AN EXPLANATION FOR UNDERESTIMATION OF SYSTOLIC PRESSURE IN CUFF-BASED MEASUREMENT

Abstract

Objective: Auscultatory blood pressure measurement is the gold standard used to calibrate other measurement devices. However, auscultatory tests systematically underestimate systolic blood pressure (SBP) and overestimate diastolic blood pressure (DBP). While there is some literature explaining overestimation, there is no physical understanding of the underestimated SBP. Our objective is to understand why auscultatory measurement results in the underestimation of SBP. The SBP is recorded when the artery first begins to reopen, during cuff deflation. Prior to this, the cuff cuts off the blood supply and the pressure in the distal part of the artery drops to the venous pressure. We propose this low pressure as a cause of underestimation and investigate this through a controlled variation of the downstream pressure in an experimental rig. Design and method: Figure 1 shows a schematic of the rig. The tube representing the artery is compressed under an inflatable cuff. The downstream pressure is varied between tests. The upstream pressure is fixed with a header tank for simplicity. Therefore, the ‘blood pressure’ measured in the rig is constant rather than oscillatory, resulting in only a single opening of the artery as the cuff is deflated, rather than repeated opening and closing. This single opening is equivalent to the first opening of the artery in the body and so represents the systolic measurement. Figure 2 depicts an example test run. The cuff around the artery is gradually deflated, and the reopening of the artery is indicated by the rise of the downstream pressure (point a). The cuff pressure at this timestamp is recorded (point b), representing the measured SBP. Results: Figure 3 shows that the recorded cuff pressure is a function of downstream pressure, with the cuff reading giving greater underestimation of SBP when the downstream pressure is lowered. Conclusions: The delay in artery reopening which results from lowering the downstream pressure provides an explanation for the systematic underestimation of SBP. This information may lead to recommendations for new measurement protocol which avoid a drop in the pressure distal to the cuff, and the resulting underestimation of SBP.