title: Analysis of P50 and oxygen transport in patients after cardiac surgery.
abstract: OBJECTIVE
  To determine whether standard P50 after cardiac surgery decreases and whether decreased P50 is related to the transfusion of red blood cells (RBCs), acid-base changes, body temperature, oxygen parameters and/or duration of cardiopulmonary bypass (CPB).
  
  
  METHODS
  Pilot study in cardiac surgery patients.
  
  
  METHODS
  University hospital.
  
  
  METHODS
  12 Consecutive elective cardiac surgery patients.
  
  
  METHODS
  Blood was taken before surgery, after CPB and in the intensive care unit until 18 h post-operatively. Cardiac output and oxygen consumption were measured. Buffy coat-poor RBCs were transfused, anticoagulated with citrate-phosphate-dextrose buffer and stored in saline-adenine-glucose-mannitol at 4 degrees C, when haemoglobin was < 5.6 mmol.l-1.
  
  
  RESULTS
  Standard P50 was calculated from measured partial pressure of oxygen and of carbon dioxide, pH and oxygen saturation in mixed venous blood (SvO2) using the Severinghaus formula. Median length of RBC storage was 25 days. Standard P50 after surgery was significantly lower than baseline value (p = 0.0001). The number of RBC units transfused and duration of CPB were conjointly associated with P50 (R2 = 0.72). Patients who received more RBCs consumed more oxygen.
  
  
  CONCLUSIONS
  Cardiac surgery patients receiving more RBC units have lower standard P50 and consume more oxygen. P50 decreased more when the CPB took longer. Because a decrease in P50 implies a low ratio of mixed venous oxygen tension (PvO2) to SvO2, a shift in P50 should be taken into account when using SvO2 as a measure of global oxygen availability. When a direct measurement of SvO2 is not available, PvO2 should be used instead of calculated SvO2.
authors.affiliation: Department of Intensive Care, Onze Lieve Vrouwe Gasthuis, Amsterdam, The Netherlands.
authors.forename: H M
authors.initials: HM
authors.lastname: Oudemans-van Straaten
authors.forename: G J
authors.initials: GJ
authors.lastname: Scheffer
authors.forename: C P
authors.initials: CP
authors.lastname: Stoutenbeek
mesh_terms: D000704:Analysis of Variance; D001831:Body Temperature; D002302:Cardiac Output; D006348:Cardiac Surgical Procedures; D002315:Cardiopulmonary Bypass; D017707:Erythrocyte Transfusion; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D008875:Middle Aged; D010100:Oxygen / Q000097:blood*; D010101:Oxygen Consumption; D011184:Postoperative Period
chemical_list: D006454:Hemoglobins; D010100:Oxygen