Cardiovascular disease

In cardiac failure, cardiac glycosides are used for their inotropic effect on the heart, but vasodilators and drugs that increase urine flow are also helpful. The reduced cardiac output resulting from heart failure leads to an increase in pressure in the veins and also to accumulation of tissue fluid (edema). Vasodilators, such as the calcium antagonist nifedipine, dilate the veins and thus lower the venous pressure, and they also increase the cardiac output by reducing the resistance of the arterial system. Diuretic drugs are used to reduce the amount of tissue fluid, but they can also have a beneficial vasodilator effect. These drugs are used in treating high blood pressure as well.

High arterial blood pressure, which is produced by excessive constriction of small arteries, is often treated with drugs. Some drugs that lower blood pressure (hypotensives) inhibit the function of the sympathetic nervous system in various ways. Hypotensive drugs include methyldopa and clonidine, which probably work at the level of the central nervous system; reserpine and guanethidine, which prevent the release of norepinephrine by sympathetic nerves; adrenoceptor-blocking drugs (e.g., propranolol, which lowers blood pressure by reducing the cardiac output; and prazosin, which blocks the vasoconstrictor action of norepinephrine). Calcium antagonists also have a use in treating hypertension, as do other vasodilators such as hydralazine. A different approach, developed in the late 1970s, consists of using an inhibitor of adrenocortical extract (captopril), thus blocking the formation of angiotensin II. This is very effective in certain types of hypertension in which rennin secretion is increased. Most antihypertensive drugs have a variety of unwanted effects, such as drowsiness, dizziness on standing (due to an excessive postural fall in arterial pressure), impotence, and allergic reactions. Though fairly often, side effects are a serious problem because of the long-term nature of antihypertensive therapy, and better drugs are constantly being sought.

Migraine is a common condition associated with severe headaches that are believed to result from excessive dilation of the arteries in the membranous covering (meninges) surrounding the brain. The cause is not known, but it is believed to involve the local release of a substance called serotonin. Ergotamine, which comes from a fungus (ergot) that infests cereal crops and has a powerful vasoconstrictor effect, is widely used to treat migraine. Accidental poisoning with the ergot fungus produces many symptoms associated with excessive vasoconstriction, including brain disturbances and gangrene, but they do not generally occur when it is used therapeutically. Other antimigraine drugs include propranolol and calcium antagonists. Tests have shown them to be effective, but it is not clear how they work.

Partial occlusion of the coronary vessels by fatty deposits (atheroma) or blood clots may result in angina pectoris, a pain that occurs when the blood supply to the heart is inadequate for its needs. Vasodilator drugs, particularly nitroglycerine tablets and calcium antagonists, are often used to relive this condition. They work in part by dilating arteries and veins, which reduce arterial blood pressure and cardiac output, thereby lowering the work and oxygen consumption of the heart. They also have some effect on the coronary vessels themselves and may direct blood toward the regions in which the flow is impaired. Propranolol is also effective because it reduces the rate and force of the heart, thus lowering its oxygen requirement.