In forty-two children undergoing cardiac surgery using hypothermic cardiopulmonary bypass (CPB), a gastric tonometer was used intraoperatively to estimate pHi, reflection of splanchnic perfusion. PHi monitoring was used to predict early postoperative complications. Intramucosal acidosis appeared after removal of the aortic clamp and remained after weaning of CPB. Fifteen children (group C) developed early postoperative life-threatening complications. Twenty-seven children (group NC) had no major complications. Retrospective data analysis showed a decrease of pHi during hypothermic CPB in the two groups but at completion of surgery, pHi was significantly lower in the group C (7.13 +/- 0.04 vs 7.31 +/- 0.001, P < 0.001). Bypass and aortic clamp times were also longer in group C and intraoperative core temperature lower. Continuous automated tonometer gas analysis appeared more accurate for the monitoring of acute haemodynamic and respiratory changes and should probably improve the predictive value of tonometry in the future. Tonometry is highly sensitive and represents an interesting and minimally invasive monitoring for critically ill children.