Calibration of Pulse Transit Time Measurements to Arterial Blood Pressure using External Arterial Pressure Applied along the Pulse Transit Path

• US 20100241011A1
• Filed: 03/20/2009
• Published: 09/23/2010
• Est. Priority Date: 03/20/2009
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: McCombie, Devin B., Asada, Haruhiko H., Reisner, Andrew T.
• Granted Patent: US 8,313,439 B2
• Time in Patent Office: Days
• US Class Current: 600/485
• CPC Class Codes: A61B 5/021 Measuring pressure in heart or blood vessels A61B5/0205 takes precedence
• CPC Class Codes: A61B 5/02125 of pulse wave propagation time

Abstract

An apparatus and methods for adaptive and autonomous calibration of pulse transit time measurements to obtain arterial blood pressure using arterial pressure variation. The apparatus and methods give pulse transit time (PTT) devices an ability to self-calibrate. The methods apply a distributed model with lumped parameters, and may be implemented, for example, using pulse transit time measurements derived from a wearable photoplethysmograph (PPG) sensor architecture with an intervening pressurizing mechanism.

Claims

1. A method for autonomous calibration of a pulse-transit-time monitoring device for measurement of arterial blood pressure, the method comprising:

measuring a pulse transit time of a circulatory feature between a proximal sensor position and a distal sensor position, the proximal and distal sensor positions disposed along a single arterial transit path on a human body;

parameterizing a modeled quantitative relationship among a set of quantities, at least one of which quantities is a quantity distributed with distance along the transit path;

performing a measurement so as to provide a plurality of quantities from among the set of quantities; and

solving the plurality of quantities, on the basis of the modeled quantitative relationship, so as to obtain the arterial blood pressure as a function of the measured pulse transit time.

2. A method in accordance with claim 1, wherein the quantity distributed with distance along the transit path is parameterized as a function of external pressure.

3. A method in accordance with claim 1, wherein the quantity distributed with distance is treated as a function that is invariant over the transit path.

4. A method, in accordance with claim 1, wherein the quantity distributed with distance is parameterized as a function of transmural pressure, pulse wave velocity, arterial blood pressure, vertical height of all loci along the transit path, and any external pressure on a body surface along the transit path.

5. A method, in accordance with claim 1, wherein the quantity distributed with distance is arterial stiffness.

6. A method in accordance with claim 1, wherein the step of measuring includes measuring a pressure.

7. A method in accordance with claim 6, wherein the step of measuring includes measuring an externally applied pressure.

8. A method, in accordance with claim 1, wherein the step of solving includes deriving a general dynamic relationship s(t) between a change in external pressure Δ

P_ex(t), applied to an artery wall between the proximal sensor position and the distal sensor position, and a resulting change in pulse transit time Δ

PTT(t).

9. A method in accordance with claim 1, wherein the set of quantities includes a plurality of quantities selected from the group including transmural pressure, pulse wave velocity, arterial blood pressure, vertical height of a plurality of loci along the transit path, and any external pressure on a body surface along the transit path.

10. A method in accordance with claim 1, wherein the transit path is treated in a lumped parameter model having at least three segments with respect to evaluation of the at least one quantity distributed with distance along the transit path.

11. A method in accordance with claim 10, wherein a first of the at least three segments corresponds to the proximal sensor position.

12. A method in accordance with claim 10, wherein a first of the at least three segments corresponds to the distal sensor position.

13. A method in accordance with claim 10, wherein one of the at least three segments is an uninstrumented segment.

14. A method in accordance with claim 10, wherein the vertical height of all segments is assumed to be equal.

15. A method in accordance with claim 1, further comprising externally varying the transmural pressure at least at one position along the arterial path connecting the proximal sensor position and the distal sensor position.

16. A method in accordance with claim 1, as above in which the quantified relationship is computed while the segments of the arterial path are moved through a plurality of heights.

17. A method in accordance with claim 15 or 16, further comprising remeasuring the pulse transit time of the circulatory feature between the proximal and distal sensor positions, subject to the externally varied transmural pressure.

18. A method in accordance with claim 17, further comprising applying the measured and remeasured pulse transit time to eliminate the at least one model variable to obtain a calibrated measure of arterial blood pressure.

19. A method in accordance with claim 15, wherein a position at which the transmural pressure is varied is at the proximal sensor position.

20. A method in accordance with claim 15, wherein a position at which the transmural pressure is varied is at the distal sensor position.

21. A method in accordance with claim 15, wherein the transmural pressure is varied by means of a cuff.

22. A method in accordance with claim 15, wherein the transmural pressure is varied by varying body posture.

23. An apparatus for performing a circulatory measurement on an extremity of a subject, the apparatus comprising:

a first sensor disposed at a first position on the extremity of the subject;

a second sensor disposed at a second position on the extremity of the subject, the second position, the first and second positions lying along a single arterial path;

a pressurizing mechanism for varying a transmural pressure at a specified position along the single arterial path between the first and second positions; and

a processor for obtaining the circulatory measurement based on a measured transit time and a calibration relating the circulatory measurement to the measured transit time on the basis of at least two distinct pressures applied by virtue of the pressuring mechanism.

24. An apparatus for performing a circulatory measurement on an extremity of a subject, the apparatus comprising:

a first sensor disposed at a first position on the extremity of the subject;

a second sensor disposed at a second position on the extremity of the subject, the second position, the first and second positions lying along a single arterial path;

a pressure sensor for measuring external pressure at least at a specified position along the single arterial path between the first and second positions; and

a processor for obtaining the circulatory measurement based on a measured transit time and a calibration relating the circulatory measurement to the measured transit time on the basis of at least the measured external pressure and a mathematical model of the arterial path.