A Comparison of Gastric and Rectal CO₂ in Cardiac Surgery Patients

Critical care nurses assess and treat clinical conditions associated with inadequate oxygenation. Changes in regional organ (gut) blood flow are believed to occur in response to a decrease in oxygenation. Although the stomach is a widely accepted monitoring site, there are multiple methodological and measurement issues associated with the gastric environment that limit the accuracy of P CO2 detection. The rectum may provide nurses with an alternative site for monitoring changes in PCO2 without the limitations associated with gastric monitoring. This pilot study used a repeated measures design to examine changes in gastric and rectal PCO2 during elective coronary artery bypass grafting with cardiopulmonary bypass (CPB) and in the immediate 4-hr postoperative period in 26 subjects. The systemic indicators explained little variation in the regional indicators during protocol. A comparison of rectal and gastric PCO2 revealed no statistically significant differences in the direction or magnitude of change over any phase of cardiac surgery (baseline, CPB, post-CPB). A reduction in both rectal and gastric PCO2 occurred during CPB, and both values trended upward during the post-CPB phase. However, poor correlation and agreement was found between the measures of PCO2 at the two sites. Although clinically important, the cause is unclear. Possible explanations include variation in CO2 production between the gastric and rectal site, differences in sensitivity of the two monitoring instruments, or the absence of hemodynamic complications, which limited the extent of change in PCO2. Further investigation using patients with more profound changes in oxygenation are needed to identify response patterns and possible mechanisms.