METHOD OF MEASURING MEAN BLOOD PRESSURE

Abstract

The present invention relates to a method for evaluating the average arterial pressure of a patient from plethysmography measurements by calculating an estimated value PAMest of the arterial pressure from the value of a parameter obtained at time t and a calibration value Calib evaluated at time t0.

Description

This linear regression is performed by any method known in the art, taking into account the relative weight of the PAMestn values ​​for each parameter. The factors a1, a2... are preferentially recalculated at each actual measurement of average cuff pressure. This method is preferably used when a certain amount of data is already available, which allows the quality of the coefficients to be refined. Thus, coefficients previously calculated on a cohort of other patients (at least 50, preferably at least 100 patients) can be used as initial coefficients. Indeed, even if there is inter-patient variability and the coefficients obtained on this cohort are not necessarily the best for the patient in question, these previously calculated coefficients can be used before being refined based on the data obtained for the patient. Thus, during each calibration, we calculate the PAMest with the coefficients previously used (the initial coefficients (coming from the cohort) during the first calibration) we compare this value to the measured PAM value we readjust the coefficients by giving more and more weight to the values ​​measured for the patient, in the regression as we have the PAM data measured in this patient. Thus, the methods described above are based on the fact that the mean arterial pressure can be measured as a function of parameters that can be measured continuously, preferably from a plethysmography measurement. In particular, the height of the dicrotic wave (or the ratio to the total value of the systolic wave and / or the diastolic wave) or the logarithm of the inverse of the perfusion index (increased by 1) are proportional to the mean arterial pressure. The method for obtaining the mean arterial pressure is therefore much simpler than those described in the prior art, while still being reliable. As mentioned above, the methods described are particularly useful for monitoring a patient's blood pressure continuously while the patient is under general anesthesia. This allows the doctor to to be able to act quickly in the event of too low pressure, without having to wait for the actual measurement obtained on the cuff. Thus, it is preferred when these methods are implemented continuously throughout the duration of a patient's general anesthesia. Furthermore, and in order to ensure patient safety, it is possible for a signal to be emitted when the MAPest value is below a predetermined threshold (this can be considered if the mean arterial pressure is below 65 mm Hg). Such a signal emitting step can be integrated into a method as described above. The signal can be a graphical signal (such as a representation of the mean arterial pressure by a color code different from the classic color code (red in case of alert instead of green or yellow). It is also possible to consider displaying the value with different color codes on the monitoring monitor, to alert the physician to a risk of hypotension. The alert signal emitted can also or alternatively be an audible signal (long beep or other) when the average arterial pressure falls below a predetermined value. This also alerts the surgeon to a problem, and the need for the anesthesiologist to perform the medical procedure to correct this hypotension. The invention also relates to a computer product / program comprising program code instructions recorded on a computer-readable medium, for implementing the steps of the methods described above, when said program is executed on a computer. This program may also contain code instructions for displaying the mean arterial pressure value on a monitor. It may also include code instructions for emitting a visual and / or audible signal if the estimated mean arterial pressure value is below a predetermined threshold (pre-programmed or entered by the practitioner). It may also include code instructions for an actual blood pressure measurement to be taken on the patient if the estimated value is below a predetermined value. Thus, the program can request and order a blood pressure measurement before the prescribed time, without the intervention of a human being. The invention also relates to a computer-readable recording medium on which is recorded a computer program comprising program code instructions for carrying out the steps of the methods as described above or programs as described above. The invention may also comprise a device for implementing a method as described above, comprising: means for receiving mean arterial pressure measurement data, in particular as taken by non-invasive method (cuff) means for receiving measurement data of one or more continuously measured parameters (in particular the height of the dicrotic wave, or the perfusion index) calculation means allowing the calculation of a Calib coefficient for each actual measurement of average arterial pressure calculation means for calculating an average arterial pressure at each time t as a function of the value of the parameters at this time t, according to the methods described above means of presenting the average arterial pressure calculated at each time t (possibly including alert means in the event that the calculated average arterial pressure is lower than a predetermined value) possibly means allowing automatic actuation of the device to measure the mean arterial pressure, in the event that the calculated mean arterial pressure is lower than a predetermined value. The computing means are essentially processors enabling the execution of the computer products / programs as mentioned above. The means for receiving data, presenting the average arterial pressure, or actuating the sphygmomanometer are conventional means in the art. The methods described above are of primary interest in the field of anesthesia and post-intervention monitoring room, but are also usable in other fields such as resuscitation (especially hyperventilated patients), cardiology, community medicine, or emergency medicine (pre-hospital and inter-hospital). The methods and devices can also be used in sports medicine. The methods and devices can also be used to assess mean arterial pressure in stress tests in a patient. BRIEF DESCRIPTION OF THE FIGURES Figure 1: Example of a graph of a sphygmomanometer signal (obtained from the site https: / / www.infirmiers.com / etudiants-en-ifsi / cours / cours-cardiologie-la-pression-arterielle-et-sa-mesure.html) Figure 2: Principle of pulse oximetry. (1): variable light absorption linked to the variation in the volume of arterial blood. (2) constant light absorption linked to the non-pulsatile part of the arterial blood. (3): constant light absorption linked to venous blood. (4): constant light absorption linked to tissues, bones, etc. From Feissel, Réanimation 16 (2007) 124-131. Figure 3: Representation of the variables that can be used in the context of the invention, based on a pulse oximeter trace. T1: duration between the start and the maximum of the pulse wave; T2: duration between the start of the pulse wave and the dicrotic wave; T3: total duration of the pulse wave; Hd: height of the dicrotic wave. Figure 4: Representations of different types of Plethysmography signals with identification of the dicrotic wave. Figure 5: flowchart representing the implementation of a method according to the invention. Figure 6: flowchart representing the implementation of another embodiment of a method according to the invention. Figure 7: A. Representation, in a representative patient, of the MAP measured by invasive method (ARTm, large dotted lines), the MAP estimated by a method according to the invention on the basis of the height of the dicrotic wave (PlethoMAP, small dotted lines), the MAP measured by the cuff sphygmomanometer (NBPm, black dots) and the calibration factor (Calib, solid line). B. Representation, for the same patient and the same period, the evolution of the systolic peak (solid line), the dicrotic (small dotted lines) and the diastole (large dotted lines) over time. A graphical representation of the superimposed pulse wave is also shown in this figure, only to improve understanding of the three points (what each value corresponds to). Note: the time scales of this representation of the pulse wave and the evolution of the peaks over time are different and this representation of the pulse wave (usually lasting 1 sec or less) is only present for informational purposes. EXAMPLES Example 1. Determination of parameters usable for continuous blood pressure measurement In cardiology, it is thus possible to measure the variation of the pulse wave in a non-invasive manner by plethysmography. This produces a curve (graph) representing the excess volume due to systolic expulsion. The perfusion index (PI) reflects the amount of blood flow measured locally and partly from the pulsed arterial flow, from the stroke volume. The perfusion index represents the area under the curve mentioned above. In the case of photoelectric plethysmography, an SpO2 value is also obtained, representing the pulsed oxygen saturation. The curve represents the profile of the pulse wave and allows us to see the dicrotic wave leaving the heart at systole (a second peak (possibly two peaks), a plateau or a break in the decrease) in the organ. This plethysmography signal can be used for continuous mean arterial pressure measurement. We thus use variations in peak size (of the dicrotic wave), area (value of the perfusion index) or timing between two events in the pulse wave profile. In particular, we can actually see the decay of the dicrotic wave, that is to say, we can detect this dicrotic wave with certainty in the plethysmography signal. In order to determine these parameters, it is possible to analyze the pulse wave. Pulse wave foot First, the pulse wave's foot can be detected. The pulse wave's foot is characterized by a rapid rise in the signal, which is translated by a peak of the second derivative of the signal. The analysis of this second derivative of the signal obtained by the plethysmograph makes it possible to obtain a signal (peak of the second derivative) at each ascending edge of the pulse wave, and therefore to detect the foot of the pulse wave, and therefore the moment corresponding to the start of the signal. Systole (peak of the pulse wave) Starting from the foot of the wave, for each cycle, we look for the maximum value of the cycle. To do this, we can divide the signal into several time windows and look at the maximum value in each time window. We can thus determine the local maximum value corresponding to the maximum of the pulse wave. We thus obtain the maximum value (absorbance value given by the pulse oximeter corresponding to the peak of the systolic wave) the time at which this maximum is reached We can therefore calculate the time between the start of the pulse wave and the maximum of the pulse wave. Dicrotic wave Once the peak of the pulse wave has been identified, the second derivative of the signal is analyzed over predetermined time windows (between 50 ms and 300 ms). The local maximum of the second derivative is sought, which is located after the systolic peak, in order to determine an area of ​​interest in which the minimum in absolute value of the first derivative is sought. The dicrotic point corresponding to the dicrotic wave corresponds to this point for which the minimum in absolute value of the first derivative is reached. Its value (absorbance value given by the pulse oximeter) can then be measured, as well as the duration between the bottom of the pulse wave and this point. Exemplification of practical application of this method Signal collection Signal collection was performed in real time from a standard patient monitor capable of providing photoplethysmography waveform as well as non-invasive blood pressure via a blood pressure cuff. Connection to the monitor is usually via an RS232 serial port or a network connection via Ethernet or Wi-Fi. In addition to these two essential parameters, the perfusion index (PI) value was also used. Communication with the monitor can be bidirectional and allow, for example, to request a new non-invasive pressure measurement on demand. Signal analysis Signal processing is based on an online algorithm that takes the measured values ​​and produces a beat-by-beat result in real time. Pulse wave foot The software uses a heartbeat detection algorithm that relies on detecting the pulse wave's foot. The pulse wave's foot is characterized by a rapid rise in the signal, resulting in a peak in the second derivative of the signal. The pulse wave foot detection is based on the second derivative of the signal. This second derivative is weighted by the first derivative to focus only on the rising part of the signal (i.e. the second derivative is given a zero value when the first derivative is not positive). The values ​​obtained are squared and then integrated in a floating manner over a centered 240ms window (average over values ​​120ms before and 120ms after the desired point). This integrated signal makes it possible to obtain a powerful signal at each rising edge of the pulse wave. This signal is compared to a threshold value. This threshold value depends on the patient, the equipment used, the shape of the plethysmography signal as well as the measurement noise. Since the measurement noise is not constant, the threshold is necessarily adaptive in real time. To calculate the threshold, we use the integral (calculated above) to which a floating average is applied with a 3s window (centered or not), the result being multiplied by 1.5. The threshold thus obtained makes it possible to define a zone of interest where the integral exceeds the threshold. Within this zone of interest, the peak of the second derivative defines the foot of the wave. Systole (peak of the pulse wave) From the foot of the wave, for each cycle, the detection of the dicrotic wave is carried out in two stages. The first stage consists of finding the systole and therefore the maximum value of the cycle. To do this, the signal is divided into 50 ms windows and the signal is advanced window by window until higher values ​​are found. Once the highest value has been exceeded, the maximum value local (corresponding to the maximum value of the pulse wave, or systole value). Dicrotic wave Once the peak of the pulse wave has been identified, we progress from this point by analyzing the second derivative of the signal, progressing through 150ms windows. We thus look for the local maximum of the second derivative which is located after the systolic peak. Once this peak is identified, it informs of an area of ​​interest in which the dicrotic wave is located. From this point, the first derivative is analyzed and the minimum in absolute value of the first derivative is sought in a window of 8ms from the peak of the second derivative. This gives the minimum of the first derivative close to the peak of the second derivative. This point is defined as the dicrotic point corresponding to the dicrotic wave and its value can be measured (total absorbance value given by the pulse oximeter). Example 2. Calibration and continuous estimation of mean arterial pressure (MAP) Calibration requires the value (Vp) of at least one of the following parameters height of the dicrotic wave value of the logarithm (natural or decimal) of the inverse of the perfusion index PI (In (1 / PI + 1)). We add 1 to the inverse of the perfusion index to avoid taking the logarithm of a value less than 1, and obtaining a negative result value of the duration between the foot of the pulse wave and the dicrotic wave and value of the duration between the foot of the pulse wave and the maximum of the pulse wave ratio of “dicrotic wave height / pulse wave height” and / or “dicrotic wave height / diastolic wave height”. total pulse wave duration These values ​​can be obtained beat by beat (i.e., for each pulse wave). Figure 3 shows how these variables are measured. Calibration also requires the mean arterial pressure (MAP) value which can be obtained in particular by a non-invasive blood pressure cuff. An average over several cycles (2, 3, 4, 5, 6, 8 or 10 cycles) of the value of the chosen parameter can be used. This allows us to avoid disturbances such as respiratory pressure variability and irregularity of the cardiac cycle. This average is preferably the statistical median rather than the arithmetic mean. A calibration factor Calib is estimated when taking non-invasive blood pressure and is obtained by Calib = MAP / Vp. It is understood that the Calib value depends on the chosen parameter and that the Calib value obtained if we choose the value of the dicrotic wave will be different from the Calib value if we choose the logarithm of (the inverse of the perfusion index (PI) + 1). Once the Calib value is obtained, the mean arterial pressure is estimated beat by beat using, as the sole data source, the photoplethysmography signal. The estimated PAM (PAMest) at time t is calculated by the formula PAMest = Calib x VPt, where Vpt is the value (possibly averaged) of the chosen parameter. Application exemplification (the parameter being the dicrotic wave) Calibration requires beat-to-beat dicrotic waveform values ​​as well as intermittent mean arterial pressure values, for example, from a non-invasive blood pressure cuff. The dicrotic pressure value obtained from the dicrotic waveform is averaged over several cycles. The number of cycles over which the value is averaged is adjustable (e.g. 5 cycles). This allows for overcoming disturbances such as respiratory pressure variability and cardiac cycle irregularity. Averaging is done using the statistical median rather than the arithmetic mean to be more robust in the presence of noise. A calibration factor is estimated when taking non-invasive blood pressure and is based on the current averaged dicrotic value (Pdic) and the measured mean arterial pressure (MAP). The calibration factor is obtained by Calib= MAP / Pdic. Continuous estimation of mean arterial pressure (MAP) Once the calibration is performed, the mean arterial pressure is estimated beat by beat using the absorption measured by photoplethysmography as the sole signal source. The estimated MAP (PAMest) is calculated by PAMest= Calib-Pdic, where Pdic is the averaged value of the dicrotic wave value as described above. Another example (Use of perfusion index), especially as signal quality The PI value serves as an indicator of signal quality. A PI value below 0.1% indicates a poor photoplethysmography signal and alerts the user to poor estimation quality and the need for more frequent calibration. Signal quality can generally be improved in these cases by properly repositioning the sensor on the patient. Integration into the estimate The PI provides information on the hemodynamic state in the mean arterial pressure in the same way as the dicrotic wave and in a complementary manner. Indeed, the PI generally evolves in the opposite direction of the mean arterial pressure. We use a measure called mPI (for modified PI) and which is calculated as follows: mPI = 10 x ln(1 / PI + 1). The mPI thus obtained varies in the same direction as the mean arterial pressure and the behavior is linearized compared to the exponential behavior of the PI. A calibration Calib is performed with the measured mean arterial pressure and mPI at time 0 and the mean arterial pressure at time t is calculated using PAMest = Calib x mPI(t). Using multiple parameters Several parameters can be used (height of the dicrotic wave, mPI, durations indicated above). We can regularly calculate a Calib for each parameter calculate the PAMest for each of the parameters define the PAMest by statistical mean (Kalman filter) by weighting these PAMest values ​​and using the value calculated at the previous time Example 3. Exemplification in real conditions These results were obtained based on the height of the dicrotic wave, similar results can be obtained with the other parameters. The study was performed in the neurosurgery operating room or during an interventional neuroradiology procedure. Patients received the usual basic care for this type of intervention, including: Monitoring Non-invasive hemodynamic monitoring of blood pressure by oscillometry, as well as continuous ECG monitoring Continuous monitoring of pulse oxygen saturation (SpO2) by photoplethysmography, as well as monitoring of exhaled CO2. Monitoring of anesthesia depth by the bispectral index (BIS) Induction and maintenance of general anesthesia using a target-controlled intravenous (TCI) method including propofol and remifentanil, and curarization before orotracheal intubation using atracurium besilate. All monitors were connected to a Philips monitor. Arterial hypotension was defined as a decrease in mean arterial pressure (MAP) of at least 20% compared to basal MAP. When arterial hypotension was observed, the anesthesiologist in charge of the patient was free to lighten the anesthesia, administer vascular filling or a vasoconstrictor (Ephedrine 9mg, Phenyephrine 50mcg or Noradrenaline 10mcg). Experimental protocol Phase 1: Preoxygenation (baseline) - Anesthetic induction. • Pre-oxygenation for 2 min: It is during this phase and before any injection that the basic values ​​of all parameters are recorded (average of 2 values, baseline) • Remifentanil at 5 ng / ml target concentration for 1 min. • Propofol 5 pg / ml target concentration. • After the BIS has fallen below 50 and verification of the absence of ciliary reflex and the patient is manually ventilable: curarization with 0.5 mg / kg of tracrium. • Wait 3 min with manual ventilation. Phase 2: Laryngoscopy-Intubation-Manual ventilation. • Direct laryngoscopy. • Orotracheal intubation. • Manual ventilation and probe fixation. Phase 3: Mechanical Ventilation-Anesthesia Maintenance. • Connecting the patient to the ventilator and starting mechanical ventilation. • Decrease in Remifentanil and Propofol targets to 3.5 ng / ml and 4 pg / ml respectively. • Continued collection for 3 min. Phase 4: Correction of possible hypotension by vasoconstrictor. • Hypotensive episode treated by administration of vasoconstrictor. • Continue collection one minute after the vasoconstrictor has taken effect. • End of the collection. During phases 1, 2 and the beginning of 3, the cuff pressure was taken every minute for an estimated average duration of 15 min. The anesthesiologist in charge of the patient was free to deviate from the initial protocol at any time if he judged that the clinical situation required it. Data collection was carried out using the Extrend data capture software (Ixellence) collecting signals at a frequency of 125Hz and all digital values. A collection point of all the following parameters was carried out every minute: - Dicrotic wave height: the calculation of the dicrotic wave height on the plethysmography signal. - IP: the perfusion index was collected beat by beat. Results: 61 patients were included in the study (median age 55 years, 32.7% men / 67.3% women). 54 out of 61 patients had at least one episode of hypotension. The incidence of hypotension, defined as a decrease in MAP >20%, in our population was 88.5%). The time spent with MAP <20% averaged 5.2 min during induction, or 44% of the time. Evolution of values ​​over the entire induction. The mean duration of the entire induction phase was 12 ± 4 min. Evolution P AM and height of the dicrotic wave PAM variations and dicrotic wave height variations were strongly linearly correlated over the entire induction duration (see Figure 7, especially the stability of the Calib value (Figure 7.A)). Analysis based on continuous mean arterial pressure measurement by invasive arterial catheterization. The arterial catheter is a device allowing arterial access in order to measure blood pressure, invasively and continuously, and to take arterial blood samples. Invasive blood pressure is an invasive technique for monitoring intravascular blood pressure using an arterial catheter. Continuous blood pressure measurement was carried out by arterial catheter and by the method according to the invention (calculation via the height of the dicrotic wave measured by plethysmography). A perfect correlation was observed between the variations in MAP measured by arterial catheter and by the method, even after the use of vasopressor drugs. The correlation was r=0.88 with a 96% agreement between the variations obtained by the technique and the actual measurement of MAP (Figure 7.A). In Figure 7.A, we can see in particular at the end a significant variation in pressure which is not detected by the cuff (due to the time between two pressure readings), but is detected by the PlethoMAP signal, and which therefore underlines the informative nature of the method using the dicrotic wave. In Figure 7.B, we can clearly see that only the measurement of the height of the dicrotic wave allows us to obtain a value of the PAM, and that the evolution of the other two parameters (systolic value or diastolic value) is not sufficiently informative. We can also see (T = 1.15h) that the drop in height of the diastolic wave is indicative of the actual drop in MAP (figure 7.A), while the systolic and diastolic pressures do not vary. The combination of these two pressures would therefore not have made it possible to detect the drop in mean arterial pressure, and to allow the anesthesiologist to take any corrective measures.

Claims

CLAIMS 1. Ex vivo method for assessing the mean arterial pressure of a patient, based on values ​​of a parameter calculated by plethysmography, comprising the steps: I. Calculate a Calib calibration value from a. From the value of the mean arterial pressure measured at time tO b. Of the VpO value linked to said parameter, measured in the patient at time tO, IL Calculate the estimated value PAMest of the patient's blood pressure at a time t after tO by the formula PAMest = Calib x Vpt, in which Vpt is the value of the parameter measurement obtained at time t characterized in that the value Vpt of the parameter is the height of the dicrotic wave.

2. Ex vivo method according to claim 1, characterized in that the Vpt value of the parameter was obtained by plethysmography taken on the finger or the earlobe.

3. Method according to claim 1 or 2, characterized in that the height of the dicrotic wave is combined with one or more elements chosen from the group consisting of the logarithm of (the inverse of the perfusion index (PI) + 1), the start of the pulse wave and the dicrotic wave, and the duration between the start of the pulse wave and the maximum of the pulse wave.

4. Ex vivo method according to one of claims 1 to 3, characterized in that the Vpt value used is a value averaged from several values ​​measured over a predetermined duration.

5. Ex vivo method for assessing mean arterial pressure in a patient, characterized in that a. the method of one of claims 1 to 4 is repeated, for different parameters, in order to obtain several PAMest values ​​at time t b. we calculate a final PAMest value by statistical estimation taking into account i. of the different PAMest values ​​calculated at time t ii. one or more PAMestfinale values ​​calculated before time t.

6. Ex vivo method according to claim 5, characterized in that the final PAMest value is calculated by Kalman filter in discrete context.

7. Computer product / program comprising code instructions program recorded on a computer-readable medium, for implementing the steps of the method according to one of claims 1 to 6, when said program is executed on a computer.

8. A computer-readable recording medium on which is recorded a computer program comprising program code instructions for carrying out the steps of the method according to one of claims 1 to 6.

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