Effect of nifedipine on regional O₂ consumption in ischemic myocardial tissue

The effect of intravenous (i.v.) nifedipine (5 μg/kg/min) on regional O₂ supply/consumption variables was determined in 14 anesthetized open-chest dogs subjected to left anterior descending coronary artery (LAD) stenosis sufficient to reduce blood flow 50%. Myocardial blood flow was measured using radioactive microspheres, and regional arterial and venous O₂ saturations were determined using microspectrophotometry. During LAD occlusion, blood flow and O₂ consumption were decreased in the ischemic region while O₂ extraction was increased. With nifedipine treatment, LAD flow (flow probe) was increased 44% as compared with the initial occluded value, with most of this increase going to the subepicardium (microspheres). Flow to the ischemic subendocardium was not changed with nifedipine treatment. Ischemic subepicardial O₂ consumption increased slightly in nifedipine-treated animals as compared with saline values, whereas O₂ extraction decreased slightly. In the subendocardial region, nifedipine resulted in a significant decrease in O₂ extraction, with a slight decrease in O₂ consumption as compared with saline controls. The O₂ supply/demand ratio was significantly improved with nifedipine only in the subendocardium of the ischemic region. This suggested that nifedipine could increase O₂ consumption in the ischemic subepicardium through proportional increase in O₂ supply while it decreased O₂ consumption in the subendocardium relative to O₂ supply.