Cardiopulmonary bypass during cardiac surgery can result in a shortfall in oxygen delivery relative to demand, marked by a decrease in muscle tissue oxygen saturation as blood flow is redistributed to vital organs. Such "tissue shock" might impair postoperative recovery.To determine the association of changes in tissue oxygen saturation with postoperative outcome in cardiac surgery patients.In 74 adults undergoing cardiac surgery, tissue oxygen saturation in the thenar eminence was recorded using near-infrared spectroscopy before and during induction of anesthesia, throughout surgery, and in the intensive care unit until extubation or for a maximum monitoring time of 24 hours. The measurements were related to postoperative outcome.Mean tissue oxygen saturation increased from 81.7% to 88.5% with induction of anesthesia and decreased to 78.9% and 69.9% during surgery and on arrival in the intensive care unit, respectively. Saturation increased to 77.8% by 6 hours after surgery and remained stable. Mean saturation during the first minutes of anesthesia and 20 minutes in the intensive care unit was lower in patients with a postoperative morbidity than in patients without such morbidity on day 15 (81.1% vs 87.6%; P = .04) and on day 3 (72.9% vs 85.5%; P = .009). No associations with other outcome measures were observed.In patients undergoing cardiac surgery, reduced tissue oxygen saturation in the thenar eminence may be associated with poor postoperative outcome. Further studies are needed to confirm these findings and to determine whether measures to improve the balance between oxygen delivery and consumption might improve both tissue oxygen saturation and outcome.