Ask the experts
To what extent do blood pressure measurements vary depending on position, i.e., sitting, standing, or lying down? Also, if the arm is allowed to hang down vertically instead of resting horizontally, what effect does this have on the blood pressure measurements? Finally, what effects do any of these variations have on the prescribed treatments for high blood pressure?
When the blood pressure (BP) is measured, the arm should be at the level of the heart. In other words, in the sitting and standing positions, the arm should be extended out straight and should be about 2-3 inches below the shoulder to approximate the level of the heart. If the arm is allowed to hang down straight, the BP may be falsely diminished by as much as 12 mm Hg. below its true value. This is not an issue when the person is lying down, as long as the arm is kept along side at the level of the body.
Normally, properly taken BP measurements show only minor variations with changes in position. Certain patients with an imbalance of the autonomic nervous system (ANS), however, may tend to have a fall in BP with the upright position. The ANS normally controls constriction (narrowing) and dilation (widening) of blood vessels and so helps regulate a person's BP. Such a fall in BP with standing is called postural hypotension (low blood pressure), which, when significant, can cause weakness, lightheadedness, and even fainting. The treatment of high blood pressure (hypertension) with medications in these patients may actually lead to an excessive fall in BP with standing. The doctor might, therefore, prescribe more limited treatment of hypertension for these patients.