Comparison of a novel patented method of measuring Blood pressure (Plethysmometry) with Sphygmomanometry

Background

We have developed a patented method that acquires photoplethysmograms (PPG) at multiple cuff-pressure levels to estimate Blood pressure (BP). The method is referred to as plethysmometry. Validation of new methods for measuring BP involves comparison with Sphygmomanometry as standard.

Objective

To compare the new method of BP measurement (plethysmometry) with Sphygmomanometry guided by international protocols for validation and to evaluate diagnostic agreement for hypertension (HT).

Methods

Ninety-two adults underwent four video/audio-documented sphygmomanometric readings (two before and two after plethysmometry). The lowest and highest of the four estimates were taken as lower and upper limits of systolic and diastolic BP reported by sphygmomanometry. The values were averaged to get average systolic and diastolic pressures for each individual.

Plethysmometry involves recording of 3 signals; cuff-pressure, and PPG from fingers of both cuffed and uncuffed arms. A brief ramp-inflation of cuff-pressure to the point at which the cuffed-arm pulse disappears provides a preliminary systolic estimate. 16 cuff-pressure steps (ranging from above the preliminary systolic estimate to 30 mmHg) were then applied, holding cuff-pressure at each level for 10 seconds. From the relationship of pulse amplitude to cuff-pressure, lower and upper limits of systolic and diastolic pressures were derived.

Average systolic and diastolic pressures and average pulse pressure were calculated. Mean arterial pressure (MAP) was obtained from the relationship between cuff-pressure and cuff-pressure oscillation amplitude. The average pressures from both methods were compared as per AAMI criterion-1 and BHS grading scheme.

A new scheme of BP classification with Plethysmometry was designed. Normative data was obtained from a subset of 77 individuals who did not have previous history of HT and whose sphygmomanometric pressures were below 140/90 mmHg. Based on the relationship of the systolic, diastolic, mean arterial and pulse pressures of the subject to the normative pressures, BP was classified as HT or normotension (NT).

Results

Plethysmometry gives two sets of arterial pressures, one proximal to the arm-cuff (closer to central pressures) and the other, in the arterial segment below or distal to the cuff (peripheral). Comparing average systolic and diastolic pressures between plethysmometry (central) and sphygmomanometry, bias(SD) was 1.18(6.66) mmHg(systolic) and -4.17(9.07) mmHg (diastolic); AAMI criterion-1 was met for central systolic but not for diastolic; criterion-2 was not applicable as there was only a single plethysmometric measurement per subject. BHS grading criteria also were met for central systolic but not for diastolic pressure. When HT diagnosis by plethysmometry with the new scheme (incorporating systolic, diastolic, mean arterial and pulse pressure) was compared with sphygmomanometry, sensitivity and specificity were more than 0.9.

Conclusions

Plethysmometry provides objective ranges of systolic and diastolic pressures and MAP from a single recording, verifiable by traces of recorded parameters. It is equivalent to multiple sphygmomanometric measurements done under ideal conditions in terms of high diagnostic accuracy for HT as well as agreement between systolic pressures. The consistent negative diastolic bias in plethysmometry may be due to the well-documented over-estimation of diastolic pressure by sphygmomanometry. Plethysmometry is a good tool for clinic BP assessment and can detect masked hypertension.