The most common method of measuring blood pressure is often inaccurate, a new study has found. This could mean people at risk of serious conditions such as heart disease are missing diagnosis and potentially life-saving treatment.
The so-called “cuff method” involves strapping an inflatable cuff over the upper arm to temporarily cut off the blood supply; then calculating the blood pressure once the cuff is relaxed.
In our study, published in the Journal of the American College of Cardiology, we found the method, which is more than a century old, is inaccurate when monitoring people with mid-range blood pressure. This is the range most common among people worldwide.
Accurate measurement of blood pressure is regarded among the most important of all medical tests. A misdiagnosis of low blood pressure can be a missed opportunity for lowering a person’s risk of cardiovascular disease, which often presents as a stroke, heart attack or kidney disease. A misdiagnosis of high blood pressure, on the other hand, could lead to people being prescribed unnecessary medication.
What is blood pressure?
Blood pressure is the force exerted in the large arteries – vessels that carry blood away from the heart – with every heartbeat. Blood pressure measurement provides a high (systolic) and a low (diastolic) value. The high value represents the peak pressure during heart contraction; the low value represents the pressure during heart relaxation.
Healthy levels of blood pressure are typically less than 120/80 mmHg (the 120 mmHg is systolic, and 80 mmHg diastolic). Decades of research clearly tell us if a person’s blood pressure is raised they are at higher risk of cardiovascular disease. The higher the blood pressure, the higher the risk.
About one in three adults have high blood pressure. Lifestyle factors such as regular exercise, normal body weight and healthy dietary choices, as well as medications, can lower blood pressure and prevent cardiovascular disease.
Although there are many factors to consider when assessing if someone has high blood pressure, the conventional threshold at which doctors might consider giving medication to lower pressure is 140/90 mmHg.
How is blood pressure measured?
The method to measure blood pressure is based on a technique invented in 1896, then refined in 1905, but the basic principal has remained virtually unchanged.
A broad cuff is placed over the upper arm and inflated until the main artery in the arm is completely occluded and blood flow is stopped. The cuff is then slowly deflated until blood flow returns into the lower arm.
A series of signals can then be measured that represent the systolic and diastolic blood pressure. These are measured by either listening with a stethoscope or, more often, using automated devices.
It’s uncertain whether cuff blood pressure accurately measures the pressure in the arteries of the arm or the major artery just outside the heart, called the aorta. This is important as blood pressure readings can be different in these two spots – a potential difference of 25 mmHg or more.
The central aorta blood pressure is a better indicator of the pressure experienced by organs, such as the heart and brain, so it is more clinically relevant.
The possibility of big blood pressure differences between the arm and the aorta could result in very different clinical decisions on diagnosis and treatment. So it is important to resolve the uncertainty as to what cuff blood pressure actually measures.
We retrieved data from studies from the 1950s until now that compared cuff blood pressure of more than 2,500 people with that of the gold standard method, called invasive blood pressure. Here, a catheter that measures pressure is inserted inside the artery either at the arm (same site as the cuff) or at the aorta.
Readings from this method were used as a reference and compared with those of the cuff method to determine the accuracy of cuff measurements.
What did we find?
Cuff blood pressure had reasonable accuracy compared with the reference standard, at either the arm or aorta, among people with low cuff blood pressure (lower than 120/80 mmHg) and high cuff blood pressure (the same or higher than 160/100 mmHg). These people are at the extreme ends of the blood pressure risk spectrum.
We found the accuracy when compared to invasive blood pressure was up to 80%.
But for the rest of the population with blood pressure in the middle range – systolic 120 to 159, and diastolic 80 to 99 mmHg – accuracy compared with invasive blood pressure at the arm or the aorta was quite low: only 50% to 57%.
Why is this important?
If people have their blood pressure measured using the cuff method and the values are either low (under 120/80 mmHg) or high (over 160/100 mmHg), we can have reasonable confidence the values are a good representation of the true (invasive) blood pressure.
But for people whose blood pressure is in the most common mid-range of 120 to 160 mmHg systolic or 80 to 100 mmHg diastolic, there is much less certainty as to whether the cuff blood pressure is truly representative of the actual blood pressure.
Our findings do not mean people should stop taking their medication or stop having their blood pressure measured using the cuff device. While this study reveals accuracy issues, the evidence from many large clinical trials clearly shows taking medication to lower blood pressure from high levels reduces the chances of stroke, heart attack and vascular disease.
Cuff blood pressure measurements are still useful, but we could help more people if we could measure blood pressure more accurately. The problem is that some people in the mid blood pressure range may fall through the diagnosis cracks.
Until the accuracy standards of pressure-measuring devices are improved, the best available confirmation of blood pressure levels comes from an average of many repeated measures over time. This is better than one or two measures, as is often the way in busy daily clinical practice, and was closest to the method examined in this study.
People can have repeated measures of blood pressure undertaken in consultation with their general practitioners or at specialist centres. These can include self-measured home blood pressure, 24-hour ambulatory blood pressure and automated unobserved blood pressure.