Cardiovascular Responses to Hemodilution and Controlled Hypotension in the Dog

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Abstract
Cardiovascular responses to acute hemodilution and controlled hypotension [due to blood loss during surgery] were studied in mongrel dogs anesthetized with halothane and paralyzed with pancuronium. Regional blood flows were determined by microsphere injections. Hemodilution to a hematocrit of 23% was produced by removal of whole blood and simultaneous infusion of Ringer''s lactate solution. Subsequently, hypotension to a mean arterial pressure of 55 mmHg was produced for 90 min by i.v. infusion of trimethaphan. The hypotension resulted entirely from a 55% decrease in total peripheral resistance. Thirty minutes after initiation of controlled hypotension, there were significant increases in blood flow to the brain, liver, skeletal muscles and diaphragm. At 30 min, calculated O2 delivery had decreased to brain (-16%), renal cortex (-51%), heart (-45%), and retina (-44%). By 90 min, retinal, adrenal and renal cortical blood flows were decreased significantly relative to control, and cerebral blood flows had returned to control levels. Absence of changes in acid-base status during the period of hemodilution and hypotension may indicate that whole body O2 delivery was maintained at adequate levels. Major decreases in calculated O2 delivery after 90 min to critical tissue beds such as renal cortex (-67%) and retina (-78%) indicate that excretion of the procedure past 30 min may involve risks that are not warranted by the benefits.