The Haldane effect - an alternative explanation for increasing gastric mucosal PCO2 gradients?

When venous oxygen saturation increases as a result of increased blood flow , changes in venous blood PCO2 and carbon dioxide content may differ becaus e of the Haldane effect. The Haldane effect may also explain increases in g astric mucosal-arterial PCO2 gradient despite major increases in splanchnic blood flow. We re-analysed data from 22 patients after cardiac surgery who were randomized to receive either dobutamine or placebo, and a separate gr oup of patients who received dobutamine for low cardiac output (n=6). Three different values of gastric mucosal oxygen extraction at baseline were ass umed (0.3, 0.5 and 0.7). In nine of 14 patients with both increasing splanc hnic blood flow and mucosal-arterial PCO2 gradient, an equal increase in mu cosal and total splanchnic blood flow, oxygen consumption and carbon dioxid e production together with the Haldane effect would have caused an increase in mucosal-arterial PCO2 gradients from a mean value of 0.53 (SD 0.88) kPa at baseline to 0.68-0.82 (0.89-0.90) kPa (P<0.01). In the remaining patien ts, disproportionate changes in flow and metabolism must have been involved in addition to the Haldane effect. We conclude that whenever major changes in mucosal tissue oxygen extraction are likely to occur, an increase in th e mucosal-arterial PCO2 gradient may be explained in part or completely by the Haldane effect, and may therefore not reflect worsening perfusion.