Retrograde Aortic Perfusion by Partial Cardiopulmonary Bypass: Effect of Mixing on Measured Arterial Oxygen Tension

Partial venoarterial bypass with return of oxygenated blood to the femoral artery can produce mixing in the aorta of the blood from the oxygenator and left ventricular sources at flow rates of less than 50% of the total body flow. This was observed in six anesthetized sheep with normal lungs maintained on controlled ventilation on partial cardiopulmonary (venoarterial) bypass. The evidence of mixing increases with the extracorporeal flow rate, reaching the aortic arch in a majority of cases at the higher rates. Management of bypass and the interpretation of its effects depend on definite knowledge of the presence or absence of mixing. Partial bypass will raise the arterial oxygen tension (Pao₂) of the blood continuing to flow through the lungs. This effect is independent of mixing (Pao₂) and may be related either to the addition of oxygen to the blood or to an enhanced pulmonary ventilation-perfusion relationship.