Cerebral oxygenation measured by near infrared spectroscopy during cardiopulmonary bypass and deep hypothermic circulatory arrest in piglets

Near infrared spectroscopy (NIRS) shows large changes in cerebral oxyhemoglobin (HbO₂), deoxyhemoglobin (Hb), and oxidation state of cytochrome aa₃ (Cyto₂) in infants undergoing cardiopulmonary bypass and deep hypothermic circulatory arrest (CPB-DHCA). To evaluate the physiologic significance of these clinical NIRS measurements, we applied the technique in a piglet model of CPB-DHCA. After an initial stabilization period on CPB, animals (n = 8) were cooled to 15°C, subjected to DHCA for 1 h, then reperfused with rewarming and monitored for 180 min. NIRS measurements were compared with determinations of cerebral blood flow (CBF). During cooling, Cyto₂ decreased markedly, whereas HbO₂ increased. DHCA was associated with a sharp decrease in HbO₂, a corresponding increase in Hb, and a smaller, less consistent further decrease in Cyto₂. NIRS measurements recovered toward baseline with reperfusion. CBF decreased during cooling and recovered to baseline levels with reperfusion. These findings are consistent with existing human data and show that 1) cooling is associated with increased oxygenation of cerebral hemoglobin despite a reduction in CBF; 2) Cyto₂ becomes more reduced during cooling, consistent with a net cellular oxygen deficit; and 3) DHCA is associated with rapid cerebral hemoglobin deoxygenation and a small further reduction of Cyto₂.