Method for determining cuff blood pressure
Most automatic cuff blood pressure (BP) measurement devices are based on oscillometry. These devices estimate BP from the envelopes of the cuff pressure oscillations using fixed ratios. The values of the fixed ratios represent population averages, so the devices may be accurate only in subjects with normal BP levels. A patient-specific oscillometric BP measurement method was developed. The idea was to represent the cuff pressure oscillation envelopes with a physiologic model and then estimate the patient-specific parameters of the model, which includes BP levels, by optimally fitting it to the envelopes.
1. A method for determining mean blood pressure fora subject, comprising:
measuring cuff pressure using an automatic cuff device during a blood pressure measure of the subject;
deriving an oscillogram from the measured cuff pressure, where the oscillogram is an amplitude of oscillations in the measured cuff pressure as a function of the measured cuff pressure;
representing the oscillogram with a mathematical model, wherein the mathematical model is defined in terms of parameters with unknown values, the parameters indicating systolic pressure and diastolic pressure and specifying a nonlinear blood volume-transmural pressure relationship of the artery underneath a cuff of the automatic cuff device;
estimating the parameters of the mathematical model by fitting the mathematical model to the oscillogram while constraining the derivative of the blood volume-transmural pressure relationship in the mathematical model, with respect to transmural pressure, with a maximum near zero and right skewed about the maximum;
constructing a blood volume waveform for the subject to within a scale factor, where the blood volume waveform is constructed from the oscillogram and the estimated mathematical model;
determining a blood pressure waveform for the subject by applying the constructed blood volume waveform and the measured cuff pressure to the estimated nonlinear blood volume-transmural pressure relationship; and
computing a mean blood pressure for the subject from the determined blood pressure waveform.
2. The method of claim 1 wherein the mathematical model is further defined as
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where t is time; P c oa (t) is amplitude of oscillations in the measured cuff pressure; P c (t) is the measure cuff pressure; SP is systolic pressure; DP is diastolic pressure; and a, b, c, and e characterize the nonlinear blood volume-transmural pressure relationship of the subject.
3. The method of claim 2 further comprises estimating the parameters of the mathematical relationship by least squares fitting of both sides of the equation to each other.
4. The method of claim 2 further comprises estimating the parameters of the mathematical model by setting a to 0-3 mmHg and constraining value of b for each value of c such that derivative of the blood volume-transmural pressure relationship with respect to transmural pressure is right skewed by 30-50 percent about its peak.
5. The method of claim 1 wherein constructing a blood volume waveform further comprises:
subtracting a lower envelope of the cuff pressure oscillations as a function of cuff pressure from the cuff pressure oscillations as a function of cuff pressure to yield a waveform with positive amplitude oscillations; and
summing the waveform with positive amplitude oscillations with the estimated nonlinear blood volume-transmural pressure relationship evaluated at estimated diastolic pressure.
6. The method of claim 1 wherein determining a blood pressure waveform for the subject further comprises deriving the blood pressure waveform by finding the root of the estimated nonlinear blood volume-transmural pressure relationship at different points in time using the constructed blood volume waveform and measured cuff pressure.
7. The method of claim 1 further comprises computing the mean blood pressure by taking a time average of the determined blood pressure waveform.
8. The method of claim 1 wherein the automatic cuff device is further defined as a sphygmomanometer.
9. An apparatus for determining blood pressure for a subject, comprising:
a blood pressure monitor configured to measure an oscillometric waveform of cuff pressure during a measure of blood pressure;
a non-transitory data store for storing a mathematical model, where the mathematical model is defined in terms of parameters with unknown values, the parameters indicating systolic pressure and diastolic pressure and specifying a nonlinear blood volume-transmural pressure relationship of the artery underneath a cuff of the blood pressure monitor; and
a signal processor configured to receive the oscillometric waveform of cuff pressure and from the blood pressure monitor, derive an oscillogram from the oscillometric waveform and estimate the parameters of the mathematical model by fitting the mathematical model to the oscillogram while constraining the derivative of the blood volume-transmural pressure relationship in the mathematical model, with respect to transmural pressure, with a maximum near zero and right skewed about the maximum, wherein the signal processor further operates to construct a blood volume waveform for the subject to within a scale factor, determine a blood pressure waveform for the subject by applying the constructed blood volume waveform and the oscillometric waveform to the nonlinear blood volume-transmural pressure relationship; and computes a mean blood pressure for the subject from the determined blood pressure waveform.
10. The apparatus of claim 9 wherein the mathematical model is further defined as
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where t is time; P c oa (t) is amplitude of oscillations in the measured cuff pressure; P c (t) is the measure cuff pressure; SP is systolic pressure; DP is diastolic pressure; and a, b, c, and e characterize the nonlinear blood volume-transmural pressure relationship of the subject.
11. The apparatus of claim 9 wherein the blood volume waveform is constructed by:
subtracting a lower envelope of the cuff pressure oscillations as a function of cuff pressure from the cuff pressure oscillations as a function of cuff pressure to yield a waveform with positive amplitude oscillations; and
summing the waveform with positive amplitude oscillations with the estimated nonlinear blood volume-transmural pressure relationship evaluated at estimated diastolic pressure.
12. The method of claim 9 wherein the blood pressure waveform is determined by finding the root of the estimated nonlinear blood volume-transmural pressure relationship at different points in time using the constructed blood volume waveform and measured cuff pressure.
13. The apparatus of claim 9 wherein the blood pressure monitor is further defined as a sphygmomanometer.