Blood pressure monitoring method and apparatus

A blood pressure monitoring system for the continuous external measurements of blood pressure in an underlying artery comprising,

• an external pressure transducer applied to a subject at a location overlying an artery, said pressure transducer including a pressure sensitive element having at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured and substantially centrally positioned over the underlying artery, said pressure sensitive element producing a continuous waveform having an amplitude which is a function of blood pressure in the underlying artery,
• means for applying a variable hold-down pressure to the transducer, the hold-down pressure being variable over a range of at least from where the underlying artery is substantially unflattened to where the artery is substantially occluded,
• means responsive to the waveform for obtaining a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude values over a range of hold-down pressures,
• means for fitting a polynomial to a set of at least one of the diastolic pressure, systolic pressure, and pulse amplitude versus hold-down pressure values to obtain coefficients of the polynomial,
• means employing the polynomial for obtaining an indication of the correct hold-down pressure required for making accurate blood pressure measurements, and
• means for maintaining the transducer hold-down pressure at substantially the indicated correct hold-down pressure for obtaining accurate blood pressure measurements from the transducer.

A machine-implemented method of obtaining a measure of the compliance of a subject's artery underlying an external pressure transducer that includes a pressure sensitive element applied to the subject using a variable hold-down pressure, the method comprising the steps of,

• (1) obtaining a continuous measurement of blood pressure with the external pressure transducer while changing the hold-down pressure on the transducer over a range from where the underlying artery is unflattened to where the artery is substantially occluded,
• (2) obtaining from the blood pressure measurement of step 1 a set of at least one of the diastolic and systolic pressure versus hold-down pressure values over said range of hold-down pressures,
• (3) fitting a polynomial of at least third-order to a set of values obtained in step 2,
• (4) determining the slope of a straight line fitted to a sub-set of values obtained in step 2 over a range of hold-down pressures below which flattening of the underlying artery occurs, and
• (5) determining the minimum slope of the graph of the polynomial fitted to said set of values, the ratio of slopes determined in steps 5 and 4 providing a measure of compliance of the underlying artery.

A blood pressure monitoring system for continuous external measurement of blood pressure in an underlying artery and for obtaining a measure of compliance of said underlying artery comprising

• a pressure transducer that includes a pressure sensitive element having at least one dimension smaller than the lumen of the underlying artery and substantially centrally positioned over the artery, said pressure sensitive element producing a continuous waveform having an amplitude which is a function of blood pressure in the underlying artery,
• means for applying a variable hold-down pressure to the transducer,
• means responsive to the waveform for obtaining a set of diastolic pressure values, a set of systolic pressure values, and a set of pulse amplitude values over a range of hold-down pressures,
• means for fitting a polynomial of at least third-order to a set of at least one of the diastolic pressure and systolic pressure versus hold-down pressure values and for fitting a polynomial of at least second-order to said pulse amplitude versus hold-down pressure values,
• means for identifying the lowest hold-down pressure at which the pulse amplitude substantially equals sixty percent of the maximum pulse amplitude on the graph of the pulse amplitude versus hold-down pressure polynomial,
• means for determining the slope of a straight line fitted to a sub-set of at least one of the diastolic pressure and systolic pressure values over a range of hold-down pressures below said hold-down pressure identified as the lowest hold-down pressure at which the pulse amplitude substantially equals sixty percent of the maximum, and
• means for determining the minimum slope of the graph of at least one of the polynomials of diastolic pressure and systolic pressure versus hold-down pressure values, the ratio of said minimum slope to the slope of said straight line providing a measure of compliance of the underlying artery.