Non-invasive blood pressure measurement using pulse wave velocity
A method and apparatus to non-invasively measure instantaneous blood pressure using pulse wave velocity are disclosed. A measurement component is affixed to a patient proximate to a blood vessel. One or more sensors, such as an ultrasound sensor, is included in the measurement component. The measurement component substantially simultaneously measures the pulse wave velocity of the vessel and the instantaneous blood velocity within the vessel. The measurement component computes the instantaneous blood pressure of the vessel using, for example, the water hammer equation. The one or more sensors may be contained in a disposable patch or collocated with another sensor, such as a patient-monitor sensor, or the like.
1. A vital sign monitor, comprising:
an ultrasound transducer comprising:
a first element configured to transmit a first ultrasound beam toward a blood vessel and to detect a reflection of the first ultrasound beam from a first portion of a wall of the blood vessel; and
a second element configured to transmit a second ultrasound beam toward the blood vessel and to detect a reflection of the second ultrasound beam from a second portion of the wall of the blood vessel, the second ultrasound beam being nonparallel to the first ultrasound beam;
a processor configured to:
identify a change in a slant range at the first portion of the wall of the blood vessel by analyzing the reflection of the first ultrasound beam;
identify a change in a slant range at the second portion of the wall of the blood vessel by analyzing the reflection of the second ultrasound beam;
determine a pulse wave velocity of the blood vessel by determining a time period between the identification of the change in the slant range at the first portion of the wall of the blood vessel and the identification of the change in the slant range at the second portion of the wall of the blood vessel;
determine a velocity of blood flowing through the blood vessel; determine a blood pressure of the blood vessel by multiplying the pulse wave velocity of the blood vessel and the velocity of the blood flowing through the blood vessel; and
an output device configured to output an indication of the blood pressure.
2. The vital sign monitor of claim 1 , wherein the first element is further configured to detect a reflection of the first ultrasound beam from the blood flowing through the blood vessel, and
wherein the processor is configured to determine the velocity of the blood flowing through the blood vessel by performing Doppler analysis on the reflection of the first ultrasound beam.
3. The vital sign monitor of claim 1 , wherein the slant range at the first portion of the wall of the blood vessel comprises a distance between the first element and the first portion of the wall of the blood vessel, and wherein the slant range at the second portion of the wall of the blood vessel comprises a distance between the second element and the second portion of the wall of the blood vessel.
4. A medical device, comprising:
an ultrasound transducer comprising elements configured to transmit nonparallel ultrasound beams toward a blood vessel and to detect reflections of the nonparallel ultrasound beams from the blood vessel; and
a processor configured to:
determine changes in slant ranges at portions of a wall of the blood vessel by analyzing the reflections of the nonparallel ultrasound beams from the blood vessel;
determine a pulse wave velocity of the blood vessel by determining a time period between the determinations of the changes in the slant ranges at the portions of the wall of the blood vessel; and
determine a blood pressure of the blood vessel by multiplying the pulse wave velocity with a velocity of blood flowing through the blood vessel.
5. The medical device of claim 4 , wherein the elements comprise an array of piezoelectric elements.
6. The medical device of claim 4 , wherein the processor is further configured to:
determine a distance between the blood vessel and the ultrasound transducer by analyzing the reflections of the nonparallel ultrasound beams from the blood vessel; and
determine a distance between the portions of the wall of the blood vessel by analyzing an angle between the nonparallel ultrasound beams and the distance between the blood vessel and the ultrasound transducer, and
wherein the processor is configured to determine the pulse wave velocity by dividing the distance between the portions of the wall of the blood vessel with the time period between the determinations of the changes in the slant ranges at the portions of the wall of the blood vessel.
7. The medical device of claim 4 , wherein the processor is further configured to determine the velocity of blood flowing through the blood vessel, and
wherein the processor is configured to determine the blood pressure of the blood vessel by multiplying the pulse wave velocity with the velocity of the blood flowing through the blood vessel and a density of the blood.
8. The medical device of claim 7 , wherein the processor is configured to determine the velocity of the blood flowing through the blood vessel by analyzing the reflections of the nonparallel ultrasound beams from the blood vessel.
9. The medical device of claim 7 , wherein one of the nonparallel ultrasound beams comprises pulses, and
wherein the processor is configured to determine the velocity of the blood flowing through the blood vessel by determining a phase difference between reflections of the pulses from blood flowing through the blood vessel.
10. The medical device of claim 4 , wherein the medical device is configured to be affixed to a neck, the blood vessel being disposed in the neck.
11. The medical device of claim 4 , further comprising:
a display screen or a speaker configured to output an indication of the blood pressure.
12. The medical device of claim 4 , further comprising:
electrocardiogram (ECG) leads or defibrillation electrodes integrated with the ultrasound transducer.
13. The medical device of claim 4 , wherein the slant ranges comprise distances between the ultrasound transducer and the portions of the wall of the blood vessel.
14. A method, comprising:
transmitting, by a first element of a transducer, a first ultrasound beam toward a blood vessel;
detecting, by the first element, a reflection of the first ultrasound beam from a first portion of a wall of the blood vessel;
determining, by a processor, a change in a slant range at the first portion of the wall of the blood vessel by analyzing the reflection of the first ultrasound beam;
transmitting, by a second element of the transducer, a second ultrasound beam toward the blood vessel, the second ultrasound beam being nonparallel to the first ultrasound beam;
detecting, by the second element, a reflection of the second ultrasound beam from a second portion of the wall of the blood vessel;
determining, by the processor, a change in a slant range at the second portion of the wall of the blood vessel by analyzing the reflection of the second ultrasound beam;
determining, by the processor, a pulse wave velocity of the blood vessel by determining a time period between the determination of the change in the slant range at the first portion of the wall of the blood vessel and the determination of the change in the slant range at the second portion of the wall of the blood vessel; and determining, by the processor, a blood pressure of the blood vessel by multiplying the pulse wave velocity with a velocity of blood flowing through the blood vessel.
15. The method of claim 14 , further comprising:
determining a distance between the blood vessel and an ultrasound transducer comprising the first element and the second element by analyzing the reflection of the first ultrasound beam or the reflection of the second ultrasound beam; and
determining a distance between the first portion of the wall of the blood vessel and the second portion of the wall of the blood vessel by analyzing an angle between the first ultrasound beam and the second ultrasound beam and the distance between the blood vessel and the ultrasound transducer,
wherein determining the pulse wave velocity comprises dividing the distance between the portions of the wall of the blood vessel with the time period between the determination of the change in the slant range at the first portion of the wall of the blood vessel and the determination of the change in the slant range at the second portion of the wall of the blood vessel.
16. The method of claim 14 , further comprising:
determining the velocity of blood flowing through the blood vessel,
wherein determining the blood pressure of the blood vessel comprises multiplying the pulse wave velocity with the velocity of the blood flowing through the blood vessel and a density of the blood.
17. The method of claim 16 , wherein determining the velocity of the blood flowing through the blood vessel comprises analyzing the reflection of the first ultrasound beam from the blood vessel.
18. The method of claim 16 , wherein the first ultrasound beam comprises pulses, and
wherein the processor is configured to determine the velocity of the blood flowing through the blood vessel by determining a phase difference between reflections of the pulses from blood flowing through the blood vessel.
19. The method of claim 14 , further comprising:
outputting, by an output device, an indication of the blood pressure.
20. The method of claim 14 , wherein the slant range at the first portion of the wall of the blood vessel comprises a distance between the first element and the first portion of the wall of the blood vessel, and wherein the slant range at the second portion of the wall of the blood vessel comprises a distance between the second element and the second portion of the wall of the blood vessel.