DISTRIBUTION OF SYSTEMIC BLOOD-FLOW IN LAMBS WITH AN AORTOPULMONARY SHUNT DURING STRENUOUS EXERCISE

We studied regional blood flows with radioactive-labeled microspheres in 12 7-wk-old lambs with an aortopulmonary left-to-right shunt [59 +/- 3% (SE) of left ventricular (LV) output] and in 11 control lambs, at rest and during exercise at 80% of predetermined peak O2 consumption. At rest, systemic blood flow was similar in the two groups. Blood flow to the heart and diaphragm was substantially higher in the shunt than in the control lambs. Blood flow to the other organs was not significantly different between the two groups. During exercise, systemic blood flow increased substantially but less in shunt (81%) than in control lambs (134%). Blood flow to the heart and diaphragm increased, that to the heart still being higher in shunt than in control lambs. Blood flow to the brain did not change, whereas that to the kidneys and splanchnic organs decreased to the same extent (25%) in shunt and control lambs. Intrahepatic and intrarenal blood flow redistribution in the shunt lambs persisted during exercise. In conclusion, myocardial blood flow is not increased at the expense of one particular organ, nor is it associated with an essential change in exercise-induced redistribution in shunt lambs

LEFT-VENTRICULAR OXYGEN AND SUBSTRATE UPTAKE IN CHRONICALLY HYPOXEMIC LAMBS

Myocardial oxygen demand may be increased in chronically hypoxemic lambs because of their increased heart rate. Therefore, we determined whether left ventricular (LV) oxygen supply, oxygen uptake and oxygen demand were proportionally increased in 6-wk-old lambs, after 4 wk of hypoxemia (n = 15), as compared with control lambs (n = 14). In addition, we determined whether LV glucose, pyruvate, lactate, FFA and ketoacids uptake were altered in hypoxemic lambs, because of alterations in arterial glucose, pyruvate and lactate concentrations, that may occur in hypoxemia. Hypoxemia was induced by the combination of an atrial septal defect and pulmonary stenosis. Arterial oxygen saturation was decreased in hypoxemic lambs (67 +/- 8 versus 91 +/- 3%, p <0.001), Hb concentration was increased, so that arterial oxygen concentration was similar in both groups of lambs. Myocardial mass (61 +/- 13 versus 44 +/- 9 g, p <0.001) and total myocardial blood flow (117 +/- 36 versus 62 +/- 27 mL . min-1, p <0.001) were increased, mainly due to right ventricular hypertrophy. LV oxygen demand, estimated by the rate pressure product (2072 +/- 465 versus 1467 +/- 358 kPa . beat . min-1, p <0.001), and oxygen uptake (723 +/- 223 versus 556 +/- 184 mumol . min-1 . 100 g-1, p <0.05) were proportionally increased in hypoxemic lambs. LV oxygen supply increased linearly with oxygen uptake (r = 0.96) in all lambs, by adjustments in LV blood flow, which was increased in hypoxemic lambs (168 +/- 41 versus 134 +/- 45 mL . min-1 . 100 g-1, p <0.05). The increase in 1,V oxygen uptake in hypoxemic lambs was proportional to the increase in heart rate (166 +/- 33 versus 118 +/- 25 beats. min-1, p <0.001). Arterial lactate, pyruvate and beta-hydroxybutyrate concentrations were slightly increased in hypoxemic lambs, but LV substrate uptake was practically unaltered as compared with control lambs. FFA and beta-hydroxybutyrate contributed most to LV substrate uptake, whereas the contribution of glucose, pyruvate and lactate was negligible. The total oxygen extraction ratios (0.45 +/- 0.43 versus 0.51 +/- 0.50) indicate that approximately 50% of the fuels for the LV were identified. We conclude that LV oxygen supply is matched to increased oxygen demand in chronically hypoxemic lambs, by the increase in LV blood flow. LV substrate uptake is unaltered in hypoxemic lambs; glucose, pyruvate and lactate uptake is negligible, despite an increased arterial pyruvate and lactate concentration. FFA and ketoacid uptake are insufficient to fuel LV oxidative metabolism