Pressure-based system and method for determining cardiac stroke volume
View Patent ↗Cardiac stroke volume (SV) of a subject is estimated as a function of a value derived from a measured arterial pressure waveform. The value may be the standard deviation, or a function of the difference between maximum and minimum pressure values, or a function of either the maximum value of the first time derivative or the absolute value of the minimum of the first time derivative of the pressure waveform, or both, or a function of the magnitude of one or more spectral components of the pressure waveform at a frequency corresponding to the heart rate. Cardiac output is then estimated as the product of the subject's heart rate and SV, scaled by a calibration constant. Arterial pressure may be measured invasively or non-invasively.
1. A method for determining a parameter proportional to the cardiac stroke volume of a subject comprising:
sensing an input signal that is proportional to arterial blood pressure;
calculating the standard deviation of the input signal over a measurement interval; and
calculating an estimate of the cardiac stroke volume as a function of the standard deviation of the input signal.
2. A method as in claim 1 , further comprising:
measuring the heart rate of the subject; and
estimating current cardiac output of the subject by calculating the product of the heart rate and the standard deviation and scaling the product by a calibration constant.
3. A method as in claim 2 , further comprising:
measuring a calibration cardiac output value; and
calculating the calibration constant as the quotient between a calibration cardiac output estimate and the product of the heart rate and the standard deviation.
4. A method as in claim 1 , further comprising sensing the input signal non-invasively.
5. A method as in claim 1 , in which the measurement interval extends over more than one cardiac cycle.
6. A method as in claim 5 , in which the measurement interval is a plurality of cardiac cycles.
7. A method as in claim 5 , further comprising:
calculating a component standard deviation value of the input signal for each of a plurality of measurement intervals;
computing a composite standard deviation value as an average of the component standard deviation values; and
using the composite standard deviation value in calculating the estimate of the cardiac stroke volume.
8. A method as in claim 5 , further comprising:
for each of a plurality of cardiac cycles, calculating a mean pressure value;
and adjusting the measurement interval as a function of change in the mean pressure value.
9. A method as in claim 5 , further comprising high-pass filtering the input signal before the step of calculating the standard deviation.
10. A method as in claim 1 , in which the input signal is a measurement of the arterial blood pressure.
11. A method as in claim 10 , further comprising:
determining a maximum value and a minimum value of the arterial blood pressure; and
calculating the standard deviation as a function of the difference between the maximum and minimum values.
12. A method as in claim 1 , in which the step of calculating the estimate of the cardiac stroke volume as a function of the standard deviation of the input signal comprises calculating the product of the standard deviation and a calibration factor.
13. A method for determining cardiac stroke volume of a subject comprising:
sensing arterial blood pressure;
converting the sensed arterial blood pressure to a pressure signal;
calculating the standard deviation of the pressure signal over a measurement interval;
calculating an estimate of the stroke volume as a function of the standard deviation of the pressure signal.
14. A method as in claim 13 , further comprising:
measuring the heart rate of the subject; and
estimating current cardiac output of the subject by calculating the product of the heart rate and the standard deviation and scaling the product by a calibration constant.
15. A method as in claim 14 , further comprising:
measuring a calibration cardiac output value; and
calculating the calibration constant as the quotient between a calibration cardiac output estimate and the product of the heart rate and the standard deviation.
16. A method for estimating cardiac output of a subject comprising:
sensing arterial blood pressure;
converting the sensed arterial blood pressure to a pressure signal;
calculating the standard deviation of the pressure signal over a measurement interval;
calculating an estimate of stroke volume as a function of the standard deviation of the pressure signal;
measuring the heart rate of the subject; and
estimating current cardiac output of the subject by calculating the product of the heart rate d the standard deviation and scaling the product by a calibration constant.
17. A system for determining a parameter proportional to the cardiac stroke volume of a subject comprising:
a sensor located in or on the body of the subject and generating a sensor signal that is proportional to arterial blood pressure;
conversion circuitry that receives the sensor signal and converts it to an input signal;
a processing system that receives the input signal and that includes processing modules for calculating the standard deviation of the input signal over a measurement interval and for calculating an estimate of the cardiac stroke volume as a function of the standard deviation of the input signal; and
a display for presenting the estimate of the cardiac stroke volume to a user.
18. A system as in claim 17 , further comprising a heart rate monitor measuring the he rate of the subject;
the processing system estimating current cardiac output of the subject by calculating the product of the heart rate and the standard deviation and scaling the product by a calibration constant.
19. A system as in claim 17 , further comprising a high pass filter connected between the sensor and the processing system.
20. A system as in claim 17 , in which the sensor is a direct blood pressure sensor.
21. A system for determining a parameter proportional to the cardiac stroke volume of a subject comprising:
a sensor located in or on the body of the subject and generating a sensor signal that is proportional to arterial blood pressure;
conversion circuitry that receives the sensor signal and converts it to an input signal;
a processing system including computer-executable code for calculating the standard deviation of the input signal over a measurement interval; and for calculating an estimate of the cardiac stroke volume as a function of the standard deviation of the input signal; and
a display for presenting the estimate of the cardiac stroke volume to a user.
22. A system as in claim 21 , further comprising a heart rate monitor measuring the he rate of the subject, the processing system further including computer-executable code for estimating current cardiac output of the subject by calculating the product of the heart rate and the standard deviation and scaling the product by a calibration constant.
23. A system as in claim 22 , further comprising a calibration system measuring a calibration cardiac output value, the processing system further including computer-executable code for calculating the calibration constant as the quotient between a calibration cardiac output estimate and the product of the heart rate and the standard deviation.
24. A system as in claim 21 , in which the sensor is non-invasive.