Effects of hemodilution and phenylephrine on cerebral blood flow and metabolism during cardiopulmonary bypass

OBJECTIVE: Hypotension resulting from hemodilution on cardiopulmonary bypass is often treated by pressor (eg, phenylephrine) infusion. The effect of phenylephrine on cerebral blood flow (CBF) in this setting is not clear. It was hypothesized that phenylephrine might decrease CBF. MEASUREMENTS and MAIN RESULTS: Six different radioactively labeled microspheres (15 microm) were used to measure CBF at 6 time points (T) in 9 pigs (mean body weight 11.3 +/- 1.2 kg): T1 baseline before bypass (mean arterial pressure MAP] 76 +/- 5 mmHg), T2 on mildly hypothermic CPB (34 degrees C, pump flow 100 mL/kg/min, hematocrit 30%, MAP 79 +/- 7 mmHg), T3 after moderate hemodilution with crystalloid (hematocrit 20%, resulting MAP 62 +/- 6 mmHg), T4 after phenylephrine administration to increase MAP to baseline values (hematocrit 20%), T5 after severe hemodilution (hematocrit 10%, resulting MAP 41 +/- 4 mmHg), and T6 after phenylephrine administration to normalize MAP (hematocrit 10%). In addition, blood flow to liver, small bowel and skeletal muscle, and pH of jugular venous blood were measured at each time point. After institution of CPB, the CBF (mL/min/100 g tissue) increased significantly to 53 +/- 9 (baseline levels 44 +/- 8, T1 v T2, p = 0.03). Hemodilution resulted in significant increases in CBF on CPB to 65 +/- 9 and 90 +/- 9 at hematocrit 20% and hematocrit 10%, respectively (T2 v T3, p = 0.03; T3 v T5, p = 0.01) and a progressive fall in jugular venous pH. At each level of hemodilution, phenylephrine resulted in an additional increase in CBF (T4, 74 +/- 8; T6, 108 +/- 12; T3 v T4, p = 0.04; T5 v T6, p = 0.01) but did not improve jugular venous pH. Changes in liver blood flow after hemodilution and vasopressor injection showed a similar pattern to CBF. However, the blood flow to small bowel and skeletal muscle increased with hemodilution but decreased significantly with phenylephrine administration. CONCLUSIONS: Phenylephrine redirects blood flow from the bowel and muscle to the brain and liver. Hemodilution increases CBF and pressor administration further increases CBF by elevating perfusion pressure. Maintenance of a higher hematocrit on CPB increases MAP and should decrease the need for vasopressor administration.