Underestimation of systolic pressure in cuff-based blood pressure measurement.

High blood pressure (hypertension) is the number one risk factor for premature death. Hypertension is asymptomatic, so blood pressure must be regularly monitored to diagnose it. In auscultatory blood pressure measurement, a patient's systolic (maximum) and diastolic (minimum) blood pressure are inferred from the pressure in an inflatable cuff wrapped around the arm. This technique is the gold standard against which all other noninvasive devices are validated. However, auscultatory measurements systematically underestimate systolic blood pressure and overestimate diastolic blood pressure. Overestimation is attributed to the increased cuff pressure needed to occlude the artery because of the surrounding tissue and arterial stiffness. In contrast, the cause of systolic underestimation, which leads to potentially a third of systolic hypertension cases being missed, has remained unclear. When the cuff is inflated beyond the systolic blood pressure, the blood flow to the vessels downstream of the cuff is cut off. The pressure in these downstream vessels drops to a low plateau. We have developed a novel experimental rig that shows that the low downstream pressure is the key cause of the underestimation of systolic blood pressure. The lower the downstream pressure, the greater the underestimation. Our results yield a simple physical model for the underestimation of systolic pressure in our rig and in the human body. Understanding the physics behind the underestimation of systolic blood pressure paves the way for developing strategies to mitigate this error.