Method for measuring the volume of rebreathed exhaled gas in a ventilation system

Abstract

A method for determining rebreathed gas fractions in a respiratory gas supply system using specific respiratory or ventilation parameters, wherein the expiratory flow and the leakage flow are recorded during a respiratory cycle, the expiratory and leakage volumes resulting from the integration of these two flows are determined, and the rebreathing fraction (VR) is determined as the difference between the expiratory volume and the leakage volume (VR = VE - VL), and an alarm is generated if a critical value for the determined FiCO2 is exceeded or if VR > 0, characterized in that, if VR > 0, the fraction of CO2 in the subsequent inspiration that is formed at the end of inspiration by the mixing of rebreathed air with fresh air in relation to the total inspired volume in the lungs (FiCO2) is estimated, and wherein the estimation of FiCO2 is carried out taking into account anatomical dead spaces.

Description

The invention relates to a method for detecting a rebreathed exhaled gas volume in a ventilation system. Open leakage systems for NIV require minimum pressures to guarantee CO2 washout for all possible combinations of respiratory rate, I:E ratio, leakage, pressure, volume, and flow rate. These minimum pressures are naturally based on worst-case combinations of the input parameters. A standard value for such a minimum pressure is currently 4 hPa for common device combinations (device + patient interface) in non-invasive ventilation. Ventilation devices for, e.g., apnea patients are known from the prior art. During ventilation, the patient wears a ventilation mask connected via a tube or tubing system to a gas supply and metering device, allowing a breathing gas, such as oxygen-enriched air, to be delivered into the patient's airways. During inspiration, the patient inhales the oxygen-rich gas and exhales the oxygen-depleted air during expiration. Depending on the mask design, this exhaled air either partially enters the mask chamber or reaches the tubing and is thus available for reintroduction into the airways during a subsequent inspiration. To prevent the exhaled air, which has a high carbon dioxide (CO2) content, from being re-inhaled and thus immediately exerting its toxic effect, respiratory masks have devices (leakage element) that ensure the CO2-rich air is discharged as directly as possible from the mask chamber to the environment, so that it is not retained in the mask chamber or in the tubing system. A ventilation mask that has such a device for the removal of exhaled air is disclosed in DE 101 58 066 A1. The ventilation mask described here is used with a device for oxygen supply and has a mask body that is connected via a coupling element to an exhalation element (leakage element) that defines a slit-like outflow channel. Another mask is known from DE 199 03 732 A1, which describes a mask body which is connected via an exhalation element (leakage element) to a coupling element which is connected via a breathing tube to a ventilator. Special applications, such as pediatric ventilation, require expiratory pressures of less than 4 hPa (e.g., 2 hPa). Under "favorable" conditions of respiratory rate, I:E ratio, leakage, pressure, volume, and flow rate, CO2 washout via the leakage system is still possible. Ventilation procedures are implemented using state-of-the-art devices, the execution of which is essentially based on controlling and regulating the volume of respiratory gas supplied to the patient. The volume of gas exhaled by the patient is not measured; if it remains in dead spaces within the ventilation device, particularly the mask, it can be rebreathed by the patient and thus pose a health risk. A method and a device for providing ventilation support are already known from DE 697 21 330 T2. The parameters to be considered, such as pressure, flow, and volume, are explained with regard to their relationships. DE 690 21 681 T2 also discloses a method and a device for providing ventilation support. Here, too, the relationships between the parameters to be considered, as mentioned above, are explained. Another such method and device are also described in DE 100 23 473 A1. Further prior art is known from WO 2003 / 0 55 552 A1. Based on this prior art, the present invention aims to provide an improved method for measuring the volume of rebreathed exhaled gas. This problem is solved by a method with the characterizing features according to independent claim 1. Preferred embodiments are described by the dependent claims. According to the invention, in a first step the expiratory leakage is estimated against the patient's exhaled tidal volume. If the tidal volume is smaller than the expiratory leakage, the exhaled, CO2-enriched volume can be completely flushed out during expiration. If this is not the case, a certain amount of CO2 is re-inhaled. The intrapulmonary CO2 concentration can then be estimated in a second step, taking into account inspiratory leakage and a worst-case scenario analysis of respiratory dead spaces. This concentration is then compared with generally accepted limits for CO2 levels in the breathing air, and if necessary, either an alarm is generated or the ventilation pressure level is increased (EPAP or preferably IPAP and EPAP to maintain effective ventilation pressure -> increased washout). Furthermore, it is conceivable that the calculation could be refined by taking into account given dead space volumes (mask dead space, anatomical dead space). For common masks, the relevant data could, for example, be stored in a memory and used for the calculation. (Input of the system used on the ventilator.) The specific anatomical dead space volume of a patient can also be taken into account to increase accuracy. The dead space can be estimated, for example, based on body weight (e.g., dead space volume in ml is approximately twice the body weight in kg). The embodiments of the present invention disclose a method for detecting a rebreathed volume of exhaled gas. One embodiment relates to detecting the volume of exhaled gas that is breathed into the ventilation system and that, due to the prevailing pressure conditions in the system, the gas flows and volumes that can hold gas, combined with insufficient gas removal from the system via an existing leak, is rebreathed. The present method according to the invention enables the measurement of the corresponding gas flows and leakage volumes, whereby the actual exhaled gas volume is compared to the exhaled gas volume that can be removed in the given time and the leakage volume. If it turns out that the exhaled gas volume to be removed does not correspond to the actual volume removed, the system parameter pressure is changed so that the ratio of exhaled gas to gas flowing out of the system changes in such a way that the patient is advantageously protected from rebreathing the CO2-rich and toxic exhaled gas. Further embodiments of the present invention relate to the generation of an automatic alarm or signal when the flows of the system change to the detriment of the patient. Improved determination of the actual CO2 concentration in the case of rebreathing (A>B) can be achieved by calculating the dead volume using a more refined approach. This approach can consider not only the dead volume in the connecting tube and mask, but also the dead volume in any exhalation piece, if present, as well as the anatomical dead volume. The determination of the anatomical dead volume can, for example, be based on the prior determination of other anatomical factors such as body weight. The relationship between body weight and anatomical dead volume can be determined as a factor and entered into the calculation and control unit, so that if the patient's weight changes, the new anatomical dead volume is easily taken into account by the ventilator device, simply by multiplying the new body weight by the factor to obtain the new dead volume to be considered. There are several ways to calculate dead space. ▪ In addition, dead spaces (anatomical dead space and dead spaces of the interface used) that lead to additional pendulum air movement are estimated. ▪ The specific anatomical dead space (based on 1 kg body weight) can be approximated using the following formula: adapted from: Numa AH, Newth CJ. Division of Pediatric Critical Care, Children's Hospital, Los Angeles, University of Southern California 90027, USA. PMID: 8727530 [PubMed - indexed for MEDLINE] ▪ The physician enters the patient's age and body weight into the device, from which the device calculates the theoretically present anatomical dead space. ▪ The (functional) mask dead space can be taken into account by the user entering the mask type into the device (using stored parameters) or by the user (physician) estimating the dead space based on the mask size, possibly with the aid of a general anesthetic (GA).Both dead spaces must be subtracted from the determined VT (tidal volume) to calculate alveolar ventilation. If a gooseneck device is used in the breathing circuit, this must also be included. In one embodiment of the invention, the rebreathed gas fractions in a breathing gas supply system are determined using specific breathing or ventilation parameters (pressure, frequency, etc.). During a breathing cycle, the expiratory flow rate and the leakage flow rate are measured. By integrating these two flows over the expiratory phase, the resulting expiratory and leakage volumes are determined, as well as the rebreathing fraction (VR) as the difference between the expiratory volume and the leakage volume (VR = VE - VL). In a further embodiment of the invention, the volumes are determined by the device via the measurement of the absolute flow rate (Qabs = QLung + QLeakage). When this absolute flow rate falls below zero, the volume formed (VA) is calculated as a measure of the exhaled air temporarily stored in the system. The volume formed when the absolute flow rate subsequently exceeds zero until the start of the next inspiration (VB) is considered a measure of the volume expelled from the system. From these two volumes, the difference volume VR = VA - VB can be calculated. If VR > 0, the proportion of CO2 in the subsequent inspiration, which is formed end-inspiratory by the mixing of rebreathed air with fresh air in relation to the total inspired volume in the lungs (FiCO2), is estimated. In a preferred embodiment of the invention, an alarm is generated when a critical value for the determined FiCO2 is exceeded or when VR > 0, which is intended to alert the user or operator of the device to the critical condition. In a further preferred embodiment of the invention, if a critical value for the determined FiCO2 is exceeded or if VR > 0, the therapy pressure is increased at least temporarily during a breathing phase to increase the leakage volume and to reduce the rebreathed gas. To estimate the FiCO2 within the alveoli, the anatomical and systemic dead spaces can be taken into account in such a way that the system-related dead space (e.g. mask, tube) and / or the anatomical dead space is initially filled with CO2-rich air from the end of the previous expiration. Furthermore, although the system-related dead space and / or the anatomical dead space can be considered as O2-rich and CO2-poor at the end of inspiration, it cannot be taken into account when determining FiCO2, for example within the alveoli, since this gas does not participate in alveolar gas exchange. The device designed for supplying respiratory gas according to the above methods has at least one source (pressure or flow source) for providing respiratory gas and at least one sensor for detecting respiratory flows, as well as optional additional sensors for measuring pressure and other ventilation parameters and means for regulating and controlling ventilation parameters. The flow and pressure can be measured either at the device output, inside the device, or directly at the patient interface. Further embodiments, as well as some of the advantages associated with these and other embodiments, will be clarified and made more easily understandable by the following detailed description, supported by the figures. The figures are merely schematic representations of embodiments of the invention. They show: Fig. 1 a schematic representation of a ventilator with a ventilation mask, Fig. 2a a graph illustrating the curves of total flow and leakage flow during an inspiratory-expiratory phase, Fig. 3a a graph illustrating the curve of total flow during an inspiratory-expiratory phase, Fig. 3b a graph illustrating the curve of pressure during an inspiratory-expiratory phase, Fig. 4 a graph illustrating some components of the invention. Fig. 1 shows a basic structure of a ventilation device. The device for supplying breathing gases comprises a housing 1 with a control panel 2 and a display 3, and has a breathing gas pump located inside it. A connecting hose 5 is attached via a coupling 4. An additional pressure measuring hose 6 can run along this connecting hose 5 and can be connected to the housing 1 via a pressure inlet port 7. The housing 1 has an interface 8 to enable data transmission. An exhalation element 9 is arranged in the area of ​​an extension of the connecting hose 5 facing away from the device housing 1. An exhalation valve with a leakage element (not shown) can also be used. The ventilation mask shown in Fig. 1 is designed as a nasal mask 10. In this case, it is attached to the patient's head via a head covering 11. In the area of ​​its extension facing the connecting tube 5, the nasal mask 10 has a coupling element 12. The connecting tube thus connects the nasal mask and the device housing 1, allowing the respiratory gas to enter the mask. Figure 2a illustrates the flow profiles of the individual flows over an inspiratory-expiratory cycle. The total flow FG is shown, which initially rises sharply during the inspiratory phase, reaches a plateau, and then drops abruptly at the transition to the expiratory phase, falling below the zero line. During the expiratory phase, the flow increases, eventually entering the positive range. The leakage flow FL,I remains constant during the inspiratory phase at a level determined by the set pressure pI. Since the pressure pE is lower during the expiratory phase, the leakage flow FL,E remains constant at a lower level during expiratory phases. Subtracting the respective leakage flow from the total flow at each point yields the inspiratory and expiratory flow FI and FE, respectively. The curve of the total flow FG shown in Fig. 3a illustrates a variation of the total flow FG shown in Fig. 2a over the leakage flow FL, which follows a similar pattern to the flow shown in Fig. 2a, depending on the actual tidal volume. Fig. 3b shows the applied pressures pI and pE. Figs. 3a and 3b clearly demonstrate that rebreathing into the ventilation system, i.e., into the mask or tubing, only occurs when the total flow FG is less than zero. This is the case precisely when the leakage volume is too small relative to the total flow FG to remove the expiratory flow FE; see Fig. 3a. Therefore, if the flow measured by the device becomes less than zero, which occurs at the first intersection of the total flow curve with the zero line, area A is measured, as shown in Fig. 3a, until the second positive intersection (second intersection) of the zero line. From this second intersection of the total flow curve with the zero line to the intersection of the total flow line with the leakage flow line calculated according to (1), area B is measured. If area A is larger than area B, the gas exhaled by the patient was not completely vented through the leak. This results in rebreathing, meaning the patient is inhaling some of their own CO2-enriched exhaled air again. This is detrimental if the patient's exhaled air contains a higher CO2 concentration than ambient air considered safe under current standards. In addition to the CPAP and BIPAP applications mentioned above, the method according to the invention could also be used in medical devices such as APAP, bilevel, titration, home ventilation, emergency, or hospital ventilators. Fig. 4 shows a diagram to facilitate understanding of the device and the procedure. In addition to the evaluation unit for the expiratory volume, the device contains an evaluation unit for the expiratory leakage volume. The values ​​from these evaluation units are fed to the calculation unit for determining the CO2 concentration. If the rebreathing fraction is greater than 0 or the FiCO2 is above a certain limit, an alarm can be triggered or a pressure adjustment can be performed. The former serves to alert the users to the alarm situation, the latter to reduce the level of CO2 accumulation by changing system parameters. SHORT DESIGNATIONS CO2 Carbon dioxide cCO2 Carbon dioxide concentration cCO2, or respirate CO2 concentration PE Expiratory pressure, device-side PI Inspiratory pressure, device-side VT Tidal volume Vtot Dead space volume VG = VL + VT Total volume of gas VG,E = VT,E + VT,E Total volume of gas that flows out during expiration FGI = FLI + FpatI Inspiratory flow FGE = FLE + FpatE Expiratory flow FL Leakage flow FG = FL + Fpat Total flow, applies during inspiration and expiration FT,I Inspiratory patient flow FT,E Expiratory patient flow FL,E Expiratory leakage flow FL,I Inspiratory leakage flow FG,E Total expiratory flow Fpat Patient flow VE Expiratory volume (VT) VL Leakage volume VR Rebreathing fraction Qabs Absolute flow (measured / determined at the device) QLunge Absolute flow of the leakage QLeakage Absolute flow of the patient

Claims

A method for determining rebreathed gas fractions in a respiratory gas supply system using specific respiratory or ventilation parameters, wherein the expiratory flow and the leakage flow are recorded during a respiratory cycle, the expiratory and leakage volumes resulting from the integration of these two flows are determined, and the rebreathing fraction (VR) is determined as the difference between the expiratory volume and the leakage volume (VR = VE - VL), and an alarm is generated if a critical value for the determined FiCO2 is exceeded or if VR > 0, characterized in that, if VR > 0, the fraction of CO2 in the subsequent inspiration that forms at the end of inspiration by the mixing of rebreathed air with fresh air in relation to the total inspired volume in the lungs (FiCO2) is estimated, and wherein the estimation of FiCO2 is carried out taking into account anatomical dead spaces. The method according to claim 1, characterized in that the determination of the volumes is carried out by measuring the absolute flow rate (Qabs = Qlunge + Qleakage), wherein when the absolute flow rate falls below zero, the volume formed (VA) is used as a measure of the expiratory air temporarily stored in the system, and the volume formed (VB) when the absolute flow rate subsequently exceeds zero until the start of the next inspiration is used as a measure of the volume washed out of the system, and a difference volume VR = VA-VB is formed. Method according to claim 1, characterized in that the anatomical dead spaces are determined and / or estimated using approximation functions and patient information. Method according to one of the preceding claims, characterized in that the estimation is additionally carried out taking into account system-related dead spaces. Method according to one of the preceding claims, characterized in that the system-related dead space and / or the anatomical initial inspiratory dead space is taken into account in such a way that its volume is filled with CO2-rich air from the end of the previous expiration. Method according to one of claims 1 - 3, characterized in that the system-related dead space is to be regarded as O2-rich and CO2-poor at the end of inspiration, but is not taken into account in the determination of FiCO2, since this gas does not participate in alveolar gas exchange. A device for supplying respiratory gas, comprising at least one source for providing respiratory gas and at least one sensor for detecting respiratory flows, as well as means for regulating and controlling ventilation parameters, in which the expiratory flow and the leakage flow are detected during a respiratory cycle, wherein the expiratory and leakage volumes resulting from the integration of these two flows are determined, and the rebreathing fraction (VR) is determined as the difference between the expiratory volume and the leakage volume (VR = VE - VL), and in that, if a critical value for the determined FiCO2 is exceeded or if VR > 0, the therapeutic pressure is increased at least temporarily during a respiratory phase to increase the leakage volume and reduce the rebreathed gas, characterized in that, if VR > 0, the proportion of CO2 in the subsequent inspiration,the end-inspiratory FiCO2, which is formed by the mixing of rebreathed air with fresh air in relation to the total inspired volume in the lungs, is estimated, taking into account anatomical dead spaces. Device according to claim 7, characterized in that the determination of the volumes is carried out by measuring the absolute flow (Qabs = Qlunge + Qleakage) using a device-side flow sensor, wherein when the absolute flow falls below zero, the volume formed (VA) is used as a measure of the expiratory air temporarily stored in the system, and the volume formed (VB) when the absolute flow subsequently exceeds zero until the start of the next inspiration is used as a measure of the volume washed out of the system, and a difference volume VR = VA-VB is formed. Device according to one of claims 7 or 8, characterized in that an alarm is generated when a critical value for the determined FiCO2 is exceeded or when VR > 0. Device according to claim 7, characterized in that the anatomical dead spaces are determined and / or estimated using proximity functions and patient information. Device according to one of the preceding claims, characterized in that the estimation is additionally carried out taking into account system-related dead spaces. Device according to claim 11, characterized in that the system-related dead space is taken into account at the initial inspiratory stage in such a way that its volume is filled with CO2-rich air from the end of the previous expiration. Device according to one of claims 7 - 10, characterized in that the system-related dead space is to be regarded as O2-rich and CO2-poor at the end of inspiration, but is not taken into account in the determination of FiCO2, since this gas does not participate in alveolar gas exchange.

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