['Gastric mucosal ischemia may be a risk factor for gastrointestinal intolerance to early feeding in the critically ill.', 'To study intragastric PCO2 air tonometry and gastric residual volumes (GRV) before and after the start of gastric feeding.', 'This is a two-center study in intensive care units of a university and teaching hospital. Twenty-nine critically ill, consecutive and consenting patients scheduled to start gastric feeding were studied after insertion of a gastric tonometry catheter and prior to and after start of gastric feeding (500\xa0ml over 1\xa0h), when clinically indicated.', 'Blood gasometry and intragastric tonometry were performed prior to and 2\xa0h after gastric feeding. The intragastric to arterial PCO2 gap (normal <8\xa0mm Hg) was elevated in 41% of patients prior to feeding and measured (mean\xa0±\xa0standard deviation) 13\xa0±\xa020 and 16\xa0±\xa023\xa0mm Hg in patients with normal (<100\xa0ml, 42\xa0±\xa034\xa0ml, n\xa0=\xa019) and elevated GRV (250\xa0±\xa0141\xa0ml, n\xa0=\xa010, P\xa0=\xa00.75), respectively. After feeding, the gradient did not increase and measured 27\xa0±\xa025 and 23\xa0±\xa034\xa0mm Hg, respectively (P\xa0=\xa00.80).'] Is gastric feeding intolerance caused by mucosal ischemia measured by intragastric air tonometry in the critically ill?