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An inotrope is a drug that changes the strength of the heart's muscular contractions. This in turn can either increase or decrease the volume of blood pumped by the heart. Positive inotropes increase the heart rate, and are used to treat congestive heart failure patients. Conversely, negative inotropes decrease the strength of muscular contractions, which is clinically useful in the treatment of high blood pressure. Common inotropic agents include dopamine, milrinone, and calcium channel blockers.
The volume of blood the heart pumps must be kept at a sufficiently high level to make sure that tissues throughout the body receive enough of it. This is tied directly to the stroke volume of the heart muscle, which depends on how powerfully and efficiently the muscle contracts, a measure called cardiac inotropy. The heart's inotropic state is the total strength of its muscular contractions, which decreases during conditions like heart failure, as less oxygenated blood gets pumped into circulation. Inotropes are drugs that either weaken or strengthen the cardiac muscular contractions, with positive inotropes increasing — and negative ones, decreasing — contraction strength and, with it, stroke volume.
Many inotropic drugs effect changes in the level of calcium within the cardiac muscle cells. Calcium is necessary to the electrical activity that causes cardiac muscular contraction. Positive inotropes increase calcium levels in the cells, and are used to treat congestive heart failure. The calcium channel blockers and the beta blockers have a negative inotropic effect and reduce the heart rate. These two classes of inotrope are used to treat high blood pressure and its related conditions.
Sometimes, the neurotransmitter dopamine is used as an inotrope, as is the catecholamine hormone, epinephrine. If heart failure patients are experiencing swelling, called edema, dopamine helps relieve this problem by telling cells in the kidneys to clear out water. Dobutamine is an inotrope, often administered by intravenous drip, which increases the body's sensitivity to chemicals that raise the heart rate, like epinephrine and neurepinephrine. Over time, the body develops a tolerance to dobutamine and dopamine, requiring that their dosage be increased to continue the inotropic effects, which may disappear entirely over time.
Phosphodiesterase-iii inhibitors are another kind of inotrope. By blocking a certain metabolic enzyme, these drugs increase the strength of cardiac contractions. They are vasodilators also, which means that they cause the blood vessels to relax and allow more blood to flow through them and perfuse the body's tissues. Milrinone is a common variety of phosphodiesterase inhibitor. In severe congestive heart failure, inotrope IV treatment can be effective in relieving many debilitating symptoms of the disease, but researchers have noticed that some of these agents can increase the risk of death in patients.