Figure 172

Major actions of the nitrates on the ischemic heart and peripheral circulation. 4-, decrease; Î, increase; unchanged; Î4, variable effect.

coronary blood flow to this area. This effect of nitroglycerin on the distribution of coronary flow is important because the subendocardium is particularly vulnerable to ischemia during acute anginal attacks.

At higher concentrations, nitroglycerin also relaxes arteriolar smooth muscle, which leads to a decrease in both peripheral vascular resistance and aortic impedance to left ventricular ejection (decreased afterload). The decreased resistance to ventricular ejection may also reduce myocardial wall tension and oxygen requirements.

Thus, nitroglycerin relieves the symptoms of angina by restoring the balance between myocardial oxygen supply and demand. Oxygen demand is lowered as a consequence of the reduction in cardiac preload and af-terload, and this results in a decrease in myocardial wall tension. Oxygen supply to the subendocardium of isch-emic areas is increased because extravascular compression around the subendocardial vessels is reduced. In addition, nitroglycerin may increase blood flow to isch-emic areas by its direct vasodilator effect on eccentric epicardial coronary artery stenoses and collateral blood vessels and by its action to inhibit platelet aggregation. Other organic nitrates are thought to exert the same beneficial actions as nitroglycerin.

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