METHOD AND APPARATUS FOR PRODUCING A CENTRAL PRESSURE WAVEFORM IN AN OSCILLOMETRIC BLOOD PRESSURE SYSTEM

• US 20100152593A1
• Filed: 12/07/2009
• Published: 06/17/2010
• Est. Priority Date: 02/21/2006
• Status: Active Grant

• Current Assignee: Uscom Ltd.
• Original Assignee: Pulsecor Ltd. (Uscom Ltd.)
• Inventors: LOWE, ANDREW
• Granted Patent: US 9,433,358 B2
• Time in Patent Office: Days
• US Class Current: 600/493
• 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
• CPC Class Codes: A61B 5/022 by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
• CPC Class Codes: A61B 5/02225 using the oscillometric method
• 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 5/412 Detecting or monitoring sepsis
• CPC Class Codes: A61B 7/00 Instruments for auscultation
• CPC Class Codes: A61B 7/04 Electric stethoscopes microphones, acoustic transducers therefor H04R
• CPC Class Codes: G06F 2218/10 by analysing the shape of a waveform, e.g. extracting parameters relating to peaks
• CPC Class Codes: G16H 40/63 for local operation

Abstract

A method for estimating an intra-arterial blood pressure waveform, from pressure waveforms obtained from a blood pressure cuff, comprises the steps of:

• • a. inflating a blood pressure cuff on a brachial artery of an arm to a pressure at least as great as the diastolic pressure;
  • b. holding the molar amount of fluid in the blood pressure cuff constant;
  • c. sensing a sequence of cuff pressure waveforms associated with the brachial artery that result from at least one cardiac ejection cycle; and
  • cc. scaling the cuff pressure waveforms to produce a pseudo-arterial waveform, assuming a nonlinear relationship between the oscillatory pressure Δp_o and the pseudo-arterial pressure p_i where the nonlinearity is hyperbolic in form, according to the formula:

p_i=c₁/f(Δp_o)+c₂

where c₁ and c₂ are constant relative to Δp_o and p_i, and f is a power function, and c₁, c₂ and f are chosen such that increments in Δp_o at larger values of Δp_o result in smaller increments in p_i than equal increments in Δp_o at smaller values of Δp_o.

Claims

1. A method of estimating an intra-arterial blood pressure waveform from pressure waveforms obtained from a blood pressure cuff, said method comprising the steps of:

a. inflating a blood pressure cuff on a brachial artery of an arm to a pressure at least as great as the diastolic pressure;

b. holding the molar amount of fluid in the blood pressure cuff constant;

c. sensing a sequence of cuff pressure waveforms associated with the brachial artery that result from at least one cardiac ejection cycle; and

d. scaling the cuff pressure waveforms to produce a pseudo-arterial waveform, assuming a nonlinear relationship between the oscillatory pressure Δ

p_o and the pseudo-arterial pressure p_i where the nonlinearity is hyperbolic in form, according to the formula;

p_i=c₁/f(Δ

p_o)+c₂

where c₁ and c₂ are constant relative to Δ

p_o and p_i, and f is a power function, and c₁, c₂ and f are chosen such that increments in Δ

p_o at larger values of Δ

p_o result in smaller increments in p_i than equal increments in Δ

p_o at smaller values of Δ

p_o.

2. A method of estimating an internal, central arterial blood pressure waveform from pressure waveforms obtained from a blood pressure cuff, said method comprising the steps of:

e. inflating a proximal blood pressure cuff on the brachial artery of an arm to a suprasystolic pressure;

f. sensing a proximal cuff pressure waveform associated with at least one cardiac ejection cycle;

g. inflating a distal blood pressure cuff on the brachial artery of an arm to a suprasystolic pressure;

h. sensing a distal cuff pressure waveform associated with at least one cardiac ejection cycle;

i. calculating the time of propagation of said blood pressure pulse from the entry of the said artery to the proximal cuff;

j. calculating the time of propagation of said blood pressure pulse from the proximal cuff to the distal cuff;

k. calculating a reflection coefficient of the pressure pulse at one of the proximal and distal cuffs; and

l. assuming a physical model of wave propagation along said artery between aorta and said proximal and distal blood pressure cuffs, determining an estimated pressure waveform at the opening of the said artery.

3. The method of claim 2, where the time of propagation of said blood pressure pulse from the proximal cuff to the distal cuff is calculated by taking the difference in time between the incidence of the blood pressure pulse at the proximal cuff and an identifiable point on a signal related to heart activity, taking the difference in time between the incidence of the blood pressure pulse at the distal cuff and an equivalent identifiable point on the same signal related to heart activity, and calculating the difference between the differences.

4. The method of claim 3, where the signal related to heart activity is measured using one of an ECG, heart sounds sensor, tonometer, photoplethysmograph and blood pressure cuff.

5. The method of claim 2, where the time of propagation of said blood pressure pulse from the entry of the said artery to the proximal cuff is calculated by taking the difference in time between the incidence of the blood pressure pulse at the proximal cuff and an identifiable point on a signal related to the entry of a pressure pulse into the subclavian artery.

6. The method of claim 2, where the time of propagation of said blood pressure pulse from the entry of the said artery to the proximal cuff is calculated using a ratio of differences between frequency-domain representations of the proximal and distal pressure waveforms.

7. The method of claim 6, where the ratio is given by:

$\mathrm{dt}=1/2<br><br>s<br><br><br><br>\ln <br><br>\left(\frac{b(P_{t<br><br><br><br>3}<br><br>\left(s\right)-\exp <br><br>\left(-s<br><br><br><br>\mathrm{\delta <br><br>}<br><br><br><br>t\right)<br><br>P_{t<br><br><br><br>4}<br><br>\left(s\right)}{\exp <br><br>\left(s<br><br><br><br>\mathrm{\delta <br><br>}<br><br><br><br>t\right)<br><br>P_{t<br><br><br><br>4}<br><br>\left(s\right)-P_{3<br><br>t}<br><br>\left(s\right)))}\right)$

where dt is the time of propagation of said blood pressure pulse from the entry of the said artery to the proximal cuff, P_t4 is the pressure at the distal cuff as a function of complex frequency, P_t3 is the pressure at the proximal cuff as a function of complex frequency, b is the pressure wave reflection coefficient, δ

t is the time for the pressure wave to propagate from the proximal to the distal cuff, and s specifies a complex frequency coordinate.

8. The method of claim 5, where the signal related to heart activity is measured using one of a tonometer and a strain sensor, placed over one of the subclavian artery, and a carotid artery.

9. The method of claim 5, where the signal related to heart activity is measured using a heart sounds sensor, and the identifiable point is a time equal to the systolic ejection period prior to the incidence of the heart sounds.

10. The method of claim 5, where the signal related to heart activity is measured using an ECG, and the identifiable point is a predetermined time after the incidence of the R-wave.

11. The method of claim 2, where the reflection coefficient of the pressure pulse at the proximal cuff is calculated according to a ratio between differences between time-shifted signals calculated from the cuff pressure waveforms.

12. The method of claim 11 where the ratio is given by:

$b=\frac{p_{t<br><br><br><br>4}<br><br>\left(\mathrm{dt}+t+\mathrm{\delta <br><br>}<br><br><br><br>t\right)-p_{t<br><br><br><br>3}<br><br>\left(\mathrm{dt}+t+\mathrm{\delta <br><br>}<br><br><br><br>t\right)}{p_{t<br><br><br><br>3}<br><br>\left(t-\mathrm{dt}\right)-p_{t<br><br><br><br>4}<br><br>\left(-\mathrm{dt}+t-\mathrm{\delta <br><br>}<br><br><br><br>t\right)}$

where b is the reflection coefficient, p_t4 is the pressure at the distal cuff as a function of time, p_t3 is the pressure at the proximal cuff as a function of time, dt is the time for the pressure wave to propagate from the root of the subclavian artery to the proximal cuff, δ

t is the time for the pressure wave to propagate from the proximal to the distal cuff, and t specifies a time coordinate.

13. The method of claim 2, where the reflection coefficient is a predetermined value.

14. The method of claim 2, where the time of propagation of said blood pressure pulse from the entry of the said artery to the proximal cuff is a predetermined value.

15. A method of estimating forward and reflected components of the aortic blood pressure waveform from an aortic pressure waveform, said method comprising the steps of:

m. determining the portion of the aortic pressure waveform, P, that occurs during diastole

n. finding two parameters, γ and

P_Res fitting an exponential decay to the diastolic portion of the waveform P, to determine an exponentially decaying pressure, P_Decay

o. assuming a form of the incident waveform emanating from the heart, P_Incident

p. calculating a reflected wave, P_Iliac, from P_Incident and a predetermined model of reflection from the distal aorta

q. calculating a pressure wave P_Valve representing a further reflection of the reflected wave P_Iliac from the aortic valve, wherein the further reflection substantially only occurs during diastole

r. calculating the parameters, α and

β

, that best fit an equation of the form P_Est=P_Res+α

P_Incident+β

(P_Iliac+P_Valve)+P_Decay to the measured aortic pressure waveform P

s. calculating the incident wave according to α

×

P_Incident and the reflected wave according to β

×

P_Iliac.

16. The method of claim 15, further comprising the step of refining the estimate of the shape of the incident pressure wave, P_Incident, by adding a portion of the difference between the fitted, P_Est, and measured, P, aortic pressure waveforms to the previously assumed incident wave, P_Incident.

17. The method of claim 15, further comprising the steps of:

t. reconstructing the pressure of the waveform downstream on the artery from said cuff; and

u. estimating the central blood flow waveform in the artery by determining the forces inducing blood flow.

18. The method of claim 17, where the reconstruction of the pressure waveform downstream comprises the steps of:

v. Shifting the incident pressure waveform P_Incident backwards in time by a predetermined amount

w. Shifting the reflected pressure waveform P_Iliac forwards in time by said predetermined amount

x. Shifting the secondary reflected pressure waveform P_Valve backwards in time by said predetermined amount

y. Summing the incident, reflected, secondary reflected, reserve and decay pressures to reconstruct the downstream pressure waveform.

19. The method of claim 17, where the estimation of the central blood flow waveform comprises the steps of:

z. Calculating the flow generated by the heart as the product of a predetermined factor and the difference between the incident and reflection pressure waves;

aa. Calculating the flow generated by the pressure gradient as the product of a predetermined factor and the difference between the aortic pressure waveform and the downstream pressure waveform; and

bb. Summing the calculated flows generated by the heart and pressure gradient to estimate the central blood flow waveform.