Effect of intracardiac shunts on inhalation induction

Published on 13/02/2015 by admin

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Last modified 13/02/2015

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Effect of intracardiac shunts on inhalation induction

David J. Cook, MD and Eduardo S. Rodrigues, MD

Intracardiac shunts may alter the rate of induction of inhaled anesthetic agents. This alteration depends on the direction and size of the shunt and on the solubility of the anesthetic agent used.

Induction of anesthesia is a function of the equilibration of three factors: the rate of anesthetic inflow into the lungs and equilibration with alveolar gas (as determined by tidal volume, respiratory rate, inspired fraction of anesthetic agent, and functional residual capacity), the rate of transfer of the anesthetic agent from lungs to arterial blood, and the rate of transfer of the anesthetic agent from blood to brain:

< ?xml:namespace prefix = "mml" />PA ↔ Pa ↔ Pb

where PA equals the alveolar partial pressure of the inhaled anesthetic agent; Pa, the arterial partial pressure of the inhaled anesthetic agent; and Pb, the brain partial pressure of the inhaled anesthetic agent. P equals the mixed venous partial pressure of the inhaled anesthetic agent.

Cardiac shunts primarily alter the effect of the uptake of the anesthetic agent by pulmonary arterial blood. The determinants of anesthetic uptake from alveoli are the blood-gas partition coefficient of the anesthetic agent, the cardiac output (CO), and the alveolar to mixed venous partial pressure difference of the anesthetic agent (PA – P).

The blood-gas partition coefficient is the distribution ratio of the anesthetic agent between blood and alveolar gas at equilibrium (relative solubility). For a highly soluble agent, it usually takes several passes of the blood volume through the lung before enough of the agent is absorbed that the blood is saturated to the point that the necessary Pa of the agent to achieve anesthesia is reached. A highly soluble agent, then, has a much slower induction time, compared with an agent that is not soluble (see following discussion). Assuming no change in ventilation or inspired fraction of anesthetic agent and normal tissue perfusion, the rate of induction is determined primarily by anesthetic solubility and the effective pulmonary blood flow.

Right-to-left shunt

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