Near infrared spectroscopy (NIRS) shows large changes in cerebral oxyhemoglobin (Hbo2), deoxyhemoglobin (Hb), and oxidation state of cytochromeaa3(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, Cyto2decreased markedly, whereas Hbo2increased. 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 that1) cooling is associated with increased oxygenation of cerebral hemoglobin despite a reduction in CBF;2) Cyto2becomes more reduced during cooling, consistent with a net cellular oxygen deficit; and3) DHCA is associated with rapid cerebral hemoglobin deoxygenation and a small further reduction of Cyto2.Abbreviations: CPB,cardiopulmonary bypass;CA,cardiac arrest;DHCA,deep hypothermic circulatory arrest;CBF,cerebral blood flow;MAP,mean arterial blood pressure;NIRS,near infrared spectroscopy;dpf,differential path length factor;Hbo2, oxygenated Hb;Hb,deoxygenated Hb;HbT,total Hb;Cyto2, oxidation state of cytochromeaa3;MRS,magnetic resonance spectroscopy;PCr,phosphocreatine;pHi, intracellular pH