Many people don’t formulate much thought into where their arm should be when measuring blood pressure. By neglecting this critical detail when performing one of the most significant vital signs, is a mistake. On the other hand, some people do know and correctly place their arm. Have you ever wondered, why does your arm have to be at heart level when taking BP?
Your upper arm has to be at heart level when taking blood pressure because if it’s not, the measurement will be inaccurate. If the blood pressure cuff is higher or lower than the heart, hydrostatic pressure will be increased or decreased. A higher or lower hydrostatic pressure at the BP cuff will decrease or increase your blood pressure reading.
Knowing this key fact plays an enormous role in your BP measurements. Hill and Bernard, two grandfathers of blood pressure, developed a formula stating how much your readings will differ for every inch the BP cuff is away from your heart. Keep reading and I’ll tell you by how much your pressure will differ. In addition, I’ll go into more detail about what hydrostatic pressure is and some tips on how to place your arm at heart level.
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BP Cuff at Heart Level
Keeping your upper arm or cuff at heart level is one of the many details in BP measuring. As long as no other errors are committed, by having the cuff at the correct height improves your accuracy. The reason is hydrostatic pressure 1.
Hydrostatic pressure is how much pressure is exerted by fluid because of gravity. In a cylinder, the pressure against the sides of it near the bottom is greater than near the top. Hydrostatic pressure plays a role when measuring your blood pressure 2.
Arm Higher Than Heart Level
When your upper arm and cuff are raised higher than heart level, your blood pressure reading will be lower. The higher your arm is raised, the lower the measurement will be. It doesn’t matter if you’re sitting, laying down or standing up.
By the way, it’s the same with a wrist blood pressure cuff. Whichever type of cuff is used, the outcome will be the same in regards to the height of the cuff. Keeping a wrist cuff at the correct height can be tricky. If you have one and want to know the correct procedure, I wrote a blog post on it. You can view it by clicking right here, How to Use a Wrist Blood Pressure Cuff.
Arm Lower Than Heart Level
When your cuff or upper arm is lower than the heart, your readings will be higher. This is a common mistake some people make when they hold their wrist cuff down on top of their thigh. This occurs because of hydrostatic pressure. The lower your cuff is, the more your pressure will increase.
How Much BP Changes When the Arm is Not at Heart Level
I mentioned near the top of this article how Hill and Bernard developed a formula. It predicted the change in blood pressure for every inch the cuff was higher or lower than the heart. The formula stated the measurement would differ 2 mmHg for every inch difference 3.
In addition to the formula, studies have shown differences when the arm is not at heart height. In the book, In Hypertension Primer: The Essentials of High Blood Pressure, it states a BP reading will be falsely low by 0.08 mmHg for every 1cm the arm is above the heart 4.
A study involved 240 people with high blood pressure. Their measurements were taken with their arms in various positions. When their arms were held straight down compared to at heart height, the measurements were 8 mmHg higher 5.
Similar results were found in a study that researched arm position and blood pressure. The participants blood pressure was measured with their arms at heart level and with the arm lower. With the arm below the heart, their pressure changed from 103/60 to 111/67. People in the study who had high blood pressure showed a greater increase in both systolic and diastolic pressure 6.
What Exactly is Heart Level?
I’ve mentioned heart level many times, but what exactly determines heart level? The American Heart Association states the correct height of the blood pressure cuff is the same level as the heart’s right atrium 7. That’s nice to know, but how do you determine exactly where the right atrium is?
The right atrium is the right upper chamber of your heart’s four chambers. It’s located on the same side of your body as your right arm but you don’t really have to know that when measuring your BP 8. The easiest way to determine the location of the right atrium is to know where the mid point of your sternum is.
The sternum starts at the top in the middle of your chest where the two clavicles meet. It runs down to the bottom where your ribcage turns up and meets. Measure the distance of your sternum from the top to bottom. The right atrium is located half way. This should be the height of your BP cuff when measuring your blood pressure.
Tips on Keeping Your Arm at Heart Level:
- While sitting in a chair, position your arm outwards from the body.
- Your elbow should be supported on a flat, comfortable surface.
- Check to see if the middle of the cuff is at the mid point of your sternum (right atrium).
- If not, adjust your upper arm.
- If it’s not possible then you’ll have to adjust either the height of your chair or the flat surface. If that’s unpossible, find a different chair or surface to use.
- Always make sure your elbow is supported after adjusting the height or your arm. If it’s not, your BP reading will be affected. Believe me it makes a difference! You can see how much of a difference in my blog post, Unsupported Arm Blood Pressure.
If you want to find out more details on arm positioning, check out my article, Effect of Arm Position on Blood Pressure.
If you measure your blood pressure at home, I applaud your actions! It’s one of the most important things you can do to help manage your blood pressure. Some of us may only be at the doctor once a year, that’s not enough measuring to keep a record of your pressure. This is especially true if you find you’re a little anxious when visiting the medical office resulting in inaccurate measurements.
- 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
- National Center for Biotechnology Information: Effects of Arm Position and Support on Blood-Pressure Readings
- National Center for Biotechnology Information: Arm Position and Blood Pressure: A Risk Factor for Hypertension?
- Journal Hypertension: Measurement of Blood Pressure in Humans: A Scientific Statement From the American Heart Association
- Wikipedia: Atrium (heart)