Re: Pre-med physics in practice

[brian whatcott <inet@INTELLISYS.NET>]

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
> > position."
> >
> > Bob Sciamanda (W3NLV)
> > Physics, Edinboro Univ of PA (em)
> > trebor@velocity.net
> > www.velocity.net/~trebor
>
>
> Jim Green
> mailto:JMGreen@sisna.com
> http://users.sisna.com/jmgreen

brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!