High blood pressure, or hypertension, is the top risk factor for premature death, associated with heart disease, strokes and heart attacks. However, inaccuracies in the most common form of blood pressure measurement mean that as many as 30% of cases of high blood pressure could be missed.

The researchers, from the University of Cambridge, built an experimental model that explained the physics behind these inaccuracies and provided a better understanding of the mechanics of cuff-based blood pressure readings.

The researchers say that some straightforward changes, which don't necessarily involve replacing standard cuff-based measurement, could lead to more accurate blood pressure readings and better results for patients. Their results are reported in the journal PNAS Nexus.

Anyone who has ever had their blood pressure taken will be familiar with the cuff-based method. This type of measurement, also known as the auscultatory method, relies on inflating a cuff around the upper arm to the point where it cuts off blood flow to the lower arm, and then a clinician listens for tapping sounds in the arm through a stethoscope while the cuff is slowly deflated.

Blood pressure is inferred from readings taken from a pressure gauge attached to the deflating cuff. Blood pressure is given as two separate numbers: a maximum (systolic) and a minimum (diastolic) pressure. A blood pressure reading of 120/80 is considered 'ideal'.

"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 which 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.

The research was supported by the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI).