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Re: Blood Pressure



I'm glad this topic has come back.

I recently asked an emergency physician about the necessity of taking
blood pressures while the patient is in a particular position. Her
response was "It doesn't matter in practice." Note the qualification.
She knew that there were variations due to body position, but that the
variation was not large enough to be important diagnostically.

In fact, there is a diagnostic test that involves taking a BP while the
patient is prone, then again after the patient has been standing for
five minutes. The difference in BP can be used to estimate blood volume
losses. However, if the BP is taken immediately after the patient
stands, the body can compensate for the blood loss, and the resulting BP
measurement is not enough different from the prone BP to draw any
conclusions. The body can't compensate for significant blood loss for
long, however, so that the five minute standing wait is sufficient.

The original discussion included many laments about the poor physics
understanding of physicians. While true, that many don't know as much
physics as they should (see laser story below) they do seem to have a
thorough knowledge of what works. I suspect that the physician quote
that started the whole discussion was from an over-brief description of
the PRACTICAL importance of body position in measuring blood pressures.
Due to compensations in the body, the vascular system isn't a simple
hydraulic system, so the basic physics we all know isn't the whole
story.

JEG

(the promised laser story)
Some years ago while preparing topics for a course for non-physics
majors called "Lasers in the Natural Sciences" I interviewed some
physicians about the use of lasers in surgery. (There were going to be a
number of premeds in the course, as the place I was teaching had a
ten-week all of Halliday/Resnick firehose intro course. To take the
required two courses for med school, the premeds had to take a second
course, which was necessarily either my course or Modern Physics with
the physics majors.)

One doc gave me a stack of papers about the early research of lasers in
surgery. About half the papers only discussed the power level of the
lasers, not the power/unit area. As a result, you could draw next to no
conclusions about what was actually being done with the laser. This was
a pretty basic omission, to say the least.

--
John E. Gastineau john@gastineau.org
140 Tenderfoot Road (301) 387-8494
Oakland MD 21550-6957 (301) 387-8495 fax
USA http://gastineau.org