Blood pressure (BP) is the force of circulating blood pushing against the walls of your arteries, measured as two numbers: systolic (when the heart beats) over diastolic (when the heart rests between beats). The universally accepted standard for taking a blood pressure measurement is with the patient seated comfortably, back supported, and feet flat on the floor. While the seated position is the routine standard, BP can absolutely be taken lying down, but this is typically done for specific medical purposes rather than a routine check.
The Necessity of Supine Measurement
Measuring blood pressure in the supine (lying flat on the back) position is not the default, but it is required in several clinical situations. The primary reason for a supine measurement is to assess for orthostatic hypotension, a condition where blood pressure drops significantly upon standing up. This assessment involves comparing BP readings taken while lying down, immediately upon standing, and after one to three minutes of standing to detect a drop of 20 mmHg or more in systolic pressure.
A supine measurement also serves to identify supine hypertension, which is high blood pressure detected only when the patient is lying flat. Recent research suggests that high BP when lying down can be a strong predictor of cardiovascular events, including stroke and heart failure, even if the seated BP is normal. For patients who are critically ill, immobilized, or undergoing certain diagnostic procedures, lying down may be the only feasible position for an accurate measurement, providing a baseline for their current circulatory status.
Physiological Impact of Posture on Readings
The difference in blood pressure between lying down and sitting or standing is a direct result of how the body manages gravity, a concept known as hydrostatic pressure. When a person is seated or standing, gravity pulls blood downward, causing a temporary pooling in the lower extremities. This pooling reduces the amount of blood returning to the heart, which triggers a reflex in the autonomic nervous system to constrict blood vessels and increase heart rate to maintain blood pressure to the brain.
When a person lies down, the effects of gravity are minimized because the heart, the blood vessels in the arm, and the measurement cuff are all on a relatively horizontal plane. This horizontal alignment eliminates the gravitational resistance the blood vessels must overcome, which allows for more efficient blood return to the heart. The resulting reading is often slightly different from a seated measurement, with the systolic pressure sometimes measuring about 8 mmHg higher and the diastolic pressure about 5 mmHg lower in the supine position compared to a seated measurement. The body’s internal pressure sensors, called baroreceptors, adjust the circulatory system to maintain stability, but the physical reality of the horizontal body position causes a measurable difference in the force against the arterial walls.
Correct Technique for Supine Measurement
To obtain an accurate blood pressure reading in the supine position, a specific technique must be followed. The patient should lie flat on their back (dorsal recumbency), with the body and arm fully supported and relaxed. The patient must rest quietly in this position for a minimum of five minutes before the measurement is taken.
The most critical step is ensuring the cuff is placed on the upper arm at the exact level of the heart. Because the patient is lying down, the arm should not simply rest on the bed, as this would position the cuff below the heart level and lead to a falsely elevated reading. Instead, the arm must be supported, often with a pillow or a rolled towel, so that the center of the cuff is level with the mid-sternum. Inaccurate positioning, even a difference of a few inches, can result in measurement errors of 10 mmHg or more. After the arm is correctly positioned, the measurement should be taken, ensuring the patient remains silent and still, and at least two readings should be recorded, separated by one to two minutes.