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 (DO2) and consumption (VO2) measured before CPB and then underwent CPB at 35 degrees C. During CPB, measurements of DO2 and VO2 were obtained at an MAP of approximately 50 mmHg and a pump flow of 2.2 L/min/m2. 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, DO2 was 375 +/- 83 mL/min/m2 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/m2 and an MAP of 53 mmHg, DO2 was 218 +/- 40 mL/min/m2. Increasing perfusion pressure to an MAP of 72 mmHg with phenylephrine and maintaining pump flow constant (2.2 L/min/m2) did not change DO2 (222 +/- 37 mL/min/m2), 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/m2 resulted in a significantly greater DO2, and the OER normalized to the pre-CPB value.

CONCLUSIONS

During CPB with conventional flow rates, DO2 is decreased. Supporting MAP with increases in pump flow better maintains DO2 than the administration of an alpha-agonist.