Effect of sympathomimetic amines on distribution of blood flow in the normal and in the ischemic myocardium (A experimental study using cross-thermocouples)

Effects of sympathomimetic amines on the regional blood flow (RMBF) in the intact and ischemic myocardium of the left ventricular free wall were studied using thirty-nine anesthetized open-chest dogs. The regional myocardial blood flow in the inner third-(subendocardial) and the outer third-(subepicardial) layer of the left ventricular wall was continuously monitored by heated cross-thermocouples designed according to Grayson's heat exchange method. The left circumflex coronary artery was constricted by a screw type constrictor until myocardial reactive hyperemia following 15 second-occlusion of the vessel nearly disappeared. The drugs were injected into the right femoral vein. The results were as follows: 1) No significant difference in RMBF between the inner and outer-layer was shown in the normal myocardium. After coronary constriction, the flow ratio through the inner to the outer-layer (I/O ratio) in the ischemic myocardium decreased. 2) Adrenaline (0.2μg/kg) produced the flow increment through the inner- as much as the outer-layer in the normal myocardium. In the ischemic myocardium, adrenaline produced changes of RMBF in the same manner. 3) Noradrenaline (0.4μg/kg) produced the flow increment in the inner- and outer-layer of the normal myocardium. I/O ratio was not changed. In the ischemic myocardium, the flow through the inner-layer was not increased, while the flow through the outer-layer was increased. I/O ratio was reduced. 4) Isoproterenol (0.2μg/kg) produced an increase in RMBF through both layers in the normal myocardium, whereas I/O ratio was reduced for less increase in the flow through the inner layer. In the ischemic myocardium, RMBF in the both layers were reduced according to a fall in aortic blood pressure. I/O ratio in the ischemic myocardium was further lowered after isoproterenol. 5) Methoxamine (0.1mg/kg) produced no significant change of I/O ratio in normal myocardium, since the blood flow through the outer-layer was increased as much as the inner-layer. In ischemic myocardium, however, I/O ratio was slightly increased with a rise in aortic blood pressure and a decrease in heart rate. These results might suggest that heart rate and/or blood pressure have closed relations to the flow through the inner-layer, especially in the ischemic myocardium