Does your blood pressure cause your emotions to have much ups and downs? If so, I know how you’re feeling. To complicate matters, if your arm is raised or lowered, your pressure will have its ups and downs too. You can avoid some false lower readings by placing your arm at the standard height. Have you ever wondered, why is blood pressure lower when the arm is raised?
Blood pressure is lower when the arm is raised due to hydrostatic pressure. At more elevated levels the hydrostatic pressure of the column of blood in between the heart and the point of the measurement is lowered.
When the arm is lower than the heart, the reverse occurs and blood pressure is more elevated. In this blog post, I’ll explain to you what hydrostatic pressure is and how much of a blood pressure difference you can expect. In addition, I’ll inform you the best way to ensure your arm is at the appropriate height the next time you use your BP monitor.
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A Raised Arm Can Cause Lower Blood Pressure
We’ve all thought of the nightmare of driving a car into a body of water. As the car sinks, it becomes impossible to open the car doors. This is due to the hydrostatic pressure in the water that increases the deeper you travel down.
Hydrostatic pressure is the pressure exerted by a fluid due to the force of gravity. The pressure increases in proportion to depth measured from the surface. This is because of the increasing weight of the fluid above it. If you fill a glass with water, the pressure of the water against the sides of the glass is greater at the bottom than near the top 1.
Hydrostatic pressure comes into play when you raise your arm above heart level. The higher the arm pressure cuff, the lower the blood pressure reading will be. It doesn’t matter if you’re laying down, standing or sitting. If the arm is raised higher than the heart, your measurements will be lower 2.
How Much A Raised Arm Lowers Blood Pressure
How much lower can your pressure be? Hill and Barnard, two grandfathers of blood pressure, developed a formula that predicts the change in blood pressure with lowering or raising the arm. It states that BP changes by 2 mmHg per inch of distance from the correct heart level 3.
In Hypertension Primer: The Essentials of High Blood Pressure, the importance of avoiding errors caused by differences in hydrostatic pressure between the point of compression by the BP cuff and the heart is discussed. It states for each 1cm the arm is above heart level, the reading will be falsely low by 0.8 mmHg 4.
An article published in the BMJ stated raising the arm above heart level results in a lower blood pressure reading. The false reading from a raised arm can be as great as 10 mmHg for systolic and diastolic pressures. In addition, concern in regards to wrist monitors was discussed because how easy it is not to position the cuff at heart level 2. I discuss this in detail in my wrist monitor blog post, Are Wrist Blood Pressure Monitors Accurate?
How To Prevent A Raised Arm During Blood Pressure Measurement
While many articles state the blood pressure cuff should be at heart level, I don’t see enough written about how to properly accomplish it. I think many people sit down, wrap the cuff around their upper arm and don’t pay attention to the height of the arm and cuff. Many people think if their arm is resting on something, their arm positioning is good enough.
The arm should be positioned outwards from the body and resting on a comfortable surface. This position raises the arm to heart level if the height of the surface is at the correct height. If not, adjustments should be made. If the arm is lower than heart level, you can rest your arm on a pillow to raise it more. If the arm is raised above heart level use a different surface that’s lower in height or a chair that’s taller.
How do you know what exactly is heart level? The correct level is having the cuff at the same height of the right atrium of your heart 5. The right atrium, the upper right side of your heart, is located midpoint of your sternum. The sternum or breastbone begins at the bottom where your ribcage meets in the middle, up to where your clavicles meet.
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- Dictionary.com: Hydrostatic pressure
- National Center for Biotechnology: 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
- Hypertension: Measurement of Blood Pressure in Humans: A Scientific Statement From the American Heart Association