The effects of nitroglycerin on the coronary blood flow and oxygen consumption of the myocardium in anesthetized dogs

With an open-chest dog preparation, which makes it possible to measure simultaneously the cardiac output, the coronary flow and the systomic arterial pressure, the actions of nitroglycerin on the coronary circulation were invcstigated. The results show that nitroglycerin, injected as a single intravenous dose or by slow infusion, causes a reduction of the arterial blood pressure and of cardiac work, an increase of the coronary blood flow, and a reduction of oxygen consumption in the myocardium proportional to the reduced cardiac work. However, when the cardiac frequency and arterial pressure are kept constant, nitroglycerin does not modify the oxygen consumption of the myocardium, while it increases the coronary flow very markedly. Also, when nitroglycerin is injected after increasing the coronary flow and the oxygen consumption of the myocardium with noradrenalin, the drug causes a definite reduction of the mean arterial pressure and of oxygen consumption with no changes in coronary flow. When the arterial pressure and cardiac frequency are kept constant, nitroglycerin does not modify the effects of noradrenaline on the oxygen consumption rate of the myocardium. It ean therefore be concluded that nitroglycerin acts on the coronary circulation of the dog by lowering the arterial blood pressure, and by reducing the oxygen consumption of the myocardium as a consequence of the reduced cardiac work rather than by direct action. However, unlike other hypotensive drugs, nitroglycerin does not reduce the coronary flow as it does the arterial pressure because it dilates the coronary vessels concurrently.