Q: My doctor usually checks my blood pressure in my left arm. When I went for my checkup yesterday, he used my right arm, and the reading was 10 points higher than usual. I asked him to check my left arm, and it was 8 points lower. Does the difference matter? And which is my real pressure?
A: Blood pressure readings vary from day to day, even minute to minute. They also vary from arm to arm, even when both arms are checked simultaneously. The difference is usually just a few millimeters of mercury (mm Hg, the standard unit for measuring blood pressure), but various studies report that 10 percent to 39 percent of people have a disparity of 10 mm Hg or more. More often than not, the readings are higher in the right arm than the left. Your blood pressure readings put you in good company, and your right-left disparity is nothing to worry about.
Unlike you, a few people have much larger differences, perhaps 40 mg Hg or more. Such large gaps are a clue to blockages in the subclavian artery or other blood vessels, so they call for investigation.
Like other experts, the American Heart Association suggests that doctors take blood pressure readings in both arms when they first examine a patient. If there is a difference, the arm with the higher reading should be used. But researchers conducting a study of 147 patients took nine readings in both arms, then repeated the procedure when 91 of the patients returned for a second visit. On average, the reading in the right arm was 2 to 3 mm Hg higher, but the higher reading shifted from arm to arm in many patients. The disparity in systolic blood pressure exceeded 10 mm Hg in 15 percent of the patients.
If you have your pressure checked in both arms, your doctor should use the higher reading to classify you as normal (120/80 or lower), prehypertensive (121-139/81-89), or hypertensive (140/90 or higher). If your systolic pressure (the higher number, recorded when your heart is pumping blood to your arteries) and diastolic pressure (the lower number, recorded when your heart is refilling with blood between beats) would put you in different categories, use the higher category as your diagnosis.
In blood pressure – unlike politics – the issue is not right or left, much less right or wrong; what really matters is up or down.
Dr. Harvey B. Simon, Editor, Harvard Men’s Health Watch
