There is little doubt concerning the effectiveness of ni-troglycerin in the treatment of angina pectoris.
However, the exact mechanism by which the drug acts to reduce myocardial ischemia is still controversial (Fig. 17.2).Although nitroglycerin dilates both peripheral capacitance and resistance vessels, the effect on the venous capacitance system predominates. Dilation of the capacitance vessels leads to pooling of blood in the veins and to diminished venous return to the heart (decreased preload). This reduces ventricular diastolic volume and pressure and shifts blood from the central to the peripheral compartments of the cardiovascular system. These effects of nitroglycerin and other organic nitrates are similar to those of mild phlebotomy, which has been shown clinically to relieve acute anginal attacks by decreasing circulating blood volume.
According to Laplace's law, a reduction in ventricular pressure and heart size results in a decrease in the myocardial wall tension that is required to develop a given intraventricular pressure and therefore decreases oxygen requirement. Since blood flow to the subendo-cardium occurs primarily in diastole, the reduction in left ventricular end diastolic pressure induced by nitro-glycerin reduces extravascular compression around the subendocardial vessels and favors redistribution of
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