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November 1996

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From:
"Lee Nix@CARDIAC" <[log in to unmask]>
Date:
Wed, 20 Nov 1996 08:48:00 -0600
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Re the question of why patients get faint when standing:

  Remember all the stories about people standing at attention and
passing out
as a result of lack of blood return to the heart secondary to muscle
contraction in the legs?  The noninvasive test depends on the phenomenon
of
collection, then contraction, the patient depends on circulating blood
volume
to remain conscious.

  Because of the tremendous capacitance of the veins in the leg, even
short
periods of standing can cause relatively huge volumes of blood to
accumulate
in them.  Approximately 500 cc additional volume of blood accumulates in
the
legs  upon standing.  According to Stegall and Rushmer (as quoted in
Hemodynamics for Surgeons by Strandness and Sumner) 250 cc more
accumulates
within a minute.  In addition an amazing amount of fluid can be shifted
into
the extravascular tissues from the veins in a relatively short period of
time.  

  Is this what you are looking for?

Lee Nix
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