ANALYSIS OF P-50 AND OXYGEN-TRANSPORT IN PATIENTS AFTER CARDIAC-SURGERY

Objective: To determine whether standard P-50 after cardiac surgery de creases and whether decreased P-50 is related to the transfusion of re d blood cells (RBCs), acid-base changes, body temperature, oxygen para meters and/or duration of cardiopulmonary bypass (CPB). Design: Pilot study in cardiac surgery patients. Setting: University hospital. Patie nts: 12 consecutive elective cardiac surgery patients. Interventions: Blood was taken before surgery, after CPB and in the intensive care un it until 18 h postoperatively. Cardiac output and oxygen consumption w ere measured. Buffy coat-poor RBCs were transfused, anticoagulated wit h citrate-phosphate-dextrose buffer and stored in saline-adenine-gluco se-mannitol at 4 degrees C, when haemoglobin was <5.6 mmol . l(-1). Me asurements and results: Standard P-50 was calculated from measured par tial pressure of oxygen and of carbon dioxide, pH and oxygen saturatio n in mixed venous blood (SvO(2)) using the Severinghaus formula. Media n length of RBC storage was 25 days. Standard P-50 after surgery was s ignificantly lower than baseline value (p=0.0001). The number of RBC u nits transfused and duration of CPB were conjointly associated with P- 50 (R(2) = 0.72). Patients who received more RBCs consumed more oxygen . Conclusion: Cardiac surgery patients receiving more RBC units have l ower standard P-50 and consume more oxygen. P-50 decreased more when t he CPB took longer. Because a decrease in P-50 implies a low ratio of mixed venous oxygen tension (PvO(2)) to SvO(2), a shift in P-50 Should be taken into account when using SvO(2) as a measure of global oxygen availability. When a direct measurement of SvO(2) is not available, P vO(2) should be used instead of calculated SvO(2).