The other day I was sought about arm position while measuring blood pressure. Many myths exist on this topic, and it can steer people in the wrong direction. When I explained the arm cuff should be at heart height, I was asked, what would happen to the blood pressure reading with the arm below the heart?
A blood pressure reading with the arm below the heart results in a higher blood pressure measurement. This occurs due to hydrostatic pressure. With the arm cuff at lowers levels, the hydrostatic pressure of the column of blood in between the heart and the point of measurement is higher.
Whether your blood pressure is measured at a doctor’s office or home, the importance of the proper arm position cannot be underestimated. Receiving false blood pressure readings because of a lower arm, may result in a diagnosis of high blood pressure. In addition, it can change dosages in your medication or blood pressure treatments you don’t really need.
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Blood Pressure With the Arm Below The Heart
Hydrostatic pressure increases the more depth exists between the point of measurement and the surface. The increasing weight of the fluid above causes the pressure increase 1.
A swimming pool filled with water endures more pressure against the sides of the pool at the bottom than against the sides near the top.
Hydrostatic pressure can affect your blood pressure. The more the arm cuff is lowered below the heart during measurement, the higher the reading will be 2. If you’re wondering how much more the reading can be, keep reading.
How Much Blood Pressure Readings Increase With the Arm Below The Heart
A formula was developed over 100 years ago by two men, a well-known physiologist and surgeon. Hill and Barnard’s formula predicts the change in blood pressure when the arm is raised or lowered below heart level. For every inch the arm is moved below the right atrium; BP will increase by 2 mmHg 3.
In the book, Hypertension Primer: The Essentials of High Blood Pressure, it states a blood pressure reading will increase 0.8 mmHg for each 1 cm the arm is lowered below heart height. In addition, the book notes how important arm position is because of the differences in hydrostatic pressure between the point of measurement and the heart 4.
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1: A study of 240 people evaluated the differences in blood pressure with arm position. Blood pressure averaged 8 mmHg higher for systolic and diastolic when the upper arm was straight down below heart height 5.
2: Another study researched two arm positions and their effect on blood pressure. 100 emergency room patients were checked with their arm held away from their body and with their upper arms lower at their side. With the lower arm, patients were classified with high blood pressure 41 percent of the time compared to 22 percent with the arm higher 6.
3: A study published in 2004 researched arm position and blood pressure. While sitting, blood pressure was measured two ways, with the arm at heart level and with the arm lowered. People with and without hypertension were measured. In those without high blood pressure, their pressure increased from 103/60 to 111/67 mmHg with the arm below the heart.
For people with high blood pressure, the reading increased from 143/78 to 166/88 mmHg with the arm below the heart 7.
4: The recommendation to have your upper arm at heart level was investigated in this study. 401 healthy men and women had their pressure measured with the arm at heart level and again with the arm lower and parallel to the sternum. In the men, the pressure was 9.4 systolic higher and 13.6 diastolic higher with the arm below the heart. The women experienced similar increases of 8.2 systolic and 12.4 diastolic 8.
Correct Arm Position in Relation to the Heart
The studies and research indicate a blood pressure reading will be higher with the arm below the heart. In addition, it can result in a false classification of high blood pressure. The last thing you want to do is make a mistake with arm positioning.
On the flip side, if you position your arm above the heart, your reading will be false to the low side. This can result in high blood pressure not classified which can be just as dangerous. The following is the correct procedure for your arm when measuring blood pressure 9.
- Position your arm outward from the body and rest it on a comfortable surface.
- Check the height of your upper arm and cuff to the mid-point of your sternum (the right atrium of your heart).
- If it’s lower than heart level, find a higher surface or a lower chair.
- If the cuff is higher than your heart, then locate a lower surface or a higher chair.
If you’re interested in more measuring tips and procedures then check out my blog post, Causes of False Blood Pressure Readings.
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- Dictionary.com: Hydrostatic pressure
- National Center for Biotechnology Information: Blood pressure measurement
- Wiley Online Library: Arm Position During Ambulatory Blood Pressure Monitoring: A Review of the Evidence and Clinical Guidelines
- Google books: Hypertension Primer: The Essentials of High Blood Pressure Chapter C109 Blood Pressure Measurement
- National Center for Biotechnology Information: Effects of Arm Position and Support on Blood-Pressure Readings
- UCSDNEWS: Arm Position Matters in Blood Pressure Readings According To UCSD Medical Researchers
- National Center for Biotechnology Information: Arm Position and Blood Pressure: A Risk Factor for Hypertension?
- National Center for Biotechnology Information: Epidemiological Perspective of Body Position and Arm Level in Blood Pressure Measurement
- Hypertension: Measurement of Blood Pressure in Humans: A Scientific Statement From the American Heart Association