Inotropes are substances used to alter the energy of muscle contractions. These substances can be negatively inotropic or positively inotropic. Negatively inotropic agents decrease the energy of muscle contractions, and positively inotropic agents increase the energy of muscle contractions.
The most common association of inotropes is with contractions of the heart muscle. More precisely, inotropic agents typically are drugs that are used to affect the strength of the heart muscle contractions. These muscular contractions can be increased or decreased as necessary by using inotropic agents.
Natural conditions resulting from inotropic activity also exist. Increased inotropic activity can result from an enlarged heart muscle in a condition called ventricular hypertrophy. Decreased inotropic activity can be caused by myocardial infarction or dead heart muscle tissue, which is more commonly known as a heart attack.
Inotropes, whether they are positive or negative, are used to help manage cardiac conditions. Most inotropes work based on the amount of calcium in the cytoplasm of the muscle cell. This level is increased by positive inotropes and decreased by negative inotropes. Not all drugs work based on the release of calcium, and even among the ones that do, the mechanism of release can vary.
Negative inotropic agents are used to decrease myocardial contractility. This approach is used to relieve a strained or overworked heart. Care is exercised, because this treatment can lead to or exacerbate heart failure. Negative inotropic agents include certain beta blockers that have been proven to reduce mortality and morbidity in congestive heart failure. Negative inotropes include beta blockers, calcium channel blockers, class IA antiarrhythmics and class IC antiarrhythmics.
Conversely, to increase myocardial contractility, one would use a positive inotropic agent. This approach is used to treat several different heart conditions. Cardiomyopathy, decompensated congestive heart failure, myocardial infarction, cardiogenic shock and septic shock are the ailments most typically treated by positive inotropes.
Positive inotropic agents are many and varied. This group includes berberine, bipyridine derivatives, calcium, calcium sensitisers and catecholamines. The group also includes eicosanoids, phosphodiesterase inhibitors, glucagon and cardiac glycosides such as digoxin. Digoxin, or digitalis, is an extract from the foxglove plant.
Positive and negative inotropes can affect any types of muscles. They have shown the greatest usefulness in treating conditions of the heart muscle. The extreme importance of safe heart function means that inotropes likely will continue to have the most impact in the treatment of cardiac conditions.
