Failure of nitroglycerin to alleviate acute myocardial ischemia or improve collateral flow when MVO₂ is held constant in the heart lung preparation

Nitroglycerin (TNG) reduces S-T segment elevation after acute coronary occlusion in the intact dog, an effect not mediated by changes in arterial pressure (AP) or heart rate (HR). To determine whether increased coronary collateral flow contributed to this effect, canine heart lung preparations were studied at 5, 10, and 15 min following acute LAD occlusion. Total coronary flow (TCF), coronary A-V O₂ difference, and MVO₂ were measured continuously. Heart rate, AP, and MVO₂ were held constant, thereby excluding peripherally mediated TNG action. Retrograde flow (QR) was collected from the distal segment of the ligated LAD. Epicardial S-T segment elevation was measured with an 8 lead electrode positioned on the ischemic zone. Seven dogs served as controls and 4 dogs received TNG at 11 min (100 μg bolus, then 75 μg/min). The authors found that 1) the magnitude of Qr was a strong determinant of S-T segment change, suggesting that coronary collaterals protect against ischemic injury and 2) although TNG increased TCF from 105 to 143 ml/min (P<.02), it had no significant effect on Qr or S-T segment elevation. Since TNG does not alter collateral function or ischemic ECG changes when MVO₂ is constant, data suggest that TNG probably alleviates acute ischemia in the intact dog by altering the peripheral hemodynamic determinants of MVO₂ or Qr.