Diarrhea, the principal manifestation of intestinal infection among the critically ill, affects approximately one-third of all patients (reported incidence of 2% to 63%) admitted to the intensive care unit (ICU). Although many definitions exist in the literature, diarrhea is best defined as bowel movements that, because of increased frequency, abnormal consistency (the normal water content of stool is 60% to 85% of total weight), or increased volume (stool volume output <250 mL or 250 g/day), are deleterious to the well-being of the patient. Potentially deleterious consequences in these patients include perianal and sacral skin ulcers with secondary superinfections and decreased absorption of enterally administered medications. For the critically ill patient, the dehydration that accompanies severe diarrhea strains a circulatory system already limited by impaired cardiac function and septic hemodynamics, which might ultimately culminate in multisystem organ failure. Diarrhea can also precipitate metabolic derangements including electrolyte imbalances and anion gap acidosis with all their debilitating consequences.

Three main types of diarrhea include secretory, osmotic, and inflammatory diarrhea.