Interaction of hindlimb blood flow,A-V shunting and A-V oxygen difference

The interactions of blood flow, A-V O₂ difference (AVDO₂), and A-V shunting were measured in normal hindlimbs of nine anesthetized dogs. An aorto-iliac nonpulsatile perfusion pump was used to change femoral artery blood flow from zero (collateral flow only) to twice its baseline level. Femoral AVDO₂ was measured by in-line spectrophotometric O₂ analysis. A-V shunting was measured with radio-labeled microspheres. Systemic hemodynamic parameters and temperature remained constant during the experiments. Despite changes in femoral mean arterial pressure (160 to 54 mm Hg) and AVDO₂ (1.8 to 8.2 ml O/₂/dl) that occurred with femoral blood flow reduction, peripheral A-V shunting remained constant at 4.1–5.5%. Alpha-adrenergic ablation (sympathectomy) was used to increase A-V shunting (up to 20%) during part of this experiment. When hindlimb blood flow was normal or increased, autoregulation of O₂ extraction maintained constant hindlimb O₂ consumption, despite sympathectomy-induced changes in A-V shunting. Subnormal femoral artery blood flow reduced hindlimb O₂ consumption, and in this setting the increased A-V shunting further decreased femoral AVDO₂ and O₂ consumption. Since AVDO₂ is dependent upon both blood flow and the variable efficiency of cellular O₂ extraction, it cannot be used as an accurate indicator of A-V shunting. Direct microsphere techniques should be applied to A-V shunt measurements in clinical settings where A-V shunting is suspected.