The relationships between cell metabolism and both hemodynamics and oxygen transport/utilization (Vo2/Do2) pattern were evaluated intra and postoperatively in eight patients undergoing major valvular heart surgery with the aid of moderately hypothermic cardiopulmonary bypass (CPB). Quadriceps femoris specimens were obtained by the needle biopsy technique for muscle ATP, ADP, AMP, phosphocreatine (PCr), creatine and lactate determination at anesthesia induction, after CPB, as well as in the ICU 18 h after surgery. Moreover, hemodynamic variables, oxygen transport and utilization indices, and plasma lactate were measured at the same intervals and throughout the CPB period.After CPB, muscle ATP and PCr contents were reduced (p< .05) as compared to those of both pre-CPB patients and healthy control subjects; muscle and plasma lactate levels were increased (p< .05). Mean Vo2and Do2values measured during CPB significantly decreased (p< .05), but Vo2reduction was proportionally greater than that of Do2(-62% vs. −41%). No correlation was found between Vo2and Do2at that time, but a significant relationship (p< .05) was found at the end of CPB. A further decrease in muscle ATP and PCr levels was measured in the ICU, as muscle and plasma lactate levels were still elevated. At that time, Vo2and Do2were not significantly different from pre-CPB values, but were significantly (p< .05) correlated with each other.Our investigation suggests that in patients undergoing major valvular heart surgery, the use of a moderately hypothermic low-flux, low-pressure CPB can be associated with a condition of inadequate tissue oxygenation, and that the oxygen deficit persists into the postoperative recovery period, leading to marked alterations of cell oxidative metabolic pathways and to the activation of anaerobic metabolism.