Modes of action

• Increase force of myocardial contraction, either by stimulating cardiac p1 adrenoreceptors (catecholamines), decreasing cAMP breakdown (PDE inhibitors), increasing calcium sensitivity (Ca sensitisers), directly increasing contractility (digoxin), or inhibiting neuronal reuptake of noradrenaline (dopexamine). All agents except digoxin have, to greater or lesser degrees, associated dilator or constrictor properties via p1 and p1 adrenoreceptors, dopaminergic receptors, or KATP channels.

• Digoxin may cause splanchnic vasoconstriction and, for an inotropic effect, requires plasma levels at the top of the therapeutic range.

• The increase in cardiac work is partially offset in those drugs possessing associated dilator effects.

• Other than epinephrine (when used for its vasoconstrictor effect in cardiopulmonary resuscitation) or digoxin (for long term use in chronic heart failure), inotropes are usually given by continuous IV infusion titrated for effect.

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