How Physics Revealed Life-Saving Blood Pressure Fix

Scientists have uncovered the reason why standard cuff-based blood pressure tests often produce inaccurate results and identified ways to make them more precise. Improving these measurements could have a major impact on patient health, the journal PNAS Nexus reported.

High blood pressure, or hypertension, is the leading cause of premature death worldwide and is strongly linked to heart disease, stroke, and heart attack. Yet flaws in the most widely used method of measuring blood pressure may result in as many as 30% of hypertension cases going undetected.

A research team from the University of Cambridge developed a specialized experimental model to examine the physics behind these errors. Their work revealed new insights into how cuff-based blood pressure devices function and why they can misread results.

According to the researchers, a few simple adjustments could make these tests far more reliable without replacing the standard devices altogether. The findings point to changes that could deliver more accurate readings and better outcomes for patients.

Most people have experienced the familiar process of having a blood pressure cuff wrapped around the upper arm. In the approach known as the auscultatory method, the cuff is inflated until it temporarily stops blood flow to the lower arm. A healthcare provider then listens through a stethoscope for specific tapping sounds as the cuff slowly releases pressure.

The measurement is taken from a pressure gauge connected to the cuff and is recorded as two numbers: systolic (the higher, maximum pressure) and diastolic (the lower, minimum pressure). A reading of 120/80 is generally considered to be in the ideal range.

“The auscultatory method is the gold standard, but it overestimates diastolic pressure, while systolic pressure is underestimated,” said co-author Kate Bassil from Cambridge’s Department of Engineering. “We have a good understanding of why diastolic pressure is overestimated, but why systolic pressure is underestimated has been a bit of a mystery.”

“Pretty much every clinician knows blood pressure readings are sometimes wrong, but no one could explain why they are being underestimated — there’s a real gap in understanding,” said co-author Professor Anurag Agarwal, also from Cambridge’s Department of Engineering.

Previous non-clinical studies into measurement inaccuracy used rubber tubes that did not fully replicate how arteries collapse under cuff pressure, which masked the underestimation effect.

The researchers built a simplified physical model to isolate and study the effects of downstream blood pressure — the blood pressure in the part of the arm below the cuff. When the cuff is inflated and blood flow to the lower arm is cut off, it creates a very low downstream pressure. By reproducing this condition in their experimental rig, they determined this pressure difference causes the artery to stay closed for longer while the cuff deflates, delaying the reopening and leading to an underestimation of blood pressure.

This physical mechanism — the delayed reopening due to low downstream pressure — is the likely cause of underestimation, a previously unidentified factor. “We are currently not adjusting for this error when diagnosing or prescribing treatments, which has been estimated to lead to as many as 30% of cases of systolic hypertension being missed,” said Bassil.

Instead of the rubber tubes used in earlier physical models of arteries, the Cambridge researchers used tubes that lay flat when deflated and fully close when the cuff pressure is inflated, the key condition for reproducing the low downstream pressure observed in the body.

The researchers say that there is a range of potential solutions to this underestimation, which include raising the arm in advance of measurement, potentially producing a predictable downstream pressure and therefore predictable underestimation. This change doesn’t require new devices, just a modified protocol.

“You might not even need new devices; just changing how the measurement is done could make it more accurate,” said Agarwal.

However, if new devices for monitoring blood pressure are developed, they might ask for additional inputs that correlate with downstream pressure, to adjust what the ‘ideal’ readings might be for each individual. These may include age, BMI, or tissue characteristics.

The researchers are hoping to secure funding for clinical trials to test their findings in patients, and are looking for industrial or research partners to help refine their calibration models and validate the effect in diverse populations. Collaboration with clinicians will also be essential to implement changes to clinical practice.

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