Effects of nitroglycerin on regional myocardial ischemia induced by atrial pacing in dogs

The exact mechanism or mechanisms by which nitroglycerin exerts its beneficial effect on pacing-induced regional myocardial ischemia has not been elucidated previously. In open-chest, anesthetized canine preparation a fixed, flow limiting stenosis was applied to the left anterior descending (LAD) coronary artery and heart rate was increased by atrial pacing. Mass spectrometry was used to measure myocardial oxygen (PmO₂) and carbon dioxide (PmCO₂) tensions. Myocardial blood flow was measured by the radioactive microsphere technique. Application of the stenosis resulted in regional decreases in PmO₂ and increases in PmCO₂ of greater magnitude in the subendocardial than in the subepicardial layer. Atrial pacing resulted in a further decrease in PmO₂ and in increase in PmCO₂ as well as a reduction in subendocardial blood flow. Nitroglycerin (TNG) infusion reduced mean arterial pressure 20 mm Hg, resulting in a 14 mm Hg reduction in PmCO₂ in the more ischemic subendocardial layer (P < 0.05). Myocardial blood flow decreased in all regions; however, the magnitude of this decrease was less in the ischemic region. Addition of aortic constriction abolished both the afterload and preload lowering effects of nitroglycerin but improved ischemic zone blood flow. These data demonstrate that nitroglycerin reduces the severity of pacing-induced regional myocardial ischemia primarily by reducing the determinants of myocardial oxygen demand. We found that when these effects are counteracted, improvement in myocardial oxygen supply becomes the dominant mechanism.