The oxygen- and acid-base status during hypothermic cardiopulmonary bypass

Abstract

In ten patients undergoing coronary artery bypass grafting with hypothermic cardiopulmonary bypass (CPB) the oxygen- and acid-base status were evaluated. The CPB was performed with the pH-stat approach i.e. constant pH (=7.40) at the actual body temperature. The mean mixed venous pO₂ at 25 °C was 9.9 kPa (range 5.0 to 22.8) and the mean cBASE in the extracellular fluid was -1.9 mmol·l^-1. The mean oxygen uptake rate was 2.1 mmol·min^-1·m^-2 at 25 °C and changed 8 % per°C. The mean ratio between CO₂ elimination rate and O₂ uptake rate ranged from 0.70 to 0.93. It is concluded that the pH-stat approach to acid-base status during moderate hypothermia and CPB is safe and that the ratio between oxygen uptake and oxygen delivery may be more appropriate than during the α—stat approach, but knowledge of the fundamental mechanisms influencing the transport of oxygen to tissues during periods of reduced body temperature is still lacking.

Key-words:

• α-stat
• BASE EXCES S
• blood gases
• carbon dioxide
• cBASE
• extracorporeal circulation
• hydrogen ion
• oxygen
• pH
• pH-stat
• temperature correction