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.
The diagnosis of high blood pressure requires accurate blood pressure measurement, and it is often not appreciated how much the position of
the arm can influence the readings. In a Dutch study, 69 hypertensive patients had their blood pressure measured with the arm in two positions. In the first (the arm down position) the arm was rested on the arm of an ordinary chair while the measurement was made. In the second (the arm up position) the upper arm was supported at the level of the heart (actually the right atrium of the heart, which is at the level halfway down the breast bone or sternum).
Blood pressure was measured in two ways: using a stethoscope and mercury sphygmomanometer, and with an automatic device.
Both methods of measurement gave the same differences between the two arm positions: when the arm was down (resting on the arm of the chair) the pressure was 7-10 mmHg higher than when it was up.
The official recommendation for blood pressure measurement from organizations such as the American Heart Association is that the cuff
should be at the level of the right atrium of the heart when the measurements
are made (the arm up position). The right atrium is the low pressure chamber which receives blood returning to the heart from the veins. The reason why
the arm position makes so much difference is that the circulation is basically
a column of blood, and the weight of the blood means that the pressure will be
greater at the bottom of the column than at the top (just as the water pressure
is greater at the bottom of the sea than at the surface).
What this means in practice is that the arm should not be resting on a tabletop or the arm of a chair without some support when blood pressure is being measured—a cushion or pillow put between the arm and the tabletop should get the arm to the right level. Unfortunately many doctors and other health care professionals measure the pressure with the arm too low, which may give falsely high readings.
Where it was published
RT Netea and colleagues. Arm position is important for blood pressure measurement. Journal of Human Hypertension 1999; 13:105.