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Re: Pre-med physics in practice

Jim's response is cast in oracular form. I too divine that all
is not quite pukka. Let's see how the 'comfort zone arm position
is inconsequential' proposition holds up.

The vertical cuff variation possible in a comfort zone is probably
no more than 20 cm. If blood were no more dense than water,
the atmosphere would support a column about 10.3 meters high, or
in pressure terms used in the field, an atmospheric pressure of
760 mm Hg is a comparison with the 120/80 mmHg blood pressure
pulses often seen. A variation in head of 20cm water gage can be
represented as a pressure variation of 15 mmHg in these units.

If this pressure variation were all applied to a reading, a respectable
result of 120/80 might become 135/95 which would be cause for alarm.

We have a physician's word that this effect does not occur. Why not?
Two possible factors come to mind:
homeostasis - a mechanism for regulating blood tension. As far as I know,
this control is slow acting, and acts at the systemic level.

So, the explanation may be related to a characteristic of the
vascular system. Some non linear elastic behavior of the blood
vessels may accomodate pressure drops by constricting.
J.E Gordon certainly warns us that latex-like behavior of the
blood vessels would make aneurysms frequent.

The negative feedback cannot be perfect - the observation of
dizziness when standing up suddenly from a seated position is
attributed to reduced blood pressure after all.

But this is as far as I can evaluate the proposition until my
automatic sphygmomanometer is repaired.

Brian Whatcott

At 23:34 6/15/01 -0600, Jim Green wrote:
Oh my heavens! Wouldn't it be nice for pre-med people to pay attention in
their physics class! OTOH it would also be nice if physics instructors
taught physics in their physics class! Further, it would be nice if
physicians would retain a bit of humility through med-school. I met one
such once.

At 23:04 15 06 2001 , you wrote:
In Dr Peter Gott's (MD) sydicated column of 6/15/2001, a reader asked about
the proper vertical level of the (cuffed) arm during a blood pressure
measurement. His reply follows, and speaks for itself:

"It used to be said that the arm should be at heart-level during blood
pressure recording; in fact, the directions in most do-it-yourself BP kits
still suggest this position. In actual fact, however, the position of the
arm is probably inconsequential, providing that the patient is comfortable
and relaxed. Because the arterial circulation is basically a hydraulic
system, the pressure should be the same in arms and legs, regardless of

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)

Jim Green

brian whatcott <> Altus OK