Monday February 21 2011
Wrist-worn blood pressure monitors are not new
A watch-like device “could revolutionise blood pressure monitoring”, BBC News has reported. According to the website, the monitor can be used to measure pressure in the wrist, which can then be used to estimate pressure in the aorta, the largest artery in the body.
Although news coverage has focussed on the wrist-worn monitor, the research devised a technique to combine blood pressure readings from the wrist and upper-arm to estimate central aortic systolic pressure (CASP). This measure of pressure in the aorta is thought to be a better way of predicting heart problems than traditional measures of blood pressure, such as using an inflatable cuff around the bicep.
A device to measure blood pressure at the wrist is not new, and the method does not replace the traditional approach of using a cuff on the upper arm. However, the researchers’ method for combining the two results to estimate CASP appears to have some merit, and may filter into medical care.
Where did the story come from?
The study was carried out by researchers from the University of Leicester, the National Institute for Health Research, Gleneagles Medical Centre in Singapore and Healthstats International in Singapore. The study was financially supported by the Leicester National Institute for Health Research Biomedical Research Unit in Cardiovascular Diseases. The study was published in the peer-reviewed Journal of the American College of Cardiology.
What kind of research was this?
Blood pressure has been measured through a blood vessel in the upper arm – the brachial artery – for many years. However, there is a current debate about whether the blood pressure measured in the arm accurately represents the corresponding pressure in the aorta, the large blood vessel that carries oxygenated blood away from the heart. Recent evidence suggests that central aortic systolic pressure (CASP), the pressure exerted as blood is pumped out of the heart and into the aorta, is a better predictor of structural damage to the heart and blood vessels.
In this study, researchers tested a mathematical algorithm to determine whether it could accurately estimate CASP using measurements of pressure in the radial artery at the wrist. The brachial artery in the upper arm divides at around the elbow, and the radial artery is one of the main branches of this, supplying blood to the forearm, wrist and hand.
The best way to measure CASP is to insert a pressure sensor into the aorta, but this is invasive and is usually only done when people undergo a procedure known as cardiac catheterisation. The procedure involves making an incision into the groin or upper arm to gain access to the arterial system, then feeding a guide wire and sheath through the arterial system so that a tiny pressure sensor can be positioned into the aorta or heart.
There are other ways to estimate CASP, such as reading the pressure in the radial artery and applying mathematical functions, called generalised transfer functions. Although this method is widely used, the application of generalised transfer functions has been criticised. In this study, researchers explored a different mathematical model to estimate CASP from the radial pressure.
This study had three separate parts, each involving a different group of participants. In the first, the researchers tested some key mathematical properties of their model. In the second, they compared their new way of estimating CASP with a well-known, accepted mathematical method. In the final part, they compared their non-invasive estimates of CASP with measures taken with cardiac catheterisation in people undergoing surgery.
What did the research involve?
The researchers tested the application of a mathematical approach called the n-point moving average. This is commonly used in other fields of study to help filter data and find underlying trends.
With each beat the heart contracts and relaxes, resulting in fluctuations in blood pressure over short periods of time. In their first experiment the researchers needed to determine how many tiny measurements of radial pressure their model would need to make within the cycle of a heartbeat. They enrolled 217 volunteers to help with this aspect of their model development.
In the second study, they used blood pressure readings taken as part of a large study that ran in Leicester over the course of five years. From this, they had 5,349 individual blood pressure readings to validate their new approach to calculating the central aortic pressure.
The final part of the experiment included 20 adults undergoing routine diagnostic cardiac catheterisation at the Gleneagles Medical Centre in Singapore. Their CASP was measured near the aortic valve directly in the heart. At the same time, a device was attached to their wrists to measure the radial pressure and a device was placed over the bicep of the same arm to measure the brachial pressure. The researchers were able to compare the measures of CASP from their model with the direct CASP measures in real time for up to three minutes.
What were the basic results?
The researchers determined the best structure for their model in the first group of volunteers. This structure led to estimates of CASP that closely approximated those derived through the more standard CASP estimation models. The accuracy of their estimates was not affected by age, gender, presence of diabetes or hypertension treatment in the participants.
In the invasive blood pressure experiment, brachial blood pressure (the standard measure in the upper arm) overestimated blood pressure compared to direct measurement of pressure in the aorta. There was strong correlation and agreement between direct measures of CASP and those estimated by the researchers’ mathematical model.
How did the researchers interpret the results?
The researchers concluded that their study demonstrates that a “simple moving average method” can be applied to brachial blood pressure measured across the wrist to estimate CASP.
Conclusion
This complex study involved the application of mathematical approaches to derive a measure of CASP from both radial artery pressure and brachial artery pressure. The researchers have developed a mathematical algorithm that appears to accurately predict CASP.
In commenting that a sensor strapped to the wrist that can “measure the pressure in the aorta”, The Independent misunderstood the mechanics of measures of radial pressure. The researchers measured pressure at both the wrist and at the upper arm and use mathematical approaches to convert them into an estimate of the pressure in the aorta.
The HealthSTATS sensor mentioned in news coverage is certainly not the first device of its kind, and there are several monitors which can be strapped to the wrist to measure radial pressure. However, it seems that the study’s technique of combining it with a traditional upper-arm cuff readings to create an estimate of CASP has some merit. The researchers make it very clear that this technology does not replace the traditional inflatable cuff, and that both methods are needed.
Past research suggests that CASP is a better marker of cardiovascular problems than upper-arm blood pressure readings. As such, more accurate measures of it, such as that allowed through this new approach, are likely to play a growing role in clinical practice
The study claimed non-smokers were more likely to be obese 
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