Cardiac output augmentation during hypoxemia improves cerebral metabolism after hypothermic cardiopulmonary bypass

Background. Hypothermic circulatory arrest (HCA) impairs cerebral oxygen delivery (CDO₂) and cerebral oxygen consumption (CMRO₂), which are further reduced by perioperative hypoxemia. This study investigates if continuous hypothermic low-flow cardiopulmonary bypass (HLF) or intermittent hypothermic low-flow cardiopulmonary bypass (IHLF) can prevent reductions in CDO₂ and CMRO₂ during hypoxemia. Methods. Eighteen neonatal piglets, cooled to 16° to 18°C with cardiopulmonary bypass (CPB), were randomly assigned into three groups: HCA, HLF (50 cc·kg ^-1·min^-1), or IHLF (1 minute of HLF for every 15 minutes of HCA). After 60 minutes of hypothermia, normothermic CPB (100 cc·kg^-1·min^-1) was established and cerebral perfusion data measured at hyperoxemia (PaO₂ 150 to 250 mm Hg), hypoxemia (PaO₂ 50 to 60 mm Hg), and severe hypoxemia (PaO ₂ 30 to 40 mm Hg), and with increased CPB flow (200 cc·kg ^-1·min^-1) during severe hypoxemia. Results. The CMRO₂ (in mL O₂·100 g^-1·min ^-1) was lower after HCA (2.5 ± 0.3), compared with HLF (4.1 ± 0.5, p = 0.02) and IHLF (6.2 ± 0.8, p = 0.002). Within groups, the change from hyperoxemia to severe hypoxemia resulted in decreased CMRO ₂: HCA (1.3 ± 0.2, p = 0.004), HLF (3.0 ± 0.5, p = 0.01), and IHLF (2.9 ± 0.5, p = 0.01). During severe hypoxemia, increasing CPB flow (from 100 cc·kg^-1·min^-1 to 200 cc·kg^-1·min^-1) improved CMRO ₂: HCA (1.9 ± 0.5, p = 0.05), HLF (4.2 ± 0.5, p = 0.05), and IHLF (7.4 ± 0.5, p = 0.04). Conclusions. Hypoxemia reduces CDO₂ and CMRO₂ despite the method of hypothermic CPB. Increased CPB flow during hypoxemia can restore both CDO₂ and CMRO₂ to values found with hyperoxemia and slower CPB flows. Augmenting cardiac output during periods of perioperative hypoxemia may prevent cerebral injury after exposure to hypothermic cardiopulmonary bypass.