Wearable pulse wave velocity blood pressure sensor and methods of calibration thereof

• US 7,674,231 B2
• Filed: 08/17/2007
• Issued: 03/09/2010
• Est. Priority Date: 08/22/2005
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: McCombie, Devin B., Shaltis, Phillip, Asada, Haruhiko H., Reisner, Andrew T.
• Primary Examiner(s): Mallari; Patricia C
• Application Number:
  US11/840,654
• Publication Number:
  US 20080039731A1
• Time in Patent Office: 935 Days
• Field of Search: 600485-490, 600/500, 600/323, 600/509, 600/481, 600/504
• US Class Current: 600/485
• CPC Class Codes: A61B 5/02125 of pulse wave propagation time
• CPC Class Codes: A61B 5/02255 the pressure being controlled by plethysmographic signals, e.g. derived from optical sensors
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives

Abstract

An apparatus and methods for performing a circulatory measurement on an extremity, such as a hand, of a subject. The circulatory measurement results in the derivation of an output circulatory metric that may encompass blood pressure or various other circulatory metrics. An indicator of an input circulatory metric at a locus on the extremity is measured, such as a pulse transit time, and calibrated to account for the hydrostatic component of blood pressure arising due to vertical displacement of the extremity with respect to the heart.

Claims

1. A method for deriving an output circulatory metric at an extremity of a subject, the method comprising:

measuring a first plethysmographic signal at a first position on the extremity of the subject and measuring a second plethysmographic signal at a second position on the extremity of the subject, each of the signals being measured at a plurality of heights, by means of a sensor disposed on the extremity of the subject, the second position characterized by a distance between the first position and the second position;

establishing a functional relationship using a digital computer, the functional relationship based at least on the measurements of the first and second plethysmographic signals at the plurality of heights, the functional relationship providing a quantitative relationship between the first and second plethysmographic signals and the output circulatory metric absent a direct measurement of the output circulatory metric at each of the plurality of heights; and

calculating a transit time of a circulatory pressure wave based on the first and second plethysmographic signals;

deriving a wave speed based on the calculated transit time and the distance between the first and second positions; and

applying the functional relationship to derive the output circulatory metric.

2. The method of claim 1, wherein the output circulatory metric is an arterial blood pressure of the subject.

3. The method of claim 2, wherein the arterial blood pressure of the subject is at least one of systolic, diastolic, and mean arterial blood pressure.

4. The method of claim 1, wherein the first position is at a wrist of the subject.

5. The method of claim 1, wherein the first plethysmographic signal is based upon measurement of blood volume in an ulnar artery of the subject.

6. The method of claim 1, wherein the first plethysmographic signal is based upon measurement of blood volume in a radial artery of the subject.

7. The method of claim 1, wherein the second plethysmographic signal is based upon measurement of blood volume in a digit of the subject.

8. The method of claim 1, wherein the step of measuring first and second plethysmographic signals include obtaining circulatory waveforms at the first and second positions on the extremity.

9. The method of claim 8, further comprising identifying a point of comparison between the waveform of the first plethysmographic signal and the waveform of the second plethysmographic signal.

10. The method of claim 8, wherein identifying a point of comparison includes defining a measure based on a specified fraction of the range between a maximum value of the waveform and a minimum value of the waveform.

11. The method of claim 1, wherein the step of establishing a functional relationship includes identifying a response function of a sensor signal as a function of intra-arterial pressure.

12. The method of claim 11, wherein the step of identifying a response function of the sensor signal as a function of intra-arterial pressure includes application of adaptive noise cancellation.

13. The method of claim 1, further comprising identifying a value of pulse transit time at zero pressure that is independent of the output circulatory metric.

14. The method of claim 13, further comprising measurement of a value related to a derivative of pulse velocity with respect to arterial pressure.