ObjectiveTo compare the influence of gastric and postpyloric enteral feeding on the gastric tonometric PCO2gap (tonometric PCO2- PaCO2).DesignA prospective, clinical trial.SettingTwo intensive care units in a university hospital.PatientsTwenty patients undergoing mechanical ventilation and enteral feeding without catecholamines, sepsis, or sign of hypoxia.InterventionsPatients were randomized to receive feeding through the tonometer (gastric group), or through a postpyloric tube (postpyloric group).Measurements and Main ResultsThe patients received tube feeding at a rate of 50 mL/hr during 4 hrs. Baseline measurements included: mean arterial pressure, heart rate, tonometric parameters, arterial gases, and arterial lactate concentration. Except for lactate concentration, these measurements were repeated after 1 and 4 hrs of enteral feeding and 2 hrs after stopping enteral feeding. During the study, arterial pH and PaCO2did not change. During enteral feeding, the PCO2gap increased in the gastric group from a mean of 7 +/- 5 to 17 +/- 14 (SD) torr (0.9 +/- 0.7 to 2.3 +/- 1.9 kPa) (p < .01) and did not change in the postpyloric group (5 +/- 5 to 3 +/- 1 torr [0.7 +/- 0.7 to 0.4 +/- 0.1 kPa]). Two hours after stopping enteral feeding, the PCO2gap was still increased in the gastric group (15 +/- 9 vs. 7 +/- 5 torr [2.0 +/- 1.2 vs. 0.9 +/- 0.7 kPa]) (p < .01).ConclusionThe results indicate that gastric enteral feeding increased the PCO2gap. However, postpyloric enteral feeding does not interact with gastric tonometric measurements and should be used when using gastric tonometry in enterally fed patients. (Crit Care Med 1998; 26:1991-1994)