Methods and apparatus for determining cardiac output

• US 7,815,578 B2
• Filed: 09/22/2003
• Issued: 10/19/2010
• Est. Priority Date: 02/10/2003
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology, The Board of Trustees of Michigan State University (Michigan State University)
• Original Assignee: Massachusetts Institute of Technology, The Board of Trustees of Michigan State University (Michigan State University)
• Inventors: Cohen, Richard J., Mukkamala, Ramakrishna, Sherman, Derin A.
• Primary Examiner(s): BOCKELMAN, MARK
• Application Number:
  US10/667,956
• Publication Number:
  US 20040158163A1
• Time in Patent Office: 2,584 Days
• Field of Search: 600/513, 600/526, 600/301, 600/483, 600/500
• US Class Current: 600/526
• CPC Class Codes: A61B 5/02007 Evaluating blood vessel condition, e.g. elasticity, compliance
• CPC Class Codes: A61B 5/02255 the pressure being controlled by plethysmographic signals, e.g. derived from optical sensors
• CPC Class Codes: A61B 5/029 Measuring or recording blood output from the heart, e.g. minute volume A61B8/065 takes precedence
• CPC Class Codes: A61B 5/352 Detecting R peaks, e.g. for synchronising diagnostic apparatus; Estimating R-R interval
• CPC Class Codes: A61B 5/726 using Wavelet transforms

Abstract

The present invention provides methods and apparatus for determining a dynamical property of the systemic or pulmonary arterial tree using long time scale information, i.e., information obtained from measurements over time scales greater than a single cardiac cycle. In one aspect, the invention provides a method and apparatus for monitoring cardiac output (CO) from a single blood pressure signal measurement obtained at any site in the systemic or pulmonary arterial tree or from any related measurement including, for example, fingertip photoplethysmography.

According to the method the time constant of the arterial tree, defined to be the product of the total peripheral resistance (TPR) and the nearly constant arterial compliance, is determined by analyzing the long time scale variations (greater than a single cardiac cycle) in any of these blood pressure signals. Then, according to Ohm'"'"'s law, a value proportional to CO may be determined from the ratio of the blood pressure signal to the estimated time constant. The proportional CO values derived from this method may be calibrated to absolute CO, if desired, with a single, absolute measure of CO (e.g., thermodilution). The present invention may be applied to invasive radial arterial blood pressure or pulmonary arterial blood pressure signals which are routinely measured in intensive care units and surgical suites or to noninvasively measured peripheral arterial blood pressure signals or related noninvasively measured signals in order to facilitate the clinical monitoring of CO as well as TPR.

Claims

1. An apparatus for determining cardiac output to within a constant scale factor comprising a computer system that includes:

(a) computer readable media having code comprising computer-executable process steps; and

(b) a processor that executes the process steps to;

(i) accept an input representing a measurement of an arterial blood pressure signal over a plurality of cardiac cycles;

(ii) estimate a function that represents the response of the arterial blood pressure to a single cardiac contraction;

(iii) fit the function of step (ii) to an exponential-like function over a time period that begins a selected amount of time following the maximum value of the function;

(iv) estimate the time constant of the function of step (ii) as the time constant of the exponential function of step (iii); and

(v) determine cardiac output to within a constant scale factor by dividing average arterial blood pressure by the time constant obtained in step (iv).

2. The apparatus of claim 1, further comprising an analog-to-digital converter.

3. The apparatus of claim 1, wherein the apparatus includes a buffer system.

4. The apparatus of claim 1, wherein the apparatus includes a display device.

5. The apparatus of claim 1 wherein arterial blood pressure is measured invasively or non-invasively at any site in the systemic or pulmonary arterial tree.

6. The apparatus of claim 1 wherein arterial blood pressure is further defined as systemic arterial pressure.

7. The apparatus of claim 1 wherein arterial blood pressure is further defined as pulmonary blood pressure.

8. An apparatus for determining cardiac output to within a scale factor comprising a computer system that includes:

(g) computer readable media having code comprising computer-executable process steps; and

(h) a processor that executes the process steps to;

i. accept an input representing a measurement of an arterial blood pressure signal over a plurality of cardiac cycles;

ii. capture long time scale information by estimating an impulse response which when convolved with cardiac contractions fits the arterial blood pressure signal;

iii. obtain a time constant by fitting an exponential-like function to the estimated impulse response over a time period that begins a selected amount of time following its maximum value;

iv. determine cardiac output to within a scale factor by dividing a measure of the average arterial blood pressure by the time constant.

9. The apparatus of claim 8 further comprising an analog-to-digital converter.

10. The apparatus of claim 8 wherein the apparatus includes a buffer system.

11. The apparatus of claim 8 wherein the apparatus includes a display device.

12. The apparatus of claim 8 wherein arterial blood pressure is measured invasively or non-invasively at any site in the systemic or pulmonary arterial tree.

13. The apparatus of claim 8 wherein arterial blood pressure is further defined as systemic arterial pressure.

14. The apparatus of claim 8 wherein arterial blood pressure is further defined as pulmonary blood pressure.