What Is a Positive Inotrope?

An inotrope is a drug which increases or reduces muscle contraction. Two main types of inotropes exist, and these are described as positive and negative. A positive inotrope increases muscle contraction, while negative inotropes reduce it. Typically, positive inotropes are used to treat cardiac conditions in which the heart is not pumping blood around the body efficiently. A positive inotrope affects the heart by binding to what are called beta1 receptors on cardiac muscle.

There are numerous uses of inotropes, both positive and negative. The positive inotrope known as dobutamine is most frequently used to treat heart failure. Adrenaline, another positive inotrope, is used in emergencies such as a severe allergic reaction. It is sometimes used in the treatment of heart attacks when the heart stops beating and during cardiopulmonary resuscitation, when a patient's breathing and heart beat may have ceased. Negative inotropes include drugs such as beta blockers, which are used to treat high blood pressure and the condition known as angina, in which the heart muscle has an inadequate blood supply.

The side effects of inotropes result from their stimulation of other receptors, apart from those which are providing the drug's useful effects. There are three main receptor types in the sympathetic nervous system, which are known as alpha receptors, beta1 and beta2 receptors. Dobutamine can cause blood vessels to dilate due to its effect on beta2 receptors. When beta2 receptors are stimulated, this leads to muscle relaxation in the walls of respiratory passages and blood vessels. In the case of adrenaline, high doses cause blood vessels to constrict, due to the drug's action on alpha receptors.

When used to treat heart failure, a positive inotrope is normally given as an infusion into a major vein. If the drug is administered into a smaller vein in an extremity, there is the possibility that blood vessel constriction could occur, cutting off the blood supply to an area of tissue. Starting with a low dose, the amount given is gradually increased until the drug takes effect. Sometimes the fact that the heart is working harder leads to a higher demand for oxygen, and patients are monitored to ensure that they have adequate oxygen levels in the blood. The demand for oxygen can be reduced by eliminating stressful factors such as pain and by arranging the environment so that the patient is required to move as little as possible.