SummaryThe relationship between breath hydrogen excretion and intestinal ischemia was investigated in nine mechanically ventilated dogs under pentobarbital anesthesia. An ileal segment was isolatedin situ, ligated at each end, and insufflated with hydrogen. Expired air was collected at intervals. Blood volume was reduced 30% by three successive equivalent hemorrhages 10 min apart. Local bowel ischemia was produced by clamping the blood supply to the isolated segment for 10 min. Graded hemorrhage produced stepwise reductions in breath hydrogen concentration, to 77 ± 13, 66 ± 15, and 35 ± 8% (mean ± S.E.) of baseline after the first, second, and third hemorrhages, respectively. These reductions correlated highly (r= 0.84;P< 0.01) with declines in mean aortic blood pressure. Occlusion of blood supply caused a significant (P< 0.025) decrease in breath hydrogen concentration and excretion to 39 ± 14% of baseline. Termination of occlusion was followed within 2 min by a 7-fold increase in breath H2concentration above the original baseline, probably reflecting reactive hyperemia. Breath hydrogen measurements appear to reflect functional (hemorrhagic shock-induced) and mechanical (vascular occlusion induced) enteric ischemia in dogs.