Why blood pressure cuffs may show readings that are too low

An overlooked effect from compressing an artery may help explain why a standard blood pressure cuff can sometimes understate the top number. Researchers found that reduced pressure beyond the cuff can delay the artery from reopening, causing the reading to appear lower than it should.

This matters because blood pressure readings guide diagnosis and treatment. If the result is too low, high blood pressure may be missed, while identifying the source of the error could help improve the method without necessarily replacing the device.

Engineers at the University of Cambridge examined the problem using a physical model designed to behave more like a real artery. Their study, published in PNAS Nexus, points to a possible mechanism and suggests changes that still require clinical testing.

In the usual approach, an inflatable cuff is wrapped around the upper arm and briefly stops blood flow. As the cuff slowly releases pressure, a clinician listens for tapping sounds with a stethoscope.

The first tapping sounds are used to estimate systolic pressure, the top number in a blood pressure reading. This is the pressure created when the heart contracts, and measuring it accurately is important for detecting high blood pressure.

Scientists have long known that cuff measurements can differ from the pressure measured directly inside an artery. One unresolved question was why the first audible sounds may not appear until later than expected as cuff pressure drops.

Earlier laboratory models often relied on rubber tubes that stayed partly open under pressure. The Cambridge team instead used tubes that lay flat and could close fully, making them more like a compressed artery.

The experiments showed that stopping blood flow also lowers the pressure in vessels downstream from the cuff. That changes the conditions needed for the compressed section to reopen.

As a result, the artery may remain closed until cuff pressure falls below the true systolic pressure. By the time the tapping sounds begin, the cuff reading is already too low, which helps explain one source of underestimation.

The researchers suggested that making downstream pressure more predictable could improve accuracy. One idea they proposed for further study was raising the arm before measurement, though this is not yet an established change in routine care.

More patient research is needed to see how much this effect varies and which adjustments work best. The study also focused on the listening-based method, so it does not show how large the error may be for every automatic home monitor.

The work offers a physics-based explanation for a familiar test. If patient studies confirm how best to account for the effect, a relatively simple change in measurement or calibration could help blood pressure readings reflect what is happening inside the arteries more accurately.

Source: University of Cambridge.