Blood Pressure Vs Height of Measurement

Donald Kenney (donaldkenney@gmail.com)
Last Update: Thu Sep 1 07:40:12 2016



BLOOD PRESSURE VS HEIGHT OF MEASUREMENT

In this simple experiment, we are just confirming the well established fact that arterial blood pressure varies with the height of the measuring device above or below the heart. This becomes important if one decides to measure blood pressure with a device other than the conventional arm cuff or even with an arm cuff in an unusual postion.

The Experiment

What we have is a series of three sets of measurements made with a wrist cuff blood pressure measurement device at head height, heart height, waist height, below waist height, and as low as I could conveniently get my arm. As we can see pretty clearly, the measured blood pressure -- both systolic and diastolic -- is lower when the device is above the heart, higher when it is below the heart.

As can be seen, For Systolic BP, the variation is roughly 2.03 mm_Hg per inch of height (=0.80mm_Hg/cm). For Diastolic BP, it is roughly 1.75mm_Hg per inch of height (=0.69mm_Hg/cm). The difference in blood pressure is mostly due to gravity.

Surprisingly perhaps, I had some difficulty finding measured or theoretical values for the change in arterial blood pressure with height above or below the heart. Reference_1 gives an observed value of. 0.74mm_Hg per inch of height in Mean Arterial Pressure(MAP). MAP is a sort of hybrid of Systolic and Diastolic blood pressure that is weighted toward the diastolic. Reference_2 gives a theoretical value of 0.7468 mm_Hg = 1.90mm_Hg/inch, and points out that diastolic and systolic pressure should show the same rate of change. I worked out my own estimate based on the densities of whole blood (about 1.06) and Mercury(13.8) and came up with 1.98 mm_Hg per inch of height (=0.80mm_Hg/cm).

Let me tabulate this:
mm_Hg/Inch mm_Hg/cm Source
2.03 0.80 My measured value for Systolic Blood Pressure
1.75 0.69 My measured value for Diastolic Blood Pressure
1.89 0.74 Measured value for Mean Arterial Pressure Ref1 http://ccn.aacnjournals.org/content/22/2/60
1.90 0.7468 Engineering Analysis Ref2 www.kup.at/kup/pdf/6279.pdf
1.98 0.80 'Cocktail Napkin' estimate based on density of whole blood (1.06) and Mercury (13.8)|

Discussion

The circulatory system is a closed set of plumbing with a built in pump, the heart. Even if the heart stopped, blood at the lowest part of the body would be under considerable 'hydrostatic' pressure from the weight of blood in the blood vessels above it. With the heart running, that hydrostatic pressure is still present. For a standing human being, the heart has to pump with enough pressure to push blood up to the brain against gravity. It also needs to pump with enough pressure to push blood at the feet back up to the heart working against gravity. The pressure required to do that is around 80mm of Mercury. Blood pressure measured above or below the heart will reflect the hydrostatic pressure. It will be lower above the heart where blood has been pushed up fighting gravity. It will be higher below the heart where the weight of the blood above measurement point will add to the pressure contributed by the heart.

Typical human blood pressures are high enough to ensure circulation even for very tall people. Yes, giraffes with their long necks have rather complex circulatory systems operating at high pressures in order to push blood up those long necks and to compensate for the sudden changes in pressure if the head is lowered. No, no one knows how Mesozoic reptiles with really long necks handled these problems. And yes, the situation is different when the body is immersed in water which applies hydrostatic pressure to the outside of the body pretty much equivalent to that experienced internally. Aquatic animals can have rather low blood pressures compared to terrestrial animals. Some aquatic snakes can't pump blood to their head if they are removed from the water and suspended vertically. Otherwise similar tree climbing snakes have higher blood pressures.

This explains why, if at all possible, blood pressure is measured with an arm cuff with the subject sitting or standing since that puts the cuff roughly at heart level. It is worth remembering this chart if you use a wrist or finger measuring device or are tempted to measure blood pressure while laying down, standing on your head, or some other unusual position. It's important to have the measuring device at heart height or to correct the readings if you can't measure at heart level.

Here's the data

And here (BPVSHT.CSV) is the raw data

References