Measuring Blood Pressure
Blood pressure is commonly measured in the seated or supine position; however, the two positions give different measurement values. With that in mind, any time a value is recorded, body position should also be recorded. It is widely accepted that diastolic pressures while sitting are higher than when a patient is supine by as much as 5 mmHg. When the arm is at the level of the heart, systolic pressure can be 8 mmHg higher, such as when a patient is in the supine position rather than sitting. A patient supporting their own arm (isometric exercise) may increase the pressure readings. If the patient’s back is not supported (i.e., when a patient is seated on an exam table instead of a chair) the diastolic pressure may be increased by 6 mmHg. Crossing the legs also may raise systolic pressure by 2-8 mmHg. Arm position plays a dramatic role in value errors as well. If the arm is below the level of the heart, values will be too high; if the arm is above the level of the heart, values will be underestimated. For every inch the arm is above or below the level of the heart, a 2 mmHg difference will be found (Pickering et al. Circ 2005;111:697-716).
Differences in bilateral measurements
Almost all of the studies evaluating blood pressure values bilaterally have demonstrated differences between the two values in a fair percentage of patients. It is not clear why this occurs, and hand dominance (i.e., left vs. right handedness) does not seem to play a role. Approximately 20% of patients will have differences of >10 mmHg between sides. When the difference in values is greater than 10 mmHg, other secondary causes for this variation should be investigated. These can include, but are certainly not limited to, coarctation of the aorta (i.e., narrowing of the aorta), congenital obstruction of the aorta, and upper extremity occlusion (Pickering et al. Circ 2005;111:697-716).
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