Blood volume measurement method, blood volume measurement device, and program

Abstract

A blood volume measurement method for calculating a cardiac output includes a first measurement step of measuring a first value in a predetermined section of a first physiological information parameter, a second measurement step of measuring a second value in a section longer than the predetermined section of a second physiological information parameter, and the pulse pressure of the subject, a baseline determination step of determining whether or not a difference between the measured first value and the measured second value is equal to or less than a threshold value, a calibration step of, when it is determined in the baseline determination step that the difference between the first value and the second value is equal to or less than the threshold value, calculating a subject-specific coefficient of the subject based on the second value, and a calculation step of calculating the cardiac output based on the first value and the subject-specific coefficient calculated in the calibration step.

Description

BLOOD VOLUME MEASUREMENT METHOD, BLOOD VOLUME MEASUREMENT DEVICE, AND PROGRAM

[0001] The present disclosure relates to a blood volume measurement method, a blood volume measurement device, and a program.Background

[0002] In a medical facility, it is necessary to monitor the hemodynamic changes of subjects in operating rooms, intensive care units, emergency rooms, dialysis rooms, or the like as continuously as possible. In the related art, monitoring of the hemodynamic changes of subjects has been mainly performed by directly monitoring the blood pressure.

[0003] However, the body regulates the cardiac output and the vascular resistance such that the blood central pressure falls within a certain range. Therefore, in order to find the instant hemodynamic changes of subjects, it is not sufficient to monitor only the blood pressure directly, and it is necessary to know the cause when changes in the blood pressure is observed. Therefore, it is necessary to monitor the cardiac output in addition to monitoring the blood pressure. Known methods for measuring cardiac output include a thermodilution method, a dye dilution method, an ultrasonic method, and the like.

[0004] However, considering that all of the above-described methods require advanced skills from medical workers and are highly invasive to the subject, the methods cannot be easily and continuously performed. Therefore, it is difficult to continuously and constantly monitor the hemodynamic change of the subject by these methods.

[0005] On the other hand, JP 4742644 B1 discloses a blood volume measurement method that can continuously and constantly monitor the hemodynamic changes of the subject in a non-invasive manner, and that does not require a technique of a skilled medical worker, such as insertion of a catheter, and causes less pain to the subject. According to this blood volume measurement method, the cardiac output is calculated by utilizing a correlation between the pulse wave transit time and the stroke volume.

[0006] Patent Literature 1: JP 4742644 B1Summary

[0007] In the blood volume measurement method disclosed in Patent Literature 1, the cardiac output is calculated from a pulse wave transit time PWTT, a pulse pressure PP, a heart rate HR, and subject-specific coefficients α, β, and K. In order to obtain the coefficients α, β, and K (hereinafter, also referred to as calibration), it is necessary to measure a plurality of calibration parameters including the pulse wave transit time PWTT, the heart rate HR, and the pulse pressure PP.

[0008] For the pulse wave transit time PWTT, the pulse pressure PP, and the heart rate HR used to calculate the cardiac output, the instantaneous values are adopted. On the other hand, for the pulse wave transit time PWTT, the heart rate HR, and the pulse pressure PP used as the calibration parameters, moving average values over a certain interval in a time series are adopted rather than instantaneous values in order to make less susceptible to the influence of instantaneous noise such as the sudden movement of the subject. For this reason, when the calibration is implemented at the timing that the hemodynamics of the subject is changing, there is a large discrepancy between the instantaneous value and the moving average value, and the calculation value of the cardiac output may be greatly changed before and after the calibration.

[0009] Therefore, the present disclosure is to provide a blood volume measurement method, a blood volume measurement device, and a program that can implement calibration at an appropriate timing.

[0010] An illustrative aspect of the present disclosure provides a blood volume measurement method for calculating a cardiac output from physiological information parameters of a subject, the method including: a first measurement step of measuring a first value in a predetermined section of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; a second measurement step of measuring a second value in a section longer than the predetermined section of a second physiological information parameter including at least one of the pulse wave transit time, a heart rate, and the pulse pressure of the subject; a baseline determination step of determining whether or not a difference between the measured first value and the measured second value is equal to or less than a threshold value; a calibration step of, in a case where it is determined in the baseline determination step that the difference between the first value and the second value is equal to or less than the threshold value, calculating a subject-specific coefficient of the subject based on the second value; and a calculation step of calculating the cardiac output based on the first value and the subject-specific coefficient calculated in the calibration step.

[0011] An illustrative aspect of the present disclosure provides a blood volume measurement device which calculates a cardiac output from physiological information parameters of a subject, including: first measurement means for measuring a first value in a predetermined section of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; second measurement means for measuring a second value in a section longer than the predetermined section of a second physiological information parameter including at least one of the pulse wave transit time, a heart rate, and the pulse pressure of the subject; baseline determination means for determining whether or not a difference between the measured first value and the measured second value is equal to or less than a threshold value; calibration means for, in a case where the baseline determination means determines that the difference between the first value and the second value is equal to or less than the threshold value, calculating a subject-specific coefficient of the subject based on the second value; and calculation means for calculating the cardiac output based on the first value and the subject-specific coefficient calculated by the calibration means.

[0012] An illustrative aspect of the present disclosure provides a program causing a computer to execute: a first measurement step of measuring a first value in a predetermined section of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; a second measurement step of measuring a second value in a section longer than the predetermined section of a second physiological information parameter including at least one of the pulse wave transit time, a heart rate, and the pulse pressure of the subject; a baseline determination step of determining whether or not a difference between the measured first value and the measured second value is equal to or less than a threshold value; a calibration step of, in a case where it is determined in the baseline determination step that the difference between the first value and the second value is equal to or less than the threshold value, calculating a subject-specific coefficient of the subject based on the second value; and a calculation step of calculating the cardiac output based on the first value and the subject-specific coefficient calculated in the calibration step.

[0013] According to the present disclosure, it is possible to make less susceptible to the influence of instantaneous noise and prevent large fluctuations in the calculated value of the cardiac output before and after calibration. Thus, the cardiac output of the subject can be accurately calculated.

[0014] FIG. 1 is a block diagram illustrating a configuration of one aspect of a physiological information monitor including a blood volume measurement device according to the present disclosure.FIG. 2 illustrates an example of a measurement form in which electrocardiogram measurement means and peripheral pulse wave detection means are attached to a subject.FIG. 3 is a flowchart illustrating a procedure for calculating esCCO by calibrating α, β, and K.FIG. 4 illustrate a relationship between PEP, PWTTa, PWTTb, and PWTT.FIG 5 illustrates a relationship between instantaneous values and moving average values of calibration parameters.FIG. 6 illustrates a relationship between a threshold value and an elapsed time from the start of the moving average measurement.

[0015] Hereinafter, an example of embodiments of a blood volume measurement method and a measurement device according to the present disclosure will be described with reference to the accompanying drawings.

[0016] First, a principle of measuring a blood volume (cardiac output (CO)) ejected by the heartbeat of a heart will be described below. When a Windkessel model, which is a diagnostic indicator of a blood vessel, is used, the flow rate (SV - Qs) obtained by subtracting the flow rate (Qs) that flows out to the periphery during systole from the flow rate of blood that flows into the aorta during systole of the heart, that is, stroke volume (SV) can be expressed as follows by using aortic compliance (C) and pulse pressure (PP: Pulse Pressure refers to a difference between a systolic blood pressure value (maximum blood pressure) and a diastolic blood pressure value (minimum blood pressure)): SV - Qs = C * PP...(1)

[0017] The flow rate (Qd) that flows out to the periphery in the diastole of the heart is equal to SV - Qs. In addition, Qs and Qd are obtained by dividing the arterial pressures (V) in the systole and the diastole by the vascular resistance (R), respectively, and multiplying the results by the systole time Ts and the diastole time Td, respectively. However, considering that Qs and Qd are simply proportional to Ts and Td, respectively, Qs and Qd can be represented as follows: (Qd =) SV - Qs = SV * Td / (Ts + Td)...(2)

[0018] From Equations (1) and (2), SV * Td / (Ts + Td) = C * PP SV = C * PP * (1 + Ts / Td) ...(3)

[0019] Here, assuming that C and Ts / Td are constant during the measurement period, when C * (1 + Ts / Td) = K, the following Equations can be represented: SV = K * PP...(4) PP = SV * 1 / K...(5) In this way, according to the Windkessel model, the pulse pressure PP is proportional to the stroke volume SV.

[0020] The pulse pressure PP1 actually measured is composed of the pulse pressure PP2 based on Equation (5) (in Equation (5), PP is used, but PP2 is used in the following description) and an increase in pulse pressure PP3 observed when a vasoconstrictor is used, and can be represented as follows: PP1 = PP2 + PP3...(6)

[0021] Assuming that PP3 is zero, from Equations (4) and (6), SV = K * PP1...(7) is obtained, and thus SV can be measured from the actually measured blood pressure (pulse pressure PP1). However, when the vasoconstrictor is used, PP1 includes PP3, so that SV is excessively evaluated. This has been a problem when SV is calculated from the blood pressure.

[0022] In addition, as described above, regarding the measurement accuracy of the device that calculates one stroke volume and the cardiac output from the invasively measured arterial blood pressure waveform, it has been reported that "in a subject admitted to the intensive care unit (ICU) after surgery, when vascular resistance changed by approximately 60% due to the administration of the vasoconstrictor phenylephrine, the measurement values of the device mentioned above indicated a significantly higher bias than the measurement values of a thermodilution-type cardiometry used as the standard, and in such cases it is necessary to recalibrate the measurement values by using a thermodilution-type cardiometry." Further, it is known that when the vasoconstrictor is used, the pulse pressure increases due to the influence of the reflected waves from the peripheral side, and PP3 corresponds to the increase.

[0023] A pulse wave transit time (PWTT) that is an arrival time from an R wave of an electrocardiogram to a peripheral SpO2pulse wave can be represented as follows: PWTT = PEP + PWTTa + PWTTb...(8)

[0024] FIG. 4 illustrate waveforms of the pulse waves measured from the subject. As shown in the figure, PEP is the heart's pre-ejection period, which is the time from the start of electrical excitement of the heart to the time when the aortic valve is opened. In addition, PWTTa is the time from when the aortic valve is opened and then the pulse wave is generated in the aorta to when the pulse wave propagates to the peripheral artery where blood pressure is usually measured invasively. In addition, PWTTb is the time until the pulse wave propagates further from the peripheral artery to the peripheral blood vessel in which photoplethysmogram is being measured.

[0025] Using 10 adult dogs, the time (PEP + PWTTa) from an R wave of an electrocardiogram (ECG) to the rise of a femoral pulse wave was measured. The relationship between the time of PEP + PWTTa and the blood pressure was measured under each condition of the administration of the vasoconstrictor, the administration of a vasodilator, the enhancement of the contractile force of the heart, the reduction of the contractile force of the heart, and the blood removal, and it was found that there was some correlation between the pulse pressure PP1 and the time of PEP + PWTTa.

[0026] The relationship between the pulse pressure PP1 and PEP + PWTTa can be represented as follows: PEP + PWTTa = a * PP1 + b...(9) In addition, the relationship between PWTTb and PP1 is represented as follows: PWTTb = c * PP1 + d + e...(10)

[0027] It was found that when PP3 appeared due to the use of the vasoconstrictor or the like, PWTTb tended to be extended as compared with the case of the other conditions, and the portion corresponding to the extension was set as e. (Here, e was not limited to a constant.)

[0028] Hence, when Equation (8) is rewritten by Equations (9) and (10), Equation (8) can be represented as follows. PWTT = (a * PP1 + b) + (c * PP1 + d + e) PP1 = 1 / (a + c) * (PWTT - b - d - e)...(11)

[0029] When the right side of Equation (5) is substituted for PP2 of Equation (6), the following equation is obtained. PP1 = SV * 1 / K + PP3...(12) From Equations (11) and (12), the following Equation can be obtained: 1 / (a + c) * PWTT - (b + d) / (a + c) = SV * 1 / K + PP3 + e / (a+c) SV = K * (1 / (a+c) * PWTT - (b + d) / (a + c)) - K * (PP3 + e / (a + c))...(13)

[0030] As described above, it was found experimentally that PWTTb tends to be extended when PP3 appears, for example, by using the vasoconstrictor. When phenylephrine is administered thereto, PP3 appears and PP1 increases. However, the relationship between PWTTb and PP1 that is observed during blood removal or pentobarbital administration is no longer observed when phenylephrine is administered, and PWTTb tends to be extended. Therefore, it was found experimentally that since the same negative correlation as under other conditions is maintained between SV and PWTT even during the administration of phenylephrine, the second term "K * (PP3 + e / (a + c))" on the right-hand side of Equation (13) can be substantially ignored.

[0031] Hence, when 1 / (a + c) = α and -(b + d) / (a + c) = β, the following Equation can be obtained: SV = K * (α * PWTT + β)...(14) α and β are values that are calibrated and calculated using parameters obtained from the subject, such as PWTT, and are coefficients unique to the subject.

[0032] Further, the estimated continuous cardiac output (esCCO) can be calculated as follows from the relationship of SV = CO / HR (HR: Heart Rate): esCCO = K * (α * PWTT + β) * HR...(15) where esCCO is the cardiac output represented by L / min, and K is a constant unique to the subject.

[0033] Further, Equation (15) can also be replaced with the following Equation. esCCO = (αK * PWTT + βK) * HR...(16) where αK and βK are coefficients unique to the subject.

[0034] When SV and esCCO are calculated using PWTT as in Equations (14), (15), and (16), the same relationship as under other conditions is maintained between SV and PWTT even when the pulse pressure is increased by using the vasoconstrictor or the like, so that a problem exhibited when SV is calculated using the blood pressure in the related art can be solved. Therefore, CO is not excessively evaluated.

[0035] Next, one aspect of the physiological information monitor including the blood volume measurement device according to the present disclosure will be described with reference to FIG. 1. Further, in the following description, reference will be made as appropriate to FIG. 2, which illustrates an example of a measurement mode using a physiological information monitor.

[0036] The physiological information monitor M includes: a blood volume measurement device 1 that measures the blood volume of a subject; a non-invasive blood pressure measurement device 2 that measures a blood pressure during systole and diastole of the heart; a pulse wave transit time detection device 3 that detects an electrocardiogram R wave and a peripheral pulse wave; and an invasive blood pressure measurement device 4.

[0037] The non-invasive blood pressure measurement device 2 is a device that measures the blood pressure of the subject by a non-invasive blood pressure (NIBP) measurement method, and includes a cuff 21, an exhaust valve 22, an inflation pump 23, a pressure sensor 24, a cuff pressure detection unit 25, and an A / D converter 26.

[0038] Specifically, as shown in FIG. 2, the non-invasive blood pressure measurement device 2 measures the blood pressure by attaching the cuff 21 to the upper arm of the subject.

[0039] The inside of the cuff 21 is opened or closed to the atmosphere by the opening and closing of the exhaust valve 22. The exhaust valve 22 is opened and closed based on the control signal output from the blood volume measurement device 1. In addition, air is supplied to the cuff 21 from the inflation pump 23. The supply of air is controlled based on the control signal output from the blood volume measurement device 1.

[0040] In addition, a pressure sensor 24 (cuff pulse wave sensor) is connected to the cuff 21, and a sensor output thereof is detected by the cuff pressure detection unit 25. The output of the cuff pressure detection unit 25 is converted into a digital signal by the A / D converter 26 and is input to the NIBP pulse pressure measurement means 11 of the blood volume measurement device 1.

[0041] Further, the exhaust valve 22, the inflation pump 23, the pressure sensor 24, the cuff pressure detection unit 25, and the A / D converter 26 are provided in a physiological information monitor body (bedside monitor) M1 which is the body of the physiological information monitor M, shown in FIG. 2.

[0042] The pulse wave transit time detection device 3 includes reference point measurement means 31, a photoplethysmogram detection sensor 32, a pulse wave detection unit 33, and an A / D converter 34.

[0043] The reference point measurement means 31 measures the time point when the R wave occurs in the electrocardiogram, as the reference point for time interval. The output of the reference point measurement means 31 is converted into a digital signal by the A / D converter and is input to the heart rate calculation means 12 and the pulse wave transit time measurement means 13 of the blood volume measurement device 1.

[0044] Specifically, as illustrated in FIG. 2, the reference point measurement means 31 is an electrocardiogram electrode 31a (electrocardiogram measurement means) attached to on the chest of the subject. The data measured by the electrocardiogram electrode 31a is wirelessly transmitted from a measurement data transmitter 65 electrically connected to the electrocardiogram electrode 31a to the physiological information monitor body M1. The transmitted measurement data is converted into a digital signal by the A / D converter in the physiological information monitor body M1 and is input to the heart rate calculation means 12 and the pulse wave transit time measurement means 13. In this manner, an electrocardiogram waveform as shown in (a) of FIG. 4 is obtained. Note that the function of the measurement data transmitter 65 may be modularized and built in the physiological information monitor body M1.

[0045] As shown in FIG. 2, the photoplethysmogram detection sensor 32 is attached to a peripheral portion of the subject, such as a finger, and measures a pulse wave. The photoplethysmogram detection sensor 32 is electrically connected to the measurement data transmitter 65, and the data measured by the photoplethysmogram detection sensor 32 is wirelessly transmitted to the physiological information monitor body M1.

[0046] The measurement data is transmitted to the pulse wave detection unit 33 in the physiological information monitor body M1, and thus the pulse wave (photoplethysmogram) at the attachment site of the subject is detected. The output of the pulse wave detection unit 33 is converted into a digital signal by the A / D converter 34 and is input to the pulse wave transit time measurement means 13 of the blood volume measurement device 1. In this manner, a photoplethysmogram waveform (waveform of the peripheral portion) as shown in (d) of FIG. 4 is obtained.

[0047] The invasive blood pressure measurement device 4 measures a blood pressure by an invasive blood pressure (IBP) method, that is, by inserting a catheter into the blood vessel of the subject. The blood pressure data measured by the invasive blood pressure measurement device 4 is input to the invasive blood pressure / pulse pressure measurement means 16 of the blood volume measurement device 1.

[0048] The blood volume measurement device 1 is the device that measures the cardiac output, and includes NIBP pulse pressure measurement means 11, heart rate calculation means 12, pulse wave transit time measurement means 13, cardiac output calculation means 14, subject-specific coefficient calculation means 15, and invasive blood pressure / pulse pressure measurement means 16. Note that the blood volume measurement device 1 is provided in the physiological information monitor body M1.

[0049] The NIBP pulse pressure measurement means 11 measures the NIBP pulse pressure, based on the blood pressure data measured by the non-invasive blood pressure measurement device 2. The NIBP pulse pressure is calculated from a difference between a systolic (highest) blood pressure value and a diastolic (lowest) blood pressure value. The measured NIBP pulse pressure is input to the subject-specific coefficient calculation means 15.

[0050] The heart rate calculation means 12 calculates the number of beats per minute (heart rate) based on the reference point (R-wave occurrence time point) measured by the reference point measurement means 31. The calculated heart rate is input to the cardiac output calculation means 14.

[0051] The pulse wave transit time measurement means 13 measures the pulse wave transit time (PWTT) based on the reference point (R wave occurrence time point) measured by the reference point measurement means 31 and the waveform of the peripheral portion detected by the photoplethysmogram detection sensor 32. The measured pulse wave transit time is input to the cardiac output calculation means 14 and the subject-specific coefficient calculation means 15. Note that the pulse wave transit time is indicated by a time difference from an R wave of an electrocardiogram to the rise of a peripheral pulse wave as illustrated in FIG. 4.

[0052] The subject-specific coefficient calculation means 15 calculates the subject-specific coefficient, based on the physiological information parameters including at least one of the NIBP pulse pressure measured by the NIBP pulse pressure measurement means 11, the heart rate calculated by the heart rate calculation means 12, and the pulse wave transit time measured by the pulse wave transit time measurement means 13. The calculated coefficient is input to the cardiac output calculation means 14.

[0053] The cardiac output calculation means 14 calculates the cardiac output, based on the physiological information parameters including at least one of the heart rate calculated by the heart rate calculation means 12, and the pulse wave transit time measured by the pulse wave transit time measurement means 13, and the coefficient calculated by the subject-specific coefficient calculation means 15.

[0054] In addition, input means 5 is connected to the blood volume measurement device 1, and data necessary for a medical worker is externally input through the input means 5. Further, a display unit 61 is connected to the blood volume measurement device 1, and the measured physiological information of the subject is displayed on the display unit 61. Further, an alarm unit 62 is connected to the blood volume measurement device 1, and an alarm is generated via the alarm unit 62 when an abnormality is detected in the measured physiological information.

[0055] Next, a blood volume measurement method for obtaining the estimated continuous cardiac output (esCCO) from the Equation (16) will be described with reference to a flowchart of FIG. 3.

[0056] α, β, and K, which are subject-specific coefficients, are obtained by calibration to calculate esCCO. Note that the calibration of α and β is performed when the pulse pressure (PP) is not increased due to the administration of the vasoconstrictor or the like.

[0057] First, an initial value of the coefficient α is read (step 1). Subsequently, the pulse wave transit time (PWTT) and the heart rate (HR) are acquired by actual measurement (step 2). PWTT is calculated from the R wave of the electrocardiogram and the photoplethysmogram measured by the photoplethysmogram detection sensor 32 to be acquired. HR is calculated from the R wave of the electrocardiogram measured by the reference point measurement means 31 to be acquired.

[0058] Next, it is determined whether the coefficient β is present or not (step 3). α and β are coefficients unique to the subject, which are experimentally determined. In a case where the coefficient β is not present in step 3 (step 3: No), a request message to measure the calibration blood pressure is displayed on the display unit 61 (step 4).

[0059] Subsequently, the calibration blood pressure is measured, and the instantaneous values of the pulse pressure (PP), PWTT, and HR (also referred to as first physiological information parameters), and the moving average values of PP, PWTT, and HR (also referred to as second physiological information parameters) are calculated as the calibration parameters (step 5). Here, the instantaneous value means a momentary value in the time-series data, and the moving average value means an average value for a certain interval in the time-series data. The certain interval in the moving average value is, for example, 3 to 10 minutes. In addition, PP is the value measured by the NIBP pulse pressure measurement means 11 or the invasive blood pressure / pulse pressure measurement means 16.

[0060] Next, a baseline determination of whether or not the difference between the calculated instantaneous value and the calculated moving average value is equal to or less than a predetermined threshold value Th is performed (step 6), and in a case where the difference between the instantaneous value and the moving average value is greater than the threshold value Th (step 6: No), the instantaneous value and the moving average value are calculated again (step 5). In a case where the difference between the instantaneous value and the moving average value is equal to or smaller than the threshold value Th (step 6: Yes), PP, PWTT, and HR are stored in a register (not shown) inside the subject-specific coefficient calculation means 15 as PP1, PWTT1, and HR1, respectively (step 7).

[0061] Here, the instantaneous values and the moving average values of the calibration parameters will be specifically described with reference to FIG. 5. FIG. 5 illustrates a relationship between the instantaneous values and the moving average values of PWTT, which is an example of the calibration parameters. At times t1 and t2 (t2 > t1), the absolute value of the difference between the instantaneous value of PWTT and the moving average value of PWTT is represented by ΔPWTTt1and ΔPWTTt2.

[0062] For example, in a case where the instantaneous noise occurs at a time t1, the noise component is superimposed on the instantaneous value, and the noise component is superimposed on the moving average value with a delay (delay occurs). For this reason, at the time t1, there is a large discrepancy between the instantaneous value and the moving average value. In this way, when the calibration is performed at a timing when there is a large discrepancy between the instantaneous value and the moving average value, that is, at a timing at which △PWTTt1is increased, the value of esCCO calculated before and after the calibration may be greatly different from each other, so that the accuracy of esCCO degrades.

[0063] Therefore, in a case where △PWTTt1is equal to or smaller than the predetermined threshold value Th, the calibration is performed at the time t1; in a case where △PWTTt1is larger than the predetermined threshold value Th, the calibration is not performed at the time t1, and the instantaneous value and the moving average value are calculated again at a time t2. In a case where a difference △PWTTt2between an instantaneous value of PWTT and a moving average value of PWTT at the time t2 is equal to or smaller than the predetermined threshold value Th, PWTT is calibrated at the time t2. In a case where PWTTt2is larger than the predetermined threshold value Th, the instantaneous value and the moving average value are calculated at the time t2 without performing the calibration.

[0064] In addition, a method for setting the threshold value Th will be described with reference to FIG. 6. FIG. 6 illustrates a relationship between the threshold value Th and the elapsed time from the start of the moving average measurement. The threshold value Th may be a fixed value or a variable value. (a) of FIG. 6 illustrates a case where the threshold value Th is a fixed value, and (b) of FIG. 6 illustrates a case where the threshold value Th is a variable value.

[0065] As shown in (a) of FIG. 6, in a case where the threshold value Th is a fixed value, the difference between the instantaneous value and the moving average value can always be calibrated in a range of the threshold value Th or less. On the other hand, for example, within 1 minute from the start of the moving average measurement, there are few cases where there is a large discrepancy between the instantaneous value and the moving average value as shown in the time t1 in FIG. 5. For this reason, there is a risk that the baseline determination will no longer function effectively within a predetermined time from the start of the moving average measurement.

[0066] On the other hand, as shown in (b) of FIG. 6, in a case where the threshold value Th at the start of the moving average measurement is set to a lower value than in the case of the fixed value, and the threshold value Th is increased according to the elapsed time from the start of the moving average measurement to a time t3 (for example, 1 minute) to widen the allowable range, the baseline determination is performed more strictly from the start of the moving average measurement to the time t3, so that the baseline determination can be made to function effectively. This makes it possible to maintain the calculation accuracy of esCCO even when the time required for calibrating esCCO is shortened. Further, in the example shown in (b) of FIG. 6, the threshold value Th is set to a constant value after the time t3.

[0067] Return to the explanation of the flowchart of FIG. 3. The stored values of PP1 and PWTT1 and the initial value α read in step 1 are substituted into equation of β = PP1 - α * PWTT1 to calculate the coefficient β (step 8). Note that this equation is derived from Equations (4) and (14). After calculating the coefficient β, the process proceeds to step 15 to determine whether the coefficient K is present or not.

[0068] On the other hand, when the coefficient β is present in step 3 (step 3: Yes), it is determined whether to recalibrate the initial value α read in step 1 or not (step 9). In step 9, when the recalibration of α is not performed, the process proceeds to step 15, and it is determined whether the coefficient K is present or not.

[0069] In step 9, when the recalibration of α is performed, the request message to measure the calibration blood pressure is displayed on the display unit 61 (step 10).

[0070] Subsequently, the calibration blood pressure is measured in the same manner as in steps 5 to 7, and the instantaneous values of the pulse pressure (PP) and PWTT and the moving average values of PP, PWTT, and HR are calculated (step 11). It is determined whether or not the difference between the calculated instantaneous value and the calculated moving average value is equal to or less than a predetermined threshold value (step 12), and in a case where the difference between the instantaneous value and the moving average value is greater than the threshold value (step 12: No), the instantaneous value and the moving average value are calculated again (step 11). In a case where the difference between the instantaneous value and the moving average value is equal to or smaller than the threshold value (step 12: Yes), PP, PWTT, and HR are stored in a register (not shown) inside the subject-specific coefficient calculation means 15 as PP2, PWTT2, and HR2, respectively (step 13).

[0071] Next, the coefficients α and β are calculated from the following two equations by using PP1, PWTT1, PP2, and PWTT2 stored in the register (step 14). PP1 = αPWTT1 + β...(17) PP2 = αPWTT2 + β...(18)

[0072] Subsequently, in a case where NO is determined in step 9 and after the processes of step 8 and step 14, it is determined whether the coefficient K is present or not (step 15).

[0073] in a case where the coefficient K is not present in step 15 (step 15: No), the input request of a calibration CO value is displayed on the display unit 61 (step 16). Subsequently, it is determined whether the CO value for calibration is input (step 17), and in a case where the calibration CO value is not input (step 17: No), the input of the calibration CO value is awaited. On the other hand, in a case where the CO value is input (step 17: Yes), the input CO value is stored in a register (not shown) inside the subject-specific coefficient calculation means 15 as the CO1 (step 18).

[0074] Subsequently, the stored CO1 and the α, β, PWTT1, and HR1 calculated above are substituted into the following equation to calculate the coefficient K (step 19): K = CO1 / ((α * PWTT1 + β) * HR1)...(19) Further, Equation (19) is derived from the following two equations: K = CO1 / (PP1 * HR1)...(20) PP1 = α * PWTT + β...(21)

[0075] Then, the coefficients α, β, and K obtained by the calibration are substituted into Equation (15) to calculate esCCO (step 20). In addition, when the coefficient K is present in step 15 (step 15: Yes), the process proceeds to step 20 to calculate esCCO. In this way, by executing the calibration at an appropriate timing when there is no large discrepancy between the instantaneous value and the moving average value, it is possible to prevent a large fluctuation of the calculation value of the cardiac output before and after calibration and to accurately calculate the cardiac output of the subject.

[0076] The calculated esCCO is displayed on the display unit 61 (step 21). Subsequently, the process returns to step 2, and the above-described process is repeated sequentially.

[0077] As described above, according to the blood volume measurement method and measurement device according to the embodiment, cardiac output is calculated using a moving average value, which makes less susceptible to the influence of instantaneous noise, and further by implementing the calibration at an appropriate timing when there is no large discrepancy between the instantaneous value and the moving average value as shown in steps 6 to 8 and steps 12 to 14, large fluctuations in the calculated value of cardiac output before and after calibration are prevented. Thus, the cardiac output of the subject can be accurately calculated.

[0078] Although the embodiments of the present disclosure have been described above, it goes without saying that the technical scope of the present disclosure should not be interpreted as being limited by the description of the embodiments. It will be understood by those skilled in the art that the present embodiment is merely an example and that various modifications of the embodiment can be made to the various embodiments within the scope of the invention described in the claims. The technical scope of the present disclosure should be determined based on the scope of the invention described in the claims and their equivalents.

[0079] For example, in step 5 in the physiological information monitor M according to the present embodiment, as the calibration parameters, PP and PWTT are first physiological information parameters, and PP, PWTT, and HR are second physiological information parameters, but at least one of PP and PWTT may be the first physiological information parameter, and at least one of PP, PWTT, and HR may be the second physiological information parameter. Further, it is assumed that the second physiological information parameter includes at least one parameter included in the first physiological information parameter.

[0080] In addition, an example is shown in which in the step 5 to the step 6 in the physiological information monitor M according to the present embodiment, the instantaneous values of PP and PWTT and the moving average values of PP, PWTT, and HR are calculated, and it is determined whether the difference between the calculated instantaneous values and the calculated moving average values is equal to or less than the predetermined threshold value Th, but the target of the determination is not limited to the difference between the instantaneous values and the moving average values. First moving average values of PP and PWTT during a first predetermined section ta and second moving average values of PP, PWTT, and HR during s second predetermined section tb longer than the first predetermined section Ta may be calculated, and it may be determined whether or not a difference between the calculated first moving average value and the calculated second moving average value is equal to or less than a predetermined threshold value Th.

[0081] In addition, each process in the physiological information monitor M according to the present embodiment can be realized as a computer program that is executed by the blood volume measurement device 1, the non-invasive blood pressure measurement device 2, and the pulse wave transit time detection device 3.

[0082] The program can be stored in a non-transitory computer-readable medium and read by a computer. Examples of the non-transitory computer-readable medium include a magnetic recording medium, a magneto-optical recording medium, a CD-ROM, a CD-R, a CD-R / W, and a semiconductor memory (including an EPROM and a flash ROM). In addition, the program may be read into the computer from various types of temporary computer-readable media. Examples of the temporary computer-readable medium include electric signals, optical signals, and electromagnetic waves. The temporary computer-readable medium can supply the program to the computer via a wired communication path such as an electric wire and an optical fiber, or via a wireless communication path. Reference Signs List

[0083] This application claims priority to Japanese Patent Application No. 2023-173166 filed on October 4, 2023, the entire content of which is incorporated herein by reference.

Claims

1. A blood volume measurement method for calculating a cardiac output from physiological information parameters of a subject, comprising: a first measurement step of measuring a first value in a predetermined section of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; a second measurement step of measuring a second value in a section longer than the predetermined section of a second physiological information parameter including at least one of the pulse wave transit time, a heart rate, and the pulse pressure of the subject; a baseline determination step of determining whether or not a difference between the first value measured and the second value measured is equal to or less than a threshold value; a calibration step of, in a case where it is determined in the baseline determination step that the difference between the first value and the second value is equal to or less than the threshold value, calculating a subject-specific coefficient of the subject based on the second value; and a calculation step of calculating the cardiac output based on the first value and the subject-specific coefficient calculated in the calibration step.

2. The blood volume measurement method according to claim 1, wherein the first value is an instantaneous value, and the second value is a moving average value.

3. The blood volume measurement method according to claim 1, wherein the threshold value is variable.

4. The blood volume measurement method according to claim 3, wherein the threshold value is increased in accordance with an elapsed time from a start of the second value measurement step.

5. The blood volume measurement method according to claim 1, wherein in the calculation step, the cardiac output is calculated by a calculation expression CO = K(α * PWTT + β) * HR, where CO represents the cardiac output, PWTT represents the pulse wave transit time, HR represents the heart rate, and α, β, and K represent the subject-specific coefficient.

6. A blood volume measurement device which calculates a cardiac output from physiological information parameters of a subject, comprising: first measurement means for measuring a first value in a predetermined section of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; second measurement means for measuring a second value in a section longer than the predetermined section of a second physiological information parameter including at least one of the pulse wave transit time, a heart rate, and the pulse pressure of the subject; baseline determination means for determining whether or not a difference between the first value measured and the second value measured is equal to or less than a threshold value; calibration means for, in a case where the baseline determination means determines that the difference between the first value and the second value is equal to or less than the threshold value, calculating a subject-specific coefficient of the subject based on the second value; and calculation means for calculating the cardiac output based on the first value and the subject-specific coefficient calculated by the calibration means.

7. A program causing a computer to execute: a first measurement step of measuring a first value in a predetermined section of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; a second measurement step of measuring a second value in a section longer than the predetermined section of a second physiological information parameter including at least one of the pulse wave transit time, a heart rate, and the pulse pressure of the subject; a baseline determination step of determining whether or not a difference between the first value measured and the second value measured is equal to or less than a threshold value; a calibration step of, in a case where it is determined in the baseline determination step that the difference between the first value and the second value is equal to or less than the threshold value, calculating a subject-specific coefficient of the subject based on the second value; and a calculation step of calculating the cardiac output based on the first value and the subject-specific coefficient calculated in the calibration step.

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