The role of vasoactive agents in shock therapy

P Ogburn
Veterinary Clinics of North America 1976, 6 (2): 287-95
Vasoactive agents may have vasoconstrictor, vasodilator, cardiac stimulatory, or combined effects on the cardiovascular system. The intensity or degree of therapeutic effect differs with each agent. Table 1 provides a relative ranking of each discussed compound's effect regarding its ability to produce one or more of the listed effects. The effects of vasoconstrictor drugs such as methoxamine, phenylephrine, and norepinephrine have been generally unfavorable in shock because of the inhibition of tissue perfusion which results from their use. Debate still exists, however, and these agents have been shown to provide some benefit in selected cases. The rationale that shock results at least in part because of intense vasoconstriction has led to the usage of vasodilators in therapy. Currently isoproterenol, a beta adrenergic stimulating agent, is being used to elicit vasodilation in lieu of alpha blockage because the alpha blocking drugs phenoxybenzamine and chlorpromazine have longer, more irreversible effects. The merit of isoproterenol has to be evaluated in light light of its cardiac stimulatory effect. With the current antishock drugs, those which possess cardiac stimulatory effects seem to be most effective with the exception of those with alpha stimulatory properties. The importance of cardiac stimulation in treating shock is related to the fact that in many forms of shock a decrease in cardiac function is evident. Drugs which effect increases in cardiac performance will increase cardiac output and tissue perfusion. The increased excitability of the heart caused by many of the drugs is a drawback, but compounds such as dopamine seem to have less excitatory effect than does isoproterenol. It may be that vasoconstriction, vasodilation, and cardiac stimulation are all contributory to the alleviation of shock. However, it is important to remember that the use of vasoactive agents must be reserved for those deteriorating shock states in which primary and secondary factors responsible for the initial state have been adequately controlled and only when appropriate methods for judging hemodynamic performance have been instituted.

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