According to a new study, commonly used arm positions — lap and side — resulted in statistically significant higher blood pressure (BP) readings than desk-supported positions. When the patient held the arm in their lap or unsupported at their side, said the authors, there was a greater likelihood of overestimation or misdiagnosis of hypertension.

The trial involved 133 participants with a mean age of 57 who were evaluated for three arm positions — supported on a lap, supported on a desk, and unsupported with the patient’s arm hanging at their side. When the patient held their arm in their lap, readings overestimated systolic blood pressure (SBP) by 3.9 mm HG and diastolic blood pressure (DBP) by 4.0 mm Hg.  An unsupported arm overestimated systolic pressure by 6.5 mm Hg and diastolic by 4.4 mm Hg. Measurements were taken between 9 AM and 6 PM using a validated oscillometric BP device. Staff conducting measurements received standardized training and completed a BP measurement certification test. All readings were conducted in a private, quiet space.

There are several physiological mechanisms that indicate why positioning impacts BP measurements. For one, said the authors, the “vertical distance between the heart and the cuff increases when the arm is positioned in the lap or at the side.” At the same time, lower arm positions result in decreased venous return and compensatory vasoconstriction, which leads to higher BP readings. Finally, unsupported arms can lead to muscle contractions, which can cause transient BP increases.

The authors observed, “Although the error in SBP with the arm supported in the lap was less striking in magnitude than when the arm was unsupported at the side, BP readings in either position were sufficiently high to raise concerns for overdiagnosis and overtreatment.” They added that their findings were more extreme among higher-risk groups.

The statistically significant BP measurement errors detected in this study were consistent with prior research. For instance, previous studies have shown unsupported arms or arms supported below heart level can overestimate SBP by 4–23 mm HG and DBP by 3–12 mm Hg.

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