The body's position is important in obtaining accurate readings when measuring blood pressure (BP). Taking BP while sitting upright is generally preferred over doing so in a supine (lying on your back) position, which overestimates BP.
Introduction
The accuracy of BP measurements is crucial for diagnosing and managing hypertension. Take BP measurements after sitting quietly for several minutes, not immediately after walking to a chair. Don't talk since this can raise BP and HR (Freed et al., 1989; Le Pailleur et al., 2001; Lynch et al., 1982). Your arm should be at heart level (O'Brien et al., 2003; Pickering et al., 2005). For every 10 cm (about 4 inches) below heart level, your systolic and diastolic blood pressures can increase by approximately 7-10 mmHg. Conversely, for every 10 cm the arm is above heart level, the systolic and diastolic blood pressures can decrease by about 7-10 mmHg. These errors are due to the hydrostatic pressure effect, where the arm's position relative to the heart influences the pressure reading.
Different body positions can affect the readings, potentially leading to misdiagnosis or inappropriate treatment. Supine BP is generally higher than sitting BP. This is observed in systolic (i.e., heart chambers contract to pump blood) and diastolic (i.e., when they relax) measurements across various studies (Buhre et al., 2000; Cicolini et al., 2011; Lu et al., 2008; Wei et al., 2009; Zachariah et al., 1990).
The difference in BP readings between supine and sitting positions is influenced by age and sex, with older individuals showing greater differences (Lu et al., 2008; Wei et al., 2009).
The largest difference in systolic blood pressure (SBP) between the supine and sitting positions was observed in individuals aged 30 to 39 (3.6 ± 6.8 mmHg) and those over 80 (5.3 ± 7.9 mmHg). A more significant difference in diastolic blood pressure (DBP) between these positions was noted in individuals over 60 (Lu et al., 2008).
The Effect of Gravity on Blood Pressure Readings
Sitting upright allows gravity to influence the distribution of blood, which more accurately reflects the conditions experienced during daily activities. This position helps to standardize readings and makes them more comparable to the measurements used in clinical settings (Netea et al., 2003).
In the supine position, intrathoracic blood volume and cardiac output increase, leading to higher BP readings. The sitting position results in a redistribution of blood volume from the thoracic (shown below) to the extrathoracic compartments, reducing cardiac output and BP (Buhre et al., 2000).
Measurement Reliability
BP readings taken in a sitting position are more consistent and reliable. The American Heart Association recommends sitting with feet flat on the floor and back supported to ensure the most accurate and consistent results (Pickering et al., 2005).
Reduction in Variability
BP tends to be higher when measured supinely due to the lack of gravitational influence. This leads to significant variability and potentially higher readings than in the upright position (O’Brien et al., 2003).
Clinical Practice Guidelines
Clinical guidelines from various health organizations recommend the sitting position for BP measurement to minimize positional differences and improve diagnostic accuracy (Mancia et al., 2007).
Conclusion
Sitting BP is generally lower than supine due to hemodynamic changes, including reduced intrathoracic blood volume and cardiac output when sitting. This positional difference is influenced by factors such as age and sex. Standardizing the body position during BP measurement is essential for accurate clinical assessments, and significant positional differences in BP can have prognostic implications in cardiovascular health.
Measuring BP while sitting upright is generally preferred over the supine position due to the influence of gravity, consistency, reduced variability, and alignment with clinical guidelines. Adopting the upright position ensures more accurate and reliable readings, which is essential for effective hypertension management.
Glossary
blood pressure: the force exerted by circulating blood on the walls of the arteries. It is typically measured in millimeters of mercury (mmHg) and is expressed as two numbers: systolic pressure (during heartbeats) over diastolic pressure (between heartbeats).
cardiac output: the amount of blood the heart pumps per minute. It is calculated by multiplying the heart rate (beats per minute) by the stroke volume (amount of blood pumped per beat).
intrathoracic blood volume: the total blood volume within the thoracic cavity, including the heart, great vessels, and pulmonary circulation. This volume affects cardiac filling and, consequently, cardiac output.
supine: lying on the back, facing upward.
References
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Cicolini, G., Pizzi, C., Palma, E., Bucci, M., Schioppa, F., Mezzetti, A., & Manzoli, L. (2011). Differences in blood pressure by body position (supine, Fowler's, and sitting) in hypertensive subjects. American Journal of Hypertension, 24(10), 1073-9. https://doi.org/10.1038/ajh.2011.106
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