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Does the position of your arm affect blood pressure?

Blood pressure is one of the vital signs that provide important information about the state of a person’s cardiovascular health. It is measured as two numbers – the systolic pressure (the pressure in the arteries when the heart contracts) and the diastolic pressure (the pressure in the arteries when the heart relaxes between beats). Blood pressure can be affected by a variety of factors including age, weight, genetics, diet, exercise, stress levels, and underlying medical conditions. One question that sometimes arises is whether the position of a person’s arm when their blood pressure is measured can affect the reading.

How blood pressure is measured

Blood pressure is most commonly measured using a sphygmomanometer, which consists of an inflatable cuff that wraps around the upper arm and a pressure gauge to measure the pressure in the cuff. To measure blood pressure, the cuff is inflated to a pressure above the systolic pressure so that it fully occludes the brachial artery in the arm. Then, as the cuff is slowly deflated, blood begins to flow through the artery again. This produces sounds called Korotkoff sounds that can be heard with a stethoscope placed over the artery around the elbow. The cuff pressure at which the first Korotkoff sound is heard is the systolic pressure, and the point at which the sounds disappear is the diastolic pressure.

Does arm position matter?

When measuring blood pressure, standard practice is to have the patient seated with their back supported and their bare upper arm at heart level. This arm position helps provide an accurate reading because it eliminates any effect of hydrostatic pressure – the pressure exerted by a fluid due to gravity. If the arm is below heart level during measurement, the hydrostatic pressure in the arteries increases, which can artificially increase the measured blood pressure. Conversely, if the arm is above heart level, the hydrostatic pressure decreases, which can artificially lower the measured blood pressure.

For this reason, medical guidelines state that the arm being used to measure blood pressure should be supported at heart level, which is generally considered to be the midpoint of the sternum or breastbone. Having the arm at heart level equals the hydrostatic pressure throughout the circulatory system and prevents any gravitational effects on blood pressure in the brachial artery.

Studies investigating the effect of arm position

Several studies have investigated whether changing arm position significantly alters blood pressure measurements in practice:

  • A 1987 study tested three arm positions – heart level, 30 cm above heart level, and 30 cm below heart level. With the arm above heart level, systolic and diastolic pressure decreased by an average of 1.4 and 1.8 mmHg. With the arm below heart level, pressures increased by an average of 10.5 and 4.7 mmHg.
  • A 2002 study looked at arm positions 30 cm above, at, and 30 cm below heart level. Systolic and diastolic pressures differed by 3-8 mmHg between positions.
  • A 2010 meta-analysis combined data from 23 studies. Compared to heart level, systolic pressure was 3.5 mmHg lower with the arm raised and 8.1 mmHg higher with the arm lowered. Diastolic pressure was 1.5 mmHg lower raised and 5.0 mmHg higher when lowered.

Overall, these studies demonstrate that arm height relative to heart level can significantly influence blood pressure measurement. However, the effect is relatively small in most patients when the arm is maintained within about 30 cm of heart level.

Recommendations for measuring blood pressure

Based on current evidence, medical experts make the following recommendations for accurately measuring blood pressure:

  • The patient should sit quietly for 3-5 minutes before measurement.
  • Arm should be bare and unsupported.
  • Cuff should be at heart level, midpoint of the sternum.
  • Feet should be flat on the floor, back supported.
  • Legs should not be crossed.
  • Cuff size should be appropriate for the arm.
  • Neither patient nor measurer should talk during measurement.

Maintaining the arm at heart level is an important aspect of accurate measurement. Talking, crossed legs, and improper cuff size can also cause measurement errors. Following the standard procedural guidelines helps minimize discrepancies.

How much does arm position affect reading?

The effect of arm height on blood pressure measurement can vary substantially depending on the individual. However, based on published studies, some general expectations are:

  • Raising the arm 30 cm above heart level typically decreases systolic pressure by 3-4 mmHg and diastolic by 1-2 mmHg compared to the heart level position.
  • Lowering the arm 30 cm below heart level typically increases systolic pressure by 6-10 mmHg and diastolic by 3-5 mmHg.
  • Beyond 30 cm above or below heart level, the effect may be enhanced and cause more significant variation from the heart level pressure.
  • For smaller changes in arm height of 10-20 cm, the effect is roughly proportional (e.g., 20 cm above heart may drop pressure by 1-2 mmHg).
  • The elderly and people with diabetes, obesity or chronic kidney disease may be more affected by alterations in arm height.

While concerning for accuracy, these arm position effects are unlikely to substantially change clinical diagnosis in most instances. But to optimize precision, keeping the arm at heart level is advised.

Example scenarios

Here are some examples to illustrate how arm position can alter blood pressure measurements:

  • Patient A has a reference blood pressure of 122/78 mmHg with the arm at heart level. If the arm is lowered 30 cm, their pressure may increase to approximately 132/83 mmHg.
  • Patient B has a pressure of 128/84 mmHg at heart level. With the arm raised 20 cm, their pressure may decrease to approximately 126/82 mmHg.
  • Patient C has untreated high blood pressure, measuring 152/98 mmHg at heart level. Lowering the arm 30 cm may further increase their pressure to around 162/103 mmHg.

While the position effects are generally small for single measurements, consistently measuring in non-standard arm positions could lead to errors in assessing a patient’s true blood pressure over time.

How does hydrostatic pressure cause this effect?

The mechanism for the arm position effect on blood pressure measurement is hydrostatic pressure changes. Hydrostatic pressure refers to the pressure exerted by a fluid due to the forces of gravity. Blood and interstitial fluid exert hydrostatic pressure proportional to the vertical distance below the heart as described by this equation:

Hydrostatic pressure = ρ × g × h

Where ρ is fluid density, g is acceleration due to gravity, and h is the vertical distance below the heart. Since arterial pressure is measured as the height of a blood column able to be supported, the hydrostatic pressure directly alters the measured pressure. Raising the arm decreases hydrostatic pressure in the artery, reducing the measured blood pressure. Lowering the arm increases hydrostatic pressure, artificially raising the measured blood pressure.

Typical hydrostatic pressure changes

Some typical scenarios and associated hydrostatic pressure changes are:

Arm position relative to heart Distance (h) Hydrostatic pressure change*
30 cm above -30 cm -22 mmHg
20 cm above -20 cm -15 mmHg
At heart level 0 cm 0 mmHg
20 cm below 20 cm 15 mmHg
30 cm below 30 cm 22 mmHg

*Based on fluid density of 1.06 g/mL and gravity of 9.81 m/s2

This demonstrates how relatively small vertical arm displacements can alter hydrostatic pressure and blood pressure measurement due to the density of blood and gravitational forces.

Other considerations

There are a few other factors that can lead to blood pressure measurement errors related to arm position:

  • Unsupported arms can lead to isometric muscle tension that increases pressure.
  • Downward hanging arms cause vein engorgement that can enhance hydrostatic effects.
  • Measuring pressure while lying down makes defining the heart level position more difficult.
  • In obesity, the restrictive cuff may alter circulation to the arm when horizontal.

Overall, the simplest way to get an accurate reading is having the patient sit upright with bare, relaxed arm supported at heart level. This minimizes both hydrostatic and isometric effects on blood pressure in the artery under the cuff.

Conclusion

Based on the physiology, published evidence, and medical guidelines, arm position relative to heart level does have a small but significant influence on blood pressure measurement. To avoid erroneous high or low readings, the standard recommendation is that the arm used for measurement should be supported at heart level. This equals hydrostatic pressure in the circulatory system and prevents gravitational effects on blood pressure in the brachial artery under the cuff. While errors from minor positional variations may be inconsequential for any single measurement, consistently using non-heart level positions could lead to inaccuracies in assessing a patient’s true blood pressure over time. Following procedural best practices of having the bare arm supported at heart level while the patient sits upright helps ensure accuracy and reliability in determining blood pressure.