System for calibrating a blood pressure measurement based on vascular transit of a pulse wave
The invention provides a system and method for measuring vital signs (e.g. SYS, DIA, SpO2, heart rate, and respiratory rate) and motion (e.g. activity level, posture, degree of motion, and arm height) from a patient. The system features: first and second sensors configured to independently generate time-dependent waveforms indicative of one or more contractile properties of the patient's heart; and a cuff-based oscillometric module. A processing component, typically worn on the patient's body and featuring a microprocessor, receives the time-dependent waveforms generated by the different sensors and processes them to determine patient-specific calibration values for use in a continuous blood pressure measurement based on pulse wave velocity (PWV).
1. A method of determining patient-specific calibration values for use in a continuous blood pressure measurement based on pulse wave velocity (PWV), the method comprising:
positioning on the patient
(i) a sensor device on a patient, the sensor device configured to measure sensor data indicative of pulse arrival time (PAT) at a first point on an extremity of the patient, the PAT defined as a time difference between ventricular depolarization occurring in a cardiac cycle, and arrival of a pressure wave at the first point resulting from blood flow caused by the ventricular depolarization through a vascular path of length (L t ), wherein the sensor device is operably connected to a processing component, and
(ii) an inflatable cuff device on the patient's extremity at a second point on the extremity of the patient at a position on the vascular path between the heart and the first point, the inflatable cuff device comprising an inflatable cuff which is configured to cover a portion of the vascular path of length (L c ) and an inflation pump, the inflatable cuff device configured to measure a waveform indicative of cuff pressure on the patient's extremity during inflation, wherein the inflatable cuff device is operably connected to the processing component;
receiving at the processing component the sensor data and the waveform indicative of cuff pressure, and causing the processing component to calculate therefrom a series of pulse arrival times (“PATs”) as a function of cuff pressure during inflation of the inflatable cuff to above the patient's systolic blood pressure, wherein the inflation causes a compliance change in the vascular path of length L 2 , thereby causing the PAT measured by the sensor device to lengthen as cuff pressure is increased; and
determining using the processing component one or more coefficients which relate PWV to mean arterial pressure (MAP) by modeling the series of PATs measured as a function of cuff pressure as a nonlinear relationship, wherein the nonlinear relationship is expressed as
PAT
(
t
)
=
(
L
t
-
L
c
aMAP
cal
+
pwv
0
)
+
(
L
c
a
(
MAP
cal
-
P
cuff
(
t
)
)
+
pwv
0
)
wherein pWV 0 is the PWV through the portion of the vascular path covered by the inflatable cuff, and a is a constant.
2. A method according to claim 1 , wherein the sensor device comprises (a) an electrical sensor comprising: at least two electrodes configured to detect electrical signals from the patient's body, and an electrical circuit operably connected to the electrodes and configured to process the detected electrical signals, the electrical sensor generating a time dependent electrical waveform indicative of one or more contractile properties of the patient's heart, and (b) an optical sensor comprising a source of electromagnetic radiation configured to irradiate tissue of the patient with radiation emitted therefrom, and a detector configured to detect one or more properties of the electromagnetic radiation after it irradiates soft tissue, the optical sensor generating a time dependent optical waveform indicative of volumetric changes in the irradiated tissue, wherein the sensor data comprises the time dependent electrical waveform and the time dependent optical waveform.
3. A method according to claim 1 , further comprising using the processing component to determine a baseline pulse arrival time (PAT cal ) from the sensor data obtained in the absence of pressure being applied by the inflatable cuff device, and a systolic pressure (SYS cal ), diastolic pressure (DIA cal ), and mean arterial pressure (MAP cal ) from the waveform indicative of cuff pressure on the patient's extremity obtained during inflation.
4. A method according to claim 3 , further comprising using the processing component to determine a linear (or non-linear) regression equation between systolic pressure and mean arterial pressure at the time of calibration and a linear (or non-linear) regression equation between diastolic pressure and mean arterial pressure at the time of calibration:
DIA cal =R dia *MAP cal +B dia
and wherein where the intercept is negligible the ratio R sys and R dia are determined according to the equations:
R
sys
=
(
SYS
cal
MAP
cal
)
and
R
dia
=
(
DIA
cal
MAP
cal
)
.
5. A method according to claim 3 , further comprising using the processing component to determine a mean arterial pressure at a time t (MAP(t)) subsequent to determining the one or more coefficients which relate PWV to MAP according to the equation
MAP
(
t
)
=
K
·
(
1
PAT
(
t
)
-
1
PAT
cal
)
+
MAP
cal
wherein K is L t /a.
6. A method according to claim 5 , further comprising using the processing component to determine a systolic arterial pressure at a time t (SYS(t)) and a diastolic arterial pressure at a time t (DIA(t)) according to the equations
SYS( t )= R sys ·MAP( t )+ B sys
and
DIA( t )= R dia ·MAP( t )+ B dia