Pharmacologic vasodilators in the coronary circulation

Pharmacologic vasodilators are used to treat hypertension, myocardial ischemia, and congestive heart failure. These drugs may indirectly affect coronary blood flow by their effects on the peripheral circulation, effects that modify myocardial oxygen demand. They also have direct effects on the coronary circulation, with some drugs affecting mainly the large epicardial coronary arteries and others affecting mainly the small resistance vessels. Some coronary stenoses are compliant and are capable of changes in caliber. Drugs that dilate large coronary arteries, such as nitroglycerin and the calcium channel-blocking agents, can dilate these stenotic segments. This dilation may contribute to the relief of myocardial ischemia by increasing coronary blood flow. Alternatively, drugs that dilate resistance vessels without an effect on large coronary arteries may cause passive narrowing or collapse of coronary stenoses and could potentially worsen myocardial ischemia. Therefore, in the presence of coronary stenoses, a vasodilator may be beneficial or harmful depending, at least in part, on the site of action of the drug in the coronary circulation.