CARDIOVASCULAR DRUGS

These drugs may be divided into three groups: drugs that affect the heart; drugs that affect blood pressure; and drugs that prevent blood clotting.

Drugs That Affect the Heart. Drugs may affect the heart in two major ways: changing the rate and forcefulness of the heartbeat and altering the rhythm of the heartbeat.

The most common drugs used to change the rate and forcefulness of the heartbeat are the digitalis glycosides (cardiac glycosides). These drugs are used to treat patients in heart failure (when the heart is not contracting with sufficient force). Most of the digitalis glycosides are obtained from the leaf of the digitalis (foxglove) plant, either as a crude mixture or as the purified glycoside from the leaf of the plant.

The important effects of the digitalis glycosides are the strengthening of the myocardium (heart muscle) and the slowing of the rate of contrac­tion of the heart. Examples of digitalis glycosides are: digitalis, digoxin, and digitoxin.

Other drugs, which belong to the general class of sympathomimetics, are used to increase heart rate and the force of contraction. These include isoproterenol and epinephrine.

Drugs used to correct abnormal heart rhythm are called antiarrhythmics. Examples of these drugs are quinidine, procainamide, lidocaine (Xylocaine), and propranolol. These drugs help restore the heart rhythm to a regular cycle by depressing ectopic (outside, unwanted) myocardial impulses. Quinidine comes from the bark of the cinchona tree and is the primary drug used to treat arrhythmias. Quinidine decreases the number of times the heart muscle can contract in a given period of time. The cocaine derivatives procainamide and lidocaine are also useful in controlling abnormal cardiac rhythms.

Drugs That Affect Blood Pressure. Vasodilators are drugs which relax the muscles of vessel walls, thus increasing the size of blood vessels. These drugs are used in treating blood vessel diseases, heart conditions, and high blood pressure (hypertension). Blood flows more freely and blood pressure falls as blood vessels open and become dilated. Examples are sympatholytics (reserpine, guanethidine, and alpha-methyldopa) and other agents such as hydralazine.

Nitrites are drugs which are also used as vasodilators. Examples of nitrite drugs are glyceryl trinitrate (nitroglycerin) and amyl nitrite. Nitroglycerin dilates all smooth (involuntary) muscles in the body, but has a greater effect on the muscles of the coronary blood vessels. The relaxation of the muscle fibers around the blood vessels of the heart in­creases the width of these heart vessels and increases blood flow to the heart muscle. The pain (angina pectoris) caused by a lack of adequate blood flow to the heart is relieved by placing nitroglycerin under the tongue; from there the drug is quickly absorbed into the bloodstream. The other nitrite drugs work in a manner similar to that of nitroglycerin.

A third type of drug used to lower blood pressure is called a diuretic, an agent which promotes excretion of fluid and a shrinkage of the volume of blood within the vessels. An example of this type of drug is chlorothiazide (Diuril),

Vasoconstrictors are drugs which constrict muscle fibers around blood vessels and narrow the size of the vessel opening. They may act directly on the muscles of blood vessels or stimulate a region in the brain which relays the message to the vessels. Vasoconstrictors are needed to raise blood pressure, increase the force of heart action, and stop local bleeding. Examples of vasoconstrictor drugs are epinephrine (adrenaline), vasopressin.

Drugs that prevent blood clotting. These drugs are called antico­agulants. They are used to prevent the formation of clots in veins and arteries. These clots may cause occlusion (thrombosis) of the blood supply to a vital organ, such as the brain, or may travel from their point of ori­gin to a new site and produce a sudden occlusion of a distant organ (em­bolism). Anticoagulant drugs are also used to prevent coagulation in preserved blood stored for transfusions.

Heparin is an anticoagulant chemical substance found normally in human cells in the liver and lung. However, heparin can be made synthe­tically for commercial preparations by extracting it from the lungs of animals. When given intravenously or intramuscularly, heparin prevents the formation of clots within vessels. Another anticoagulant is bishydroxycoumarin (Coumadin and Dicumarol). Coumadin is given orally and is effective in preventing the for­mation of new clots, as well as retarding the extension of those already formed.