Positive inotropes are drugs which act primarily by increasing myocardial contractility, i.e. they increase the force of myocardial contraction. They require the existence of a cardiac reserve - a completely decompensated heart will not respond to these drugs. Diseased cells may or may not respond to the influence of the inotropes, depending on:
method by which the drug increases contractility
potency of the drug
type of deficit resulting in a loss of contractility
severity of the defect present in the cell
number of cells involved
The successful use of these drugs results in improvement to either the quality (ie. alleviate clinical signs at rest) or quantity (ie. increase survival time) of life.
The mechanisms of action vary from class to class. Most drugs are thought to work by increasing the concentration of free calcium ions in the sarcoplasm, usually by triggering release of the calcium stores in the sarcoplasmic reticulum through ryanodine receptors.
There are three main groups of drugs: cardiac glycosides, phosphodiesterase inhibitors and sympathomimetics. As a general rule, cardiac glycosides and phosphodiesterase inhibitors are used for chronic heart failure, sympathomimetics for acute heart failure.
sympathomimetics
phosphodiesterase inhibitors
cardiac glycosides
diagram
