Saturday, December 21, 2013

Some Issues About Evaluating Blood Pressure

A new report by an expert "guidelines committee" has shifted blood pressure norms for people over 60 years of age up to 150/90 from the previously accepted level of 140/90.  This has huge implications for many individuals presently taking anti-hypertensive medications, many of which have decidedly negative side effects.

This blog is not meant to evaluate the validity of this conclusion but to educate individuals about possible errors in blood pressure measurement.

Blood pressure is most often measured by using an inflatable cuff wrapped around the upper arm, a manometer (either a column of mercury, or a clock-face dial with a needle) and a stethoscope with which the examiner listens for the appearance and disappearance of pulse sounds in the crease of the elbow.  The blood pressure can vary based on a number of factors that affect these elements. Techniques used to record blood pressure in a doctor's office are "indirect."  The only true,"direct" method of measuring the pressure in a blood vessel is to attach a measuring device to the blood vessel directly - a procedure normally performed only in emergency rooms and hospital settings.

There is room for variability and some differences when the indirect measure of blood pressure is used. For instance:

1. The cuff may not be appropriately sized for the circumference and length of the upper arm - cuffs that are to short, or do not comfortably fit around the arm are apt to give falsely high pressures.  The reverse is true of cuffs that are too loose or to large.  The cuff must also be properly applied to the arm so that the bladder inside the cuff covers the brachial artery (the large artery in the upper arm.)
2.  Blood pressures are determined by the appearance and disappearance of sounds the blood makes as it moves under the stethoscope for which the examiner listens.  Clearly the auditory acuity of the examiner means something here. The ability to hear the initial and final sound depends not just on the examiner's ears, but also on the rate at which the air is released from the inflated cuff.
3.  Blood pressure may vary with body position.  It may also differ from one arm to the other.

Given the variables involved in the indirect recording of blood pressures, using absolute numbers as guidelines is fraught with the potential for error.  A blood pressure recorded as 154/90 could easily be an inaccurate reading of a pressure which may actually be 148/86, or even 158/94.

It would be more helpful to provide a "range" of normal (or abnormal) rather than an absolute number, given the variables discussed above.  And remember, one blood pressure recording is never enough to make a diagnosis of hypertension - there should be at least three such recordings over a period of a few weeks before a diagnosis is confirmed.

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