmonary mechanics of the ventilated patient. For example, the target pressureabove PEEP for the airway pressure may be set based on a pressure difference between the monitored airway pressure and a monitored intrathoracic pressure of the patient during an inspiratory effort by the patient. For example, the target pressure above PEEP for a coming control period may be set based on a difference between the monitored airway pressure anda monitored oesophageal pressure during a previous inspiratory effort by the patient. Thus,with reference again made to Fig.6, the target pressure for Paw LAM for a coming controlperiod may be determined based on a pressure difference or pressure offset, Poffset, betweenthe monitored airway pressure of the patient, Paw LAM, and the monitored oesophagealpressure, Pes LAM.In the above examples, and as best illustrated by Figs.3-5, the EEFRA period is initiated at the end of an expiration period, typically but not necessarily when the airway pressure ofthe patient substantially corresponds to a set PEEP level of the baseline ventilation. In Figs. 3-5, the start of the EEFRA period occurs at a point in time t1. The processor 30 of the control computer 15 may, for example, be configured to initiate the EEFRA period when the duration of the ongoing expiration phase exceeds a set expiration time (in PC or VC mode) or an apnea time (in PS or VS mode), or upon detection of a trigger event indicating an inspiration effort by the patient 3. The EEFRA period should have a duration that is long enough to allow for a noticeable increase in venous return to the heart of the patient, and thus long enough to cause a noticeable change in the monitored hemodynamic parameter. At the same time, the EEFRAperiod should be short enough to minimize the risk of hypoxia and hypercapnia. The EEFRAperiod may have a duration of 5-30 seconds. Preferably, the EEFRA period has a duration of 20seconds or less, and even more preferably a duration of 10 seconds or less. In a non-limitingexample, the EEFRA period has a duration of about 7 seconds. During both the baseline ventilation and the EEFRA period, the processor 30 of the breathing apparatus 2 monitors the at least one hemodynamic parameter related to fluid responsiveness of the patient 3. Typically but not necessarily, the monitored hemodynamic parameter is the cardiac output of the patient 3, which cardiac output may be measured or estimated using any means known in the art of cardiac output monitoring. For example, the cardiac output of the patient 3 may be measured or estimated using pulse contour analysis, echocardiography, or other hemodynamic monitoring techniques. Other non-limiting examples of hemodynamic parameters that may be monitored and used by the processor 30 in the assessment of fluid responsiveness include pulse pressure (PP), stroke volume (SV), mean arterial pressure (MAP), central venous pressure (CVP), arterial pressure, heart rate (HR), or any parameter derived therefrom. After the EEFRA period, the processor 30 determines a degree of fluid responsivenessof the patient 3 based on a change in the at least one monitored hemodynamic parameterbetween the period of baseline ventilation and the EEFRA period. For example, the processor30 may be configured to determine that a cardiac output increase of ≥5% during a 15 secondEEFRA period indicates that the patient 3 is fluid responsive, e.g. in the meaning of being likelyto respond with an increase in cardiac output to a 500 mL saline infusion. The processor 30 may further be configured to present a result of the determination to an operator of the breathing apparatus 2 or another clinician, in order to assist the clinician inthe decision on whether or not to administer intravenous fluid to the patient 3. Thepresentation may for example comprise presentation of a visual indication of a level of fluid responsiveness of the patient 3, or a recommendation on whether or not to administer fluidto the patient 3, presented on a display 32 of the breathing apparatus 2 or a display of ahemodynamic monitor 32’ or remote device 32’’ to which the breathing apparatus 2 iscommunicatively connected. Fig.7 is a flowchart illustrating a method for assessing fluid responsiveness of a patient3 connected to a breathing apparatus 2 providing mechanical ventilation to the patient 3,according to an exemplary embodiment of the present disclosure. The method is a computer- implemented method performed by the at least one processor 30 of the control computer 15 of the breathing apparatus 2 upon execution of a computer program stored in the memory 31. In a first step, S1, a respiratory pressure of the patient 3 is monitored. The respiratorypressure should be indicative of an intrathoracic pressure affecting the hemodynamic systemof the patient. In a second step, S2, a pneumatic unit 17 of the breathing apparatus 3 is controlled toprovide baseline ventilation of the patient 3 with alternating inspiration phases and expirationphases during a period of baseline ventilation. Controlling the pneumatic unit herein means to control any or both of a flow of gas flowing into an inspiratory line 5 of the breathing apparatus 2 and a flow of gas flowing out of an expiratory line 7 of the breathing apparatus. Typically, the control involves the control of an inspiratory valve 27 and / or an expiratory valve 29 of the pneumatic unit. In a third step, S3, at least one hemodynamic parameter related to fluid responsivenessof the patient 3 is monitored. The hemodynamic parameter is typically a cardiac output of thepatient 3 but may be any parameter indicative of a degree of fluid responsiveness of the patient, which parameter is affected by changes in the intrathoracic pressure of the patient. The hemodynamic parameter may be continuously or intermittently monitored during the period of baseline ventilation and the subsequent EEFRA period. In a fourth step, S4, the baseline ventilation is aborted and an EEFRA period is initiated at an end of an expiration phase. In a fifth step, S5, the pneumatic unit 17 is dynamically controlled during the EEFRAperiod based on the monitored respiratory pressure to counteract a change in theintrathoracic pressure caused by spontaneous respiratory efforts by the patient 3.In a sixth and final step, S6, the a degree of fluid responsiveness of the patient 3 isdetermined based on a change in the at least one monitored hemodynamic parameter between the period of baseline ventilation and the EEFRA period.

Claims

CLAIMS1. A method for assessing fluid responsiveness of a patient (3) connected to a breathingapparatus (2) providing mechanical ventilation to the patient (3), comprising the steps of: -monitoring a respiratory pressure of the patient (3);- controlling a pneumatic unit (17) of the breathing apparatus (3) to provide baselineventilation of the patient (3) with alternating inspiration phases and expiration phases during a period of baseline ventilation; -monitoring at least one hemodynamic parameter related to fluid responsiveness of thepatient (3); -aborting baseline ventilation and initiating an end-expiratory fluid responsivenessassessment [EEFRA] period at an end of an expiration phase, and -determining a degree of fluid responsiveness of the patient (3) based on a change inthe at least one monitored hemodynamic parameter between the period of baseline ventilation and the EEFRA period, characterised by the step of dynamically controlling the pneumatic unit (17) during theEEFRA period based on the monitored respiratory pressure to counteract a change in anintrathoracic pressure of the patient (3) during spontaneous respiratory efforts by thepatient (3).

2. The method of claim 1, wherein the dynamic control comprises controlling the pneumaticunit (17) to deliver a flow of breathing gas into an inspiratory line (5) of the breathing apparatus (2) at least during the spontaneous respiratory efforts (3).

3. The method of claim 1 or 2, wherein the dynamic control comprises controlling thepneumatic unit (17) to keep the monitored respiratory pressure substantially constant at a target pressure at least during the spontaneous inspiratory efforts by the patient (3).

4. The method of any of the preceding claims, wherein the monitored respiratory pressure isan airway pressure (Paw) of the patient (3), the dynamic control comprising controlling the pneumatic unit (17) to keep the airway pressure substantially constant at a target pressure above a set positive end-expiratory pressure [PEEP] of the baseline ventilation.

5. The method of claim 4, wherein the target pressure is:- set to a predefined fixed value above the set PEEP, or- set based on a difference between the monitored airway pressure (Paw) and amonitored intrathoracic pressure of the patient (3) during a previous respiratory effortby the patient (3).

6. The method of any of the preceding claims, wherein the dynamic control comprisesdynamically controlling any or both of an inspiratory valve (27) and an expiratory valve (29) of the pneumatic unit (17) during the EEFRA period to counteract the change inintrathoracic pressure.

7. The method of claim 6, wherein the dynamic control comprises dynamically controllingboth the inspiratory valve (27) and the expiratory valve (29) of the pneumatic unit (17)during the EEFRA period to counteract the change in intrathoracic pressure.

8. The method of claim 6, wherein the dynamic control comprises:- keeping the expiratory valve (29) of the pneumatic unit (17) open in a fixed position orby applying a constant control current to the expiratory valve, and -dynamically controlling the inspiratory valve (27) of the pneumatic unit (17) tocounteract the change in intrathoracic pressure.

9. The method of claim 6, wherein the dynamic control comprises:- delivering a fixed inspiratory flow through the inspiratory valve (27) of the pneumaticunit (17), and -dynamically controlling the expiratory valve (29) of the pneumatic unit (17) tocounteract the change in intrathoracic pressure.

10. The method of any claim 6, wherein the dynamic control comprises:- when no inspiratory effort is made by the patient (3), dynamically controlling theexpiratory valve (29) of the pneumatic unit (17) to keep an airway pressure (Paw) of thepatient substantially constant at a set PEEP level, and- when a spontaneous inspiratory effort by the patient (3) is detected, closing theexpiratory valve (29) and dynamically controlling the inspiratory valve (27) of thepneumatic unit (17) to counteract a decrease in the intrathoracic pressure during theinspiratory effort.

11. The method of any of the preceding claims, wherein the hemodynamic parameter isselected from the group consisting of cardiac output [CO], pulse pressure [PP], stroke volume [SV], mean arterial pressure [MAP], central venous pressure [CVP], arterial pressure, heart rate [HR], or any parameter derived therefrom.

12. A computer program for assessing fluid responsiveness of a patient (3) connected to abreathing apparatus (2) providing mechanical ventilation to the patient (3), characterised in that the computer program comprises computer-readable instructions which, whenexecuted by at least one processor (30) of the breathing apparatus (2), cause the breathing apparatus to perform the method of any of the preceding claims.

13. A breathing apparatus (2) for assessing fluid responsiveness of a patient (3) connected tothe breathing apparatus (2), the breathing apparatus comprising: -at least one pressure sensor (25, 25’, 25’’) for measuring a respiratory pressure of thepatient (3); -a pneumatic unit (15) for regulating a flow of respiratory gas to and / or from thepatient (3), and -at least one processor (30) configured to:o monitor the respiratory pressure of the patient (3);o control the pneumatic unit (15) to provide baseline ventilation to the patient(3) with alternating inspiration phases and expiration phases during a period of baseline ventilation; omonitor at least one hemodynamic parameter related to fluid responsivenessof the patient (3); oabort the baseline ventilation and initiate an end-expiratory fluidresponsiveness assessment [EEFRA] period at an end of an expiration phase, ando determine a degree of fluid responsiveness of the patient (3) based on a changein the at least one monitored hemodynamic parameter between the period of baseline ventilation and the EEFRA period, characterised in that the at least one processor (30) is configured to dynamically controlthe pneumatic unit (17) during the EEFRA period based on the monitored respiratory pressure to counteract a change in an intrathoracic pressure of the patient (3) duringspontaneous respiratory efforts by the patient (3).

14. The breathing apparatus (2) of claim 13, wherein the at least one processor (30) isconfigured to control the pneumatic unit (17) to deliver a flow of breathing gas into aninspiratory line (5) of the breathing apparatus (2) at least during the spontaneous respiratory efforts by the patient (3).

15. The breathing apparatus (2) of claim 13 or 14, wherein the at least one processor (30) isconfigured to control the pneumatic unit (17) to keep the monitored respiratory pressure substantially constant at a target pressure at least during the spontaneous respiratory efforts by the patient (3).

16. The breathing apparatus (2) of any of the claims 13-15, wherein the monitoredrespiratory pressure is an airway pressure (Paw) of the patient (3), the at least oneprocessor (30) being configured to dynamically control the pneumatic unit (17) to keep the airway pressure substantially constant at a target pressure above a set PEEP of the baseline ventilation.

17. The breathing apparatus (2) of claim 16, wherein the target pressure is:- set to a predefined fixed value above the set PEEP,- set based on a difference between the monitored airway pressure (Paw) and amonitored intrathoracic pressure of the patient (3) during a previous respiratory effortby the patient (3).

18. The breathing apparatus of any of the claims 13-17, wherein the at least one processor(30) is configured to dynamically control any or both of an inspiratory valve (27) and anexpiratory valve (29) of the pneumatic unit (17) to counteract the change inintrathoracic.

19. The breathing apparatus (2) of claim 18, wherein the at least one processor (30) isconfigured to apply any of the following control strategies: -a first control strategy involving the steps of dynamically controlling both theinspiratory valve (27) and the expiratory valve (29) of the pneumatic unit (17) tocounteract the change in intrathoracic pressure;- a second control strategy involving the steps of keeping the expiratory valve (29) of thepneumatic unit (17) open in a fixed position or by applying a constant control current to the expiratory valve while dynamically controlling the inspiratory valve (27) of the pneumatic unit (17) to counteract the change in intrathoracic pressure;- a third control strategy involving the steps of delivering a fixed inspiratory flowthrough the inspiratory valve (27) of the pneumatic unit (17) while dynamicallycontrolling the expiratory valve (29) of the pneumatic unit (17) to counteract thechange in intrathoracic pressure, or- a fourth control strategy involving the steps of dynamically controlling the expiratoryvalve (29) of the pneumatic unit (17) to keep an airway pressure (Paw) of the patientsubstantially constant at a set PEEP level when no spontaneous inspiratory effort is made by the patient (3), and, when a spontaneous inspiratory effort by the patient (3) is detected, closing the expiratory valve (29) and dynamically controlling theinspiratory valve (27) of the pneumatic unit (17) to counteract a decrease in theintrathoracic pressure during the spontaneous inspiratory effort.

Images