Method and apparatus for the measurement of cardiac output

• US 6,071,244 A
• Filed: 04/15/1999
• Issued: 06/06/2000
• Est. Priority Date: 01/05/1996
• Status: Expired due to Term

• Current Assignee: Lidco Group, PLC
• Original Assignee: Monitoring Technology Corporation
• Inventors: O'Brien, Terence Kevin, Linton, Robert Antony Fox, Linton, Nicholas William Fox, Band, David Marston
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US09/101,420
• Time in Patent Office: 418 Days
• Field of Search: 600/526
• US Class Current: 600/526
• CPC Class Codes: A61B 5/029 Measuring or recording blood output from the heart, e.g. minute volume A61B8/065 takes precedence

Abstract

A method for the measurement of cardiac output in a patient in which the arterial blood pressure waveform of a patient from a blood pressure monitoring device over a period of time is subjected to various transformations and corrections, including autocorrelation, whereby the pulsatility and heart rate of the patient are obtained. The nominal stroke volume is then calculated from the pulsatility and the nominal caridac output obtained by multiplying the stroke volume by the heart rate.

Claims

1. A method for the measurement of cardiac output in a patient, which method comprises:

(i) recording and storing the arterial blood pressure waveform of a patient from a blood pressure monitoring device over a period of time;

(ii) subjecting the waveform obtained in (i) to a non-linear transformation that corrects for the variation of the characteristics of the arterial system with pressure and thereby obtaining a corrected waveform;

(iii) subjecting the corrected waveform from (ii) to autocorrelation in order to derive the pulsatility which is equal to ##EQU2## where R(τ

) is the autocorrelation function and τ

_min is the value of τ

at which the autocorrelation is at a minimum and heart rate of the corrected waveform;

(iv) calculating the nominal stroke volume from the pulsatility;

(v) obtaining the nominal cardiac output by multiplying the nominal stroke volume by the heart rate; and

(vi) obtaining the cardiac output from the nominal cardiac output by means of a calibration factor.

2. A method as claimed in claim 1 wherein the arterial blood pressure is recorded and stored in (i) for a period of up to ten seconds.

3. A method as claimed in claim 2 wherein the arterial blood pressure is recorded and stored in (i) for a period of up to four seconds.

4. A method as claimed in claim 1 wherein the non-linear transformation in step (ii) is effected using a look up table, with the mean of the data then being found and subtracted from each of the values obtained in (i).

5. A method as claimed in claim 1 wherein the nominal stroke volume is obtained by multiplying the pulsatility by a predetermined calibration factor.

6. A method as claimed in claim 5 wherein the predetermined calibration factor is obtained for a particular patient using a known calibration method.

7. A method as claimed in claim 6 wherein the calibration method is an indicator dilution method.

8. A method for the measurement of cardiac output in a patient, which method comprises:

(a) recording and storing the arterial blood pressure waveform of a patient from a blood pressure monitoring device over a period of time;

(b) obtaining and subtracting the mean of the blood pressure waveform from each of the values obtained in (a) and subjecting the data so obtained to autocorrelation;

(c) transforming the data from (b) into data which relates to the pulsatility which is equal to ##EQU3## where R(τ

) is the autocorrelation function and τ

_min is the value of τ

at which the autocorrelation is at a minimum and heart rate of the waveform;

(d) calculating the nominal stroke volume from the pulsatility;

(e) obtaining the nominal cardiac output by multiplying the nominal stroke volume by the heart rate; and

(f) obtaining the cardiac output from the nominal cardiac output by means of a calibration factor.

9. A method as claimed in claim 8 wherein the arterial blood pressure is recorded and stored in (a) for a period of up to ten seconds.

10. A method as claimed in claim 9 wherein the arterial blood pressure is recorded and stored in (a) for a period of up to four seconds.

11. A method as claimed in claim 8 wherein the nominal stroke volume is obtained by multiplying the pulsatility by a predetermined calibration factor.

12. A method as claimed in claim 11 wherein the predetermined calibration factor is obtained for a particular patient using a known calibration method.

13. A method as claimed in claim 12 wherein the calibration method is an indicator dilution method.

14. Apparatus for the measurement of cardiac output in a patient, which comprises:

means for monitoring the arterial blood pressure of a patient;

means for monitoring the heart rate of a patient;

means for recording and storing the arterial blood pressure waveform of a patient;

means for transforming the blood pressure waveform to correct for the variation of the characteristics of the arterial system with pressure and thereby obtaining transformed data;

means for autocorrelating the transformed data and deriving the pulsatility which is equal to ##EQU4## where R(τ

) is the autocorrelation function and τ

_min is the value of τ

at which the autocorrelation is at a minimum and heart rate of the waveform therefrom;

means to calculate the nominal cardiac output from the measurements of heart rate of the patient and the pulsatility of the waveform; and

means to converting the nominal cardiac output into the cardiac output.

15. Apparatus as claimed in claim 14 wherein the means for transforming the pressure waveform comprises a look up table.

16. Apparatus as claimed in claim 14 which includes means to calibrate the device.

17. Apparatus for the measurement of cardiac output in a patient, which comprises:

means for monitoring the arterial blood pressure of a patient;

means for monitoring the heart rate of a patient;

means for recording and storing the arterial blood pressure waveform of a patient;

means for obtaining and subtracting the mean of the blood pressure waveform from each of the values of the blood pressure and means for subjecting the data so obtained to autocorrelation;

means for transforming the autocorrelated data into data relating to the pulsatility which is equal to ##EQU5## where R(τ

) is the autocorrelation function and τ

_min is the value of τ

at which the autocorrelation is at a minimum and heart rate of the blood pressure waveform;

means to calculate the nominal cardiac output from the measurements of heart rate of the patient and the pulsatility of the waveform; and

means for converting the nominal cardiac output into the cardiac output.