Support of mean arterial pressure during tepid cardiopulmonary bypass:: Effects of phenylephrine and pump flow on systemic oxygen supply and demand

Objective: To examine the effects of phenylephrine infusion and increases in pump flow on systemic oxygen supply and demand when they are used to support mean arterial pressure (MAP) during cardiopulmonary bypass (CPB). Design: Prospective, unblinded study. Setting: The animal cardiopulmonary laboratory at the Mayo Foundation (Rochester, MN). Participants: Twelve pigs. Interventions:Twelve pigs had systemic oxygen delivery ((D) over dot O-2) and consumption ((V) over dot O-2) measured before CPB and then underwent CPB at 35 degrees C. During CPB, measurements of (D) over dot O-2 and (V) over dot O-2 were obtained at an MAP of approximately 50 mmHg and a pump flow of 2.2 L/min/m(2). Thereafter, MAP was elevated to 70 mmHg either by increases in pump flow or by a phenylephrine infusion, and the balance between systemic oxygen supply and demand was reassessed. Measurements and Main Results: Before CPB, (D) over dot O-2 was 375 +/- 83 mL/min/m(2) and decreased with the onset of CPB mainly because of the effects of hemodilution. During CPB, with a pump flow of 2.2 L/min/m(2) and an MAP of 53 mmHg, (D) over dot O-2 was 218 +/- 40 mL/min/m(2). Increasing perfusion pressure to an MAP of 72 mmHg with phenylephrine and maintaining pump flow constant (2.2 L/min/m(2)) did not change (D) over dot O-2 (222 +/- 37 mL/min/m(2)), and the oxygen extraction ratio (OER) was increased relative to pre-CPB levels. In contrast, increasing MAP to 71 mmHg by increasing pump flow to 3.2 L/min/m(2) resulted in a significantly greater (D) over dot O-2 and the OER normalized to the pre-CPB value. Conclusions: During CPB with conventional flow rates, (D) over dot O-2 is decreased. supporting MAP with increases in pump flow better maintains (D) over dot O-2 than the administration of an alpha-agonist. Copyright (C) 1999 by W.B. Saunders Company.