Why measure blood pressure on the right arm!

Frank Russo - March 9, 2007.

A lot of blood pressure monitors have been designed for use on the left arm, and most doctors in my experience tend to routinely always prefer the left arm . The right arm is usually only measured when the left is a bit high, as a sort of second opinion. I guess often the orientation of the doctor's desk to the patient's chair has a lot to do with which arm the doctor will use.

In my researching however, I have found that one should always prefer the right arm and only measure the left as a sort of second opinion. The blood pressure readings in the right arm are far more stable than those in the left. Food and drinks tend to affect the right very little, whereas they greatly affect the readings in the left arm. This effect of food is well acknowledged by the manufacturers of the Omron monitor, so much so that they stipulate in their guidelines that one should only measure after half an hour from any food ingestion.

The effect of the location does not seem to affect the systolic as much as the diastolic. The latter can often be 10 'points' lower on the right, and this is crucial not only because the diastolic has a much lower value so that any change is far more significant, but also because the diastolic is far more stable and a much better indicator: its value is very reflective of peripheral resistance whereas the systolic varies much more according to cardiac output.

Initially I thought that this stereoisomeristic variance may have been due to the stomach's enlargement due to food ingestion. This proposition was however unrealistic as the stomach is far removed from the left arm and a fair way down in the thorax. I then remembered that in my first research paper from 1981 I mentioned that in most people the thoracic lymphatic duct empties in the left sub-clavian rather than the right, somewhere near the shoulder and neck joint. Most medical books and doctors rarely mention the lymphatic circulation, they always concentrate on the arterial and the venous circulation!

This extra "portal" on the left is the answer as to why the readings on the left arm are a bit higher and less stable: not only is there a lot of extra fluid being introduced to the left arm area but a lot of the digested food actually can also end up there. On reflection that paper on the circulation that I wrote in 1981 has been my catalyst for many new ideas.

So in conclusion, let us remember that it's not just a matter that the medical fraternity has standardized the left arm for blood pressure measurements: there are variables that render the left arm rather unsuitable as the reference bench-mark, and as considered the right arm is a much better indicator.