Transient oxygen uptake during myocardial reactive hyperemia in the dog

Oxygen uptake during myocardial hyperemia (MRH) following occlusions of 2.5-30 s was studied in nine anesthetized open-chest dogs by continuous measurement of left anterior coronary blood flow and anterior coronary vein oxygen saturation with electromagnetic flowmeter and fiber-optic catheters, respectively. The ratio of excess oxygen uptake to debt multiplied by 100% was defined as the oxygen repayment ratio (RR) and varied between -50% and +150% (mean 78%). Application of a steady-state Fick formula, as used by previous investigators, results in a profound overestimate of the RR (70-400%, mean 295%). RR is severely distorted by the venous catheter system, and after consideration of these effects we would conclude that the "true" RR in MRH is even lower than we found. Comparing our results with steady-state exercise autoregulation leads us to the following conclusions: 1) chemical kinetic limitations in oxygen release by red blood cells are excluded, 2) a simple direct PO2 regulation of flow is unlikely, but 3) our results are not inconsistent with a more complex integral oxygen regulation or with the metabolic hypothesis.