Better calibration for cuff-based blood pressure readings

A study explains why cuff-based blood pressure readings systematically underestimate systolic blood pressure. High blood pressure is the most important risk factor for premature death. Yet the gold standard method for measuring blood pressure, the inflatable cuff, is known to systematically underestimate systolic (maximum) blood pressure and overestimate diastolic (minimum) blood pressure. To measure blood pressure, a cuff is placed around the upper arm and inflated to constrict the brachial artery, collapsing it. The pressure is then slowly released, until blood begins to flow through the artery again. With each blood pressure pulse, the blood pressure briefly exceeds the cuff pressure, and the artery opens, producing a tapping sound heard by the clinician. The cuff pressure continues to decrease, until the blood pressure exceeds the cuff pressure throughout the whole cardiac cycle. The artery is then fully open, and the tapping sounds disappear. The cuff pressures at which the tapping sounds appear and disappear are noted as the systolic and diastolic pressures, respectively. The reason for the overestimation of the minimum pressure is well understood: to fully occlude the artery, the cuff needs a little extra pressure to counteract the tissue around the artery and the stiffness of arterial walls. But the reason for the underestimation of systolic pressure has been unknown. Kate Bassil and Anurag Agarwal use a novel experimental rig that more accurately reproduces the behavior of the human circulatory system than previous rigs. The authors show that when the artery is closed, the blood pressure downstream of the cuff drops, as pressure equalizes in the isolated vasculature of the arm and hand. The cuff does not apply even pressure, and this, in combination with the low downstream pressure, changes the length of the closed section of artery under the cuff. The artery opens from the upstream side to the downstream side, but the detectable signal won’t be present until the whole length is open—and the longer it takes to open, the lower the pressure readings will have fallen before the clinician notes it. In sum, the lower the downstream pressure, the longer the section of artery that closes, the more time the artery takes to reopen, and the greater the underestimation of the systolic blood pressure. According to the authors, the study could inform improved calibration methods or refinements to the cuff protocol.

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