Classification of inotropic agents

The parenteral inotropic agents currently in use include two pharmacological classes, catecholamines and phosphodiesterase inhibitors. However, a more pragmatic classification subdivides inotropic agents on the basis of their actions on the peripheral circulation. Thus those agents which have a predominantly vasoconstrictor effect may be termed inoconstrictors. Endogenous catecholamines, for example norepinephrine (noradrenaline), epinephrine (adrenaline), and dopamine, all have peripheral vasoconstrictor effects mediated by the a rreceptor; the effect is greatest in the highest dose ranges. Agents whose predominant effect on the peripheral circulation is vasodilation may be termed inodilators. The synthetic derivatives of dopamine, for example dobutamine, dopexamine, and isoproterenol (isoprenaline), have predominant b2-agonist activity and uniformly induce peripheral vasodilation. Phosphodiesterase inhibitors (amrinone, milrinone, enoximone) are actually more potent vasodilators than inotropic agents.

The primary indication for an inoconstrictor is a situation where inotropic support is required in the face of low systemic vascular resistance, for example septic shock. Inodilators are suited to situations where low cardiac output is associated with high systemic vascular resistance, for example congestive heart failure or severe cardiomyopathy. The vasodilator effect decreases systemic vascular resistance and hence afterload, which further enhances the inotropic action in improving cardiac function.

It may be advantageous to combine an inotropic agent with a vasodilator or vasoconstrictor drug to achieve the optimal balance between inotropic effect, afterload reduction, and perfusion pressure.

In short, the rational choice of an inotropic agent or combination of agents depends on careful evaluation of global hemodynamic indices including cardiac output, mean arterial pressure, and central venous pressure, as well as the effect of the agent on heart rate.

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