For more than a century taking patients' blood pressure has been a key part of medical diagnosis and the tracking of disease progression. So it's quite a shock to learn existing measurements have much greater variation, depending on where they are taken on the body, than has been assumed. Billions of measurements taken over that time may not have been as accurate as the doctors taking them thought.
Medical staff are well aware blood pressure readings can jump around from moment to moment. Consequently, anomalous results will often be retaken. However, pressure has generally been assumed to be fairly consistent throughout the body, with the upper arm chosen most often as a matter of convenience.
Where precise blood pressure measurements are required catheters are inserted into the arteries. However, this is invasive, and it has been thought pressure cuffs give the same result to within 10 points – close enough for most purposes.
Professor DaiWai Olson and Kathrina Siaron of the University of Texas South-Western Medical Center have thrown this belief into doubt. In Scientific Reports they describe testing 80 patients attending a neuroscience intensive care unit (NSICU) for serious neurological conditions, such as having brain tumors or having suffered a stroke. "For our patients in the NSICU, blood pressure often needs to be maintained in a very narrow range,” Siaron said in a statement.
The authors put cuffs on both upper arms at once and connected them to different machines. The average difference between the left and right arm was 8 millimeters of mercury for systolic pressure (the higher figure quoted in readings, when the blood is contracting) and 6 for the diastolic (the pressure in the arteries when the heart rests between beats). When Olson and Siaron compared one upper arm with the opposite wrist the systolic difference jumped to 13 systolic, but the diastolic gap was only 5.
These are averages; in extreme cases, a patient's reading varied by 40 points depending on the site where the measurement was taken. The authors note this could drastically affect the treatments a patient received. "If we take pressure in one arm, a patient seems fine, but in the other arm, they're in a crisis," Olson said. The paper notes, “Even 5 mmHg ... is enough to make a difference when deciding to intervene or withhold treatment.”
Medical providers do sometimes check unexpected readings by conducting a second measurement on the other arm, but these findings suggest the practice might be justified far more often.
The authors acknowledge they don't know why results vary so much by location, and are planning a larger trial they hope will shed light on the topic, including using a sample of healthy people. They are unclear if one site is more reliable than others, and suggest it may be ideal to average measurements taken at several points, particularly among people whose conditions make blood pressure changes especially dangerous.