By: Thomas Pickering, MD, DPhil, FRCP, Director of Integrative and Behavioral Cardiology Program
of the Cardiovascular Institute at Mount Sinai School of Medicine, New York.
It is usually said that the blood pressure is the same in the two arms, although some studies have reported that there may be differences, with the pressure in the right arm tending to be higher than in the left. One question that has not been settled is the extent to which these differences may be due to the fact that the blood pressure is constantly changing, and another is whether the Korotkoff sounds (the sounds heard through a stethoscope when blood pressure is being measured) are harder to hear in one arm than in the other. A study performed in Ireland has looked to see if people who have blood pressure differences between the two arms when pressure is measured in the conventional way using a cuff and stethoscope also show these differences during 24 hour ambulatory monitoring.
The investigators identified 10 hypertensive people, all of whom were right handed, who showed a persistent difference of at least 10 mm Hg between the two arms (in each case higher on the right) when the pressure was measured with a mercury sphygmomanometer. The average blood pressures were 170/102 mmHg in the right arm and 154/96 in the left arm. All 10 agreed to wear two ambulatory blood pressure monitors, one on each arm, for 24 hours, and the monitors took a reading every 20 minutes during this time.
In five of the subjects, the blood pressure differences between the arms were less than 5 mmHg when the pressure was taken by the monitors, but in the other five, it was greater than 5 mmHg. For all 10 subjects, the average 24-
hour pressures were 145/84 in the right arm and 139/81 in the left.
This study makes several points. First, the differences between the arms found when blood pressure is taken by a mercury sphygmomanometer become much smaller or disappear when the pressure is taken with automatic devices which work on a different principle. Both ambulatory and digital home monitors use the oscillometric method, which does not require the detection of the Korotkoff sounds.
Second, all the differences observed between the arms were bigger for systolic than for diastolic pressure. The height of the blood pressure wave in the arteries depends partly on the wave that is reflected from the periphery, in the same way that the waves on a pond are reflected from the edge. This reflection varies in different regions of the body and explains why the systolic pressure in the legs is higher than in the arms, and also why there may be differences between the arms.
Third, clinic blood pressure is typically taken in the right arm, whereas ambulatory blood pressure (and home blood pressure) readings are often taken in the left arm, so in some people the blood pressure difference between the two arms may be one reason for the difference in the readings taken by the different techniques.
Where it was published
JC O’Shea and MB Murphy. Ambulatory blood pressure monitoring: which arm? Journal of Human Hypertension 2000; 14: 227.