What is the physiologic response to hypovolemia in anaphylaxis?

Updated: May 16, 2018
  • Author: S Shahzad Mustafa, MD; Chief Editor: Michael A Kaliner, MD  more...
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Two distinct physiologic responses occur in mammals experiencing hypovolemia. [6] The initial response to hypovolemia is a baroreceptor-mediated increase in overall cardiac sympathetic drive and a concomitant withdrawal of resting vagal drive, which together produce peripheral vasoconstriction and tachycardia.

When effective blood volume decreases by 20-30%, a second phase follows, which is characterized by withdrawal of vasoconstrictor drive, relative or absolute bradycardia, increased vasopressin, further catecholamine release as the adrenals become more active, and hypotension. Hypotension in this hypovolemic setting is independent of the bradycardia, since it persists when the bradycardia reverses with atropine administration.

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