Coronary Pressure-flow Relationship below the Range of Autoregulation

The effects of elevated aortic pressure on the pressure-flow (P/F) relationship below the pressure range of autoregulation were investigated. The effects of myocardial contraction dysfunction on coronary blood flow were also studied. In eight dogs, the left anterior descending coronary artery (LAD) was perfused by a bypass circuit from the subclavian artery. Coronary perfusion pressure (CPP) was reduced by mechanical stenosis of the bypass tube. Aortic pressure (AoP) was elevated by aortic banding. The heart rate was kept constant by SA block and pacing. Bypass flow (CoF), myocardial shortening (%SS) of the LAD region, and instantaneous diastolic P/F relation were obtained at each CPP and AoP level. When the stenosis was mild (CPP > 60 mmHg), CoF was increased by 1.2 ml/min by 10 mmHg elevation of AoP, but when the stenosis was severe (CPP < 50mmHg), CoF was reduced by 2.0ml/min by AoP elevation. The slope of P/F relation was reduced by the elevation of AoP in mild stenosis. In severe stenosis, AoP elevation increased the zero flow pressure (Pzf) and did not change the slope of the P/F relation. The %SS was significantly reduced with a rise of AoP only in severe stenosis. In five other dogs, phasic LAD flow was measured, using an ultrasonic transit time flowmeter just below the first diagonal branch. The branch was cannulated proximally to infuse adenosine (500μg/min) into the LAD to obtain maximal vasodilation. Lidocaine (0.5–4 mg) was injected into the LAD to alter the regional myocardial contraction.