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

Abstract

A blood volume measurement method for calculating a cardiac output, includes an instantaneous value measurement step of measuring an instantaneous value of a first physiological information parameter, a division moving average value measurement step of measuring a plurality of times a division moving average value in a first predetermined section of a second physiological information parameter, a comprehensive moving average value calculation step of calculating a comprehensive moving average value which is an average value of a plurality of the division moving average values when a number of times the division moving average values are measured in a second predetermined section is a threshold value or more, in the division moving average value measurement step, a calibration step of calculating a subject-specific coefficient of the subject based on the comprehensive moving average value calculated in the comprehensive moving average value calculation step, and a calculation step of calculating the cardiac output based on the instantaneous 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 central blood 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 blood pressure are 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 (hereinafter referred to as 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 PWTT, the heart rate HR, and the pulse pressure PP.

[0008] For the PWTT, the pulse pressure PP, and the heart rate HR used to calculate the cardiac output, instantaneous values are adopted. On the other hand, for the 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 noises due to sudden movement of the subject or the like. However, when intermittent noises occur in a certain interval, there is a concern that the normal moving average value cannot be measured and the burden on the subject may increase, such as an extension of the measurement time of the moving average value.

[0009] In addition, the general moving average value has a characteristic that the responsiveness to rapid fluctuations in the measurement value is poor as compared with the instantaneous value. 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.

[0010] Therefore, the present disclosure is to provide a blood volume measurement method, a blood volume measurement device, and a program that reduce a burden on a subject and have high responsiveness to the hemodynamic changes of the subject.

[0011] 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: an instantaneous value measurement step of measuring an instantaneous value of a first physiological information parameter including at least one of a PWTT and a pulse pressure of the subject; a division moving average value measurement step of measuring a plurality of times a division moving average value in a first predetermined section of a second physiological information parameter including at least one of the PWTT, a heart rate, and a pulse pressure of the subject; a comprehensive moving average value calculation step of calculating a comprehensive moving average value which is an average value of a plurality of the division moving average values when a number of times the division moving average value is measured in a second predetermined section is a threshold value or more, in the division moving average value measurement step; a calibration step of calculating a subject-specific coefficient of the subject based on the comprehensive moving average value calculated in the comprehensive moving average value calculation step; and a calculation step of calculating the cardiac output based on the instantaneous value and the subject-specific coefficient calculated in the calibration step.

[0012] 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, the device including: instantaneous value measurement means for measuring an instantaneous value of a first physiological information parameter of the subject; division moving average value measurement means for measuring a plurality of times a division moving average value in a first predetermined section of a second physiological information parameter of the subject; comprehensive moving average value calculation means for calculating a comprehensive moving average value which is an average value of a plurality of the division moving average values, when the number of times the division moving average value is measured in a second predetermined section is a threshold value or more, in the division moving average value measurement means; calibration means for calculating a subject-specific coefficient of the subject based on the comprehensive moving average value calculated in the comprehensive moving average value calculation means; and calculation means for calculating the cardiac output based on the instantaneous value and the subject-specific coefficient calculated by the calibration means.

[0013] An illustrative aspect of the present disclosure provides a program for calculating a cardiac output from physiological information parameters of a subject, the program causing a computer to execute: an instantaneous value measurement step of measuring an instantaneous value of a first physiological information parameter including at least one of a PWTT and a pulse pressure of the subject; a division moving average value measurement step of measuring a plurality of times a division moving average value in a first predetermined section of a second physiological information parameter including at least one of the PWTT, a heart rate, and a pulse pressure of the subject; a comprehensive moving average value calculation step of calculating a comprehensive moving average value which is an average value of a plurality of the division moving average values when a number of times the division moving average value is measured in a second predetermined section is a threshold value or more, in the division moving average value measurement step; a calibration step of calculating a subject-specific coefficient of the subject based on the comprehensive moving average value calculated in the comprehensive moving average value calculation step; and a calculation step of calculating the cardiac output based on the instantaneous value and the subject-specific coefficient calculated in the calibration step.

[0014] According to the present disclosure, it is possible to accurately calculate the cardiac output of the subject, by making less susceptible to the influence of instantaneous or intermittent noise. In addition, it is possible to prevent a situation in which a normal moving average value cannot be measured and the time required for calibration is extended, thereby reducing the burden on the subject or user.

[0015] 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 illustrate 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 illustrates a relationship between PEP, PWTTa, PWTTb, and PWTT.FIG. 5 illustrates a relationship between instantaneous values and moving average values of calibration parameters according to a reference example.FIG. 6 is an explanatory drawing of a calculation algorithm for moving average values of the calibration parameters according to a reference example.FIG. 7 is an explanatory drawing of a calculation algorithm for a comprehensive moving average value of calibration parameters according to the present disclosure.FIG. 8 is an explanatory drawing when weighting is performed in the calculation algorithm for the comprehensive moving average value of the calibration parameters according to the present disclosure.

[0016] 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.

[0017] 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)

[0018] 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)

[0019] From Equations (1) and (2), SV * Td / (Ts + Td) = C * PP SV = C * PP * (1 + Ts / Td) is derived...(3)

[0020] 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.

[0021] 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)

[0022] 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.

[0023] 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.

[0024] 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)

[0025] FIG. 4 illustrates 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 further propagates from the peripheral artery to the peripheral blood vessel in which photoplethysmogram is being measured.

[0026] 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.

[0027] 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)

[0028] 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.)

[0029] 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)

[0030] 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)

[0031] 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, 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.

[0032] 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 calibrated and calculated using parameters obtained from the subject, such as PWTT, and are coefficients unique to the subject.

[0033] 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.

[0034] 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.

[0035] 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 drawback exhibited when SV is calculated using the blood pressure in the related art can be solved. Therefore, CO is not excessively evaluated.

[0036] 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 to FIG. 2 as appropriate, which shows an example of a measurement mode using a physiological information monitor.

[0037] 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 PWTT detection device 3 that detects an electrocardiogram R wave and a peripheral pulse wave; and an invasive blood pressure measurement device 4.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] The PWTT 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.

[0044] 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 PWTT measurement means 13 of the blood volume measurement device 1.

[0045] Specifically, as illustrated in FIG. 2, the reference point measurement means 31 is an electrocardiogram electrode 31a (electrocardiogram measurement means) mounted 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 PWTT measurement means 13. In this manner, an electrocardiogram waveform as shown in (a) of FIG. 4 is obtained. Further, the function of the measurement data transmitter 65 may be modularized and built in the physiological information monitor body M1.

[0046] As shown in FIG. 2, the photoplethysmogram detection sensor 32 is mounted on 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.

[0047] 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 PWTT 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.

[0048] 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.

[0049] 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, PWTT 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.

[0050] 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.

[0051] 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.

[0052] The PWTT measurement means 13 measures the 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 PWTT is input to the cardiac output calculation means 14 and the subject-specific coefficient calculation means 15. Note that the PWTT 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.

[0053] 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 PWTT measured by the PWTT measurement means 13. The calculated coefficient is input to the cardiac output calculation means 14.

[0054] 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 PWTT measured by the PWTT time measurement means 13, and the coefficient calculated by the subject-specific coefficient calculation means 15.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] First, an initial value of the coefficient α is read (step 1). Subsequently, the 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.

[0059] 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).

[0060] Subsequently, the calibration blood pressure is measured, and the instantaneous values of the pulse pressure (PP), PWTT, and HR (also referred to as the first physiological information parameters), and the division moving average values of PP, PWTT, and HR (also referred to as the 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 division moving average value means an average value for each first predetermined section in the time-series data. For example, the calculation frequency of the instantaneous value may be set to once per 10 seconds, and the first predetermined section in the division moving average value may be set to 1 minute. In addition, for example, in a case where the instantaneous value can be calculated and acquired even once in the first predetermined section, the division moving average value may be calculated. 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.

[0061] Next, it is determined whether or not the number of times the division moving average value is calculated in the second predetermined section is equal to or larger than a threshold value (step 6). In a case where the number of times the division moving average value is calculated in the second predetermined section is not equal to or greater than the threshold value (step 6: No), the instantaneous value and the moving average value are calculated again (step 5). In a case where the number of times the division moving average value is calculated in the second predetermined section is equal to or larger than the threshold value (step 6: Yes), the comprehensive moving average value which is the average value of the plurality of division moving average values in the second predetermined section is calculated (step 7). 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 8).

[0062] Here, details of the calculation algorithm for the moving average value of the calibration parameters according to the present disclosure will be described. First, as comparison targets, a relationship between the instantaneous value and the moving average value of the calibration parameters according to the reference examples, and a calculation algorithm for the moving average value will be described with reference to FIGs. 5 and 6.

[0063] FIG. 5 is a graph showing a relationship between the instantaneous values and the moving average values of PWTT that 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.

[0064] Generally, the moving average value has a characteristic that the responsiveness to rapid fluctuations in the measurement value is poor as compared with the instantaneous value. For this reason, when the calibration is performed at a timing at which the hemodynamics of the subject have changed, 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. For example, when the instantaneous noise occurs at a time t1 in FIG. 5, 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. Therefore, it is required to improve the responsiveness of the moving average value to the rapid fluctuations in the measurement value.

[0065] FIG. 6 is an explanatory drawing of a calculation algorithm for a moving average value of the calibration parameters according to the reference example. In the example in FIG. 6, the instantaneous value is calculated at a frequency of once per 10 seconds, and the moving average value MA of the 18 instantaneous values is calculated when the instantaneous value is calculated 18 times from the start of the measurement. Note that the number in the square displayed in the instantaneous value of FIG. 6 represents the total number of times of the instantaneous value calculated from the start of the measurement, and the square in which the number is not described represents that the instantaneous value is not calculated due to the instantaneous noise or the like. For example, it is shown that the sixth instantaneous value from the start of measurement is calculated when the elapsed time t is 70 seconds to 80 seconds, and the instantaneous value is not calculated when the elapsed time t is 80 seconds to 90 seconds. In addition, since the 18th instantaneous value is calculated when the elapsed time t is in a range of 260 seconds to 270 seconds, the moving average value MA of the first to 18th instantaneous values is calculated. As shown in FIG. 6, in a case where intermittent noise or the like occurs and the instantaneous value cannot be calculated intermittently, it takes time to calculate the moving average value and the time required for the calibration is extended, which may increase the burden on the subject.

[0066] Next, a calculation algorithm for the moving average value according to the present disclosure will be described with reference to FIGs. 7 and 8. FIG. 7 is an explanatory drawing of an algorithm for calculating the comprehensive moving average value MA of the calibration parameters according to the present disclosure. In the example in FIG. 7, the instantaneous value is calculated at a frequency of once per 10 seconds, and when the instantaneous value is calculated even once in the first predetermined section (for example, 60 seconds), the division moving average value ma which is the moving average value of the calculated instantaneous values is calculated. Next, when the number of times the division moving average value ma is calculated in a second predetermined section (for example, 180 seconds) longer than the first predetermined section becomes equal to or larger than a threshold value Th (for example, Th = 3), a comprehensive moving average value MA which is an average value of the plurality of division moving average values ma is calculated. For example, since the instantaneous value is calculated four times in the 1st first predetermined section, the division moving average value ma1 of the first to fourth instantaneous values is calculated when the elapsed time t is 50 seconds to 60 seconds. Similarly, since the instantaneous value is calculated four times in the 2nd first predetermined section, the division moving average value ma2 of the fifth to eighth instantaneous values in the elapsed time t of 110 seconds to 120 seconds is calculated. In addition, since the division moving average value ma is calculated three times in the second predetermined section, the comprehensive moving average value MA is calculated which is the average value of the first to third division moving average values ma1 to ma3 of which the number of times of calculation is equal to or larger than a threshold value Th in the elapsed time t of 170 seconds to 180 seconds. Note that in a case where intermittent noise or the like occurs and the instantaneous value cannot be calculated intermittently, the maximum section Tmax of the second predetermined section can be provided in order to prevent the old division moving average value ma to be referred to.

[0067] As compared with the calculation algorithm for the moving average value according to the reference example of FIG. 6, the elapsed time t required for calculating the moving average value MA is 260 seconds or more in the calculation algorithm of the reference example. On the other hand, in the calculation algorithm in the present disclosure in FIG. 7, the elapsed time t required for calculating the comprehensive moving average value MA is 180 seconds or less. In this way, by using the calculation algorithm in the present disclosure, even in the case where the intermittent noise or the like occurs and the instantaneous value cannot be calculated intermittently, the calculation of the moving average value can be performed in a short time, thereby preventing the time required for the calibration from being extended and reducing the burden on the subject.

[0068] FIG. 8 is an explanatory drawing in a case where weighting is performed in the calculation algorithm for the comprehensive moving average value MA of the calibration parameters according to the present disclosure. In the example in FIG. 8, the instantaneous value is calculated at a frequency of once per 10 seconds as in the example in FIG. 7, and when the instantaneous value is calculated even once in the first predetermined section (60 seconds), the division moving average value ma of the instantaneous values calculated in the first predetermined section is calculated. Next, when the number of times the division moving average value ma is calculated in the second predetermined section (180 seconds) becomes equal to or larger than a threshold value Th (Th = 3), the comprehensive moving average value MA is calculated. At this time, in order to improve the responsiveness of the comprehensive moving average value MA to the rapid fluctuations in the measurement value, weighting is performed on the division moving average value ma.

[0069] A weighting method will be specifically described. First, the number of times bn (n is an integer of 1 to Th, in the present example, n is an integer of 1 to 3 because Th = 3) of tracing back the first predetermined section is calculated. For example, assuming that the elapsed time t of 180 seconds is the current time, at the first division moving average value ma1 (the elapsed time t is 50 seconds to 60 seconds), b3 = 2 is obtained because the first predetermined section is traced back twice from the current time. Similarly, at the second division moving average value ma2 (elapsed time t is 110 seconds to 120 seconds), b2 = 1 is obtained because the first predetermined section is traced back once from the current time. At the third division moving average value ma3 (elapsed time t is 170 seconds to 180 seconds), b1 = 0 is obtained because it is not traced back from the current time.

[0070] Next, the weighting coefficient an (n is an integer of 1 to Th, in the present example, n is an integer of 1 to 3 because Th = 3) is calculated from the following equation by using the number bn of times of calculation of the instantaneous value: an = M - bn...(17) M is a value obtained by dividing the maximum section Tmax of the second predetermined section by the first predetermined section, and represents an upper limit value of the number of times when the first predetermined section is traced back from the current time. In the example in FIG. 8, since the maximum section Tmax of the second predetermined section is set to 600 seconds and the first predetermined section is set to 60 seconds, M = 10 is obtained in Equation (17), and a1 = 10, a2 = 9, and a3 = 8 are calculated. Further, the weighting is performed on the division moving average value ma by the corresponding weighting coefficient an from the following equation to calculate the comprehensive moving average value MA: MA = (ma1 * a3 + ma2 * a2 + ma3 * a1) / (a3 + a2 + a1)...(18) In this way, the weighting is performed such that the ratio of the most recent moving average value calculated from the current time is made larger, among the plurality of the moving average values ma calculated. This makes it possible to improve the responsiveness of the comprehensive moving average value MA to the rapid fluctuations in the measurement values.

[0071] 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 9). 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.

[0072] 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 10). In step 10, when the recalibration of α is not performed, the process proceeds to step 17, and it is determined whether the coefficient K is present or not.

[0073] In step 10, when the recalibration of α is performed, the request message to measure the calibration blood pressure is displayed on the display unit 61 (step 11).

[0074] 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 division moving average values of PP, PWTT, and HR are calculated (step 12). 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 13), and in a case where the number of times of calculation of the division moving average value is less than the threshold value (step 13: No), the instantaneous value and the division moving average value are calculated again (step 12). In a case where the number of times of calculation of the division moving average value is equal to or larger than the threshold value (step 13: Yes), the comprehensive moving average value is calculated (step 14), and 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 15).

[0075] 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 16): PP1 = αPWT1 + β...(19) PP2 = αPWT2 + β...(20)

[0076] Subsequently, in a case where NO is determined in step 10 and after the process of step 9 or step 16, it is determined whether the coefficient K is present or not (step 17).

[0077] In a case where the coefficient K is not present in step 17 (step 17: No), a request for the calibration CO value is displayed on the display unit 61 (step 18). Subsequently, it is determined whether the calibration CO value is input or not (step 19), and in a case where the CO value for calibration is not input (step 19: No), the input of the calibration CO value is awaited. On the other hand, in a case where the CO value is input (step 19: Yes), the input CO value is stored in a register (not shown) inside the subject-specific coefficient calculation means 15 as CO1 (step 20).

[0078] Subsequently, the stored CO1 and α, β, PWTT1, and HR1 calculated above are substituted into the following equation to calculate the coefficient K (step 21): K = CO1 / ((α * PWTT1 + β) * HR1)...(21) Further, Equation (21) is derived from the following two equations: K = CO1 / (PP1 * HR1)...(22) PP1 = α * PWTT + β...(23)

[0079] Then, the coefficients α, β, and K obtained by the calibration are substituted into Equation (15) to calculate esCCO (step 22). In addition, when the coefficient K is present in step 17 (step 17: Yes), the process proceeds to step 22 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.

[0080] The calculated esCCO is displayed on the display unit 61 (step 23). Subsequently, the process returns to step 2, and the above-described process is repeated sequentially.

[0081] As described above, according to the blood volume measurement method and measurement device according to the embodiment, it is possible to accurately calculate the cardiac output of the subject, by making less susceptible to the influence of instantaneous or intermittent noise. In addition, it is possible to prevent a situation in which a normal moving average value cannot be measured and the time required for calibration is extended, thereby reducing the burden on the subject or user.

[0082] 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 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.

[0083] 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.

[0084] 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 PWTT detection device 3.

[0085] 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.

[0086] This application claims priority to Japanese Patent Application No. 2023-173167 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, the method comprising: an instantaneous value measurement step of measuring an instantaneous value of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; a division moving average value measurement step of measuring a plurality of times a division moving average value in a first 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 comprehensive moving average value calculation step of calculating comprehensive moving average value which is an average value of a plurality of the division moving average values when the number of times the division moving average value is measured in a second predetermined section is a threshold value or more, in the division moving average value measurement step; a calibration step of calculating a subject-specific coefficient of the subject based on the comprehensive moving average value calculated in the comprehensive moving average value calculation step; and a calculation step of calculating the cardiac output based on the instantaneous value and the subject-specific coefficient calculated in the calibration step.

2. The blood volume measurement method according to claim 1, wherein in the comprehensive moving average value calculation step, weighting is performed on the plurality of division moving average values to calculate the comprehensive moving average value.

3. The blood volume measurement method according to claim 2, wherein the weighting is increased in accordance with an elapsed time from a start of the comprehensive moving average value calculation step.

4. 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.

5. A blood volume measurement device which calculates a cardiac output from physiological information parameters of a subject, the device comprising: instantaneous value measurement means for measuring an instantaneous value of a first physiological information parameter of the subject; division moving average value measurement means for measuring a plurality of times a division moving average value in a first predetermined section of a second physiological information parameter of the subject; comprehensive moving average value calculation means for calculating a comprehensive moving average value which is an average value of a plurality of the division moving average values, when the number of times the division moving average value is measured in a second predetermined section is a threshold value or more, in the division moving average value measurement means; calibration means for calculating a subject-specific coefficient of the subject based on the comprehensive moving average value calculated in the comprehensive moving average value calculation means; and calculation means for calculating the cardiac output based on the instantaneous value and the subject-specific coefficient calculated by the calibration means.

6. A program of calculating a cardiac output from physiological information parameters of a subject, the program causing a computer to execute: an instantaneous value measurement step of measuring an instantaneous value of a first physiological information parameter including at least one of a pulse wave transit time and a pulse pressure of the subject; a division moving average value measurement step of measuring a plurality of times a division moving average value in a first 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 comprehensive moving average value calculation step of calculating comprehensive moving average value which is an average value of a plurality of the division moving average values when the number of times the division moving average value is measured in a second predetermined section is a threshold value or more, in the division moving average value measurement step; a calibration step of calculating a subject-specific coefficient of the subject based on the comprehensive moving average value calculated in the comprehensive moving average value calculation step; and a calculation step of calculating the cardiac output based on the instantaneous value and the subject-specific coefficient calculated in the calibration step.

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