25 Diarrhea
Diarrhea is one of the most common abnormal manifestations of gastrointestinal (GI) dysfunction in the intensive care unit (ICU); the reported incidence is between 2% and 63%.1 Diarrhea is best defined as bowel movements that, owing to increased frequency, abnormal consistency, or increased volume, cause discomfort to the patient or the caregiver. This definition demonstrates the subjectivity in diagnosing diarrhea, a fact that complicates interpretation of the literature and limits applicability of guidelines. The impact of diarrhea on patient care in the ICU, including its cost in morbidity and mortality, is unknown. However, it is undeniable that diarrhea remains a persistent problem in many ICUs.
Criteria
Several criteria are used to diagnose diarrhea:
1 Abnormal frequency. Normal frequency is described as one or two bowel movements per day and is in part determined by the amount of fiber in the diet. Three or more bowel movements per day are considered abnormal.1
2 Abnormal consistency. Abnormal consistency is described as either nonformed stool or stool having excessive fluid content that causes “inconvenience” to the patient, nursing staff, or caregiver. Normal stool water content is 60% to 85% of the total weight.1
3 Abnormal amount. Stool amount and volume vary significantly with the amount and type of enteral intake. Insoluble fiber adds a significant amount of bulk volume. A “normal” amount is considered to be approximately 200 grams per day.1 Abnormal amounts are considered to be greater than 300 grams/d, or volumes greater than 250 mL/d.1,2
Pathophysiology
1 Increased fluid secretion that overwhelms absorption. On average, up to 9 liters of fluid is secreted into the GI lumen in addition to the normal oral intake. Less than 1% of that fluid is contained in stool, owing to the amazingly large absorptive capacity of the small and large bowel. Within the intestinal mucosa, passive and active transport of sodium determines the amount of water that is absorbed. Stimulation of the active secretion of fluids into the GI lumen occurs when intracellular levels of the second messenger, cyclic adenosine monophosphate (cAMP), increase within enterocytes. Increased intracellular cAMP concentration promotes chloride secretion.3 Thus, diarrhea caused by excessive secretion of fluids is called secretory diarrhea. Secretory diarrhea characteristically contains large amounts of fluid and is described as watery. Secretory diarrhea is observed in certain infectious diseases such as cholera or infections with rotavirus. Secretory diarrhea also can be observed in endocrine disturbances associated with carcinoid syndrome or vasoactive intestinal peptide (VIP)-secreting tumors.
2 Increased mucous secretion from the large bowel. Overproduction of mucus by the large bowel can lead to development of diarrhea. Excessive mucus secretion is observed in colonic infections such as Clostridium difficile colitis and amebiasis.4 The incidence of infectious diarrhea in the ICU is unknown.
3 Contaminated food products. Of particular concern is the contamination of the food being given in the ICU. Contamination of enteral formulas can occur at multiple levels, including preparation of the enteral product, use of “open units,” addition of modular dietary components, and contamination of the enteral access port (i.e., feeding tube, gastrostomy tube). The incidence of diarrhea due to contaminated feeding tubes is unknown.
4 Diarrhea due to increased osmotic load. Many substances that are taken orally and are not fully absorbed can exert a significant osmotic force, overwhelming the physiologic absorptive capacity of the GI tract. A significant number of patients with diarrhea in the ICU fall into this category.
a Osmotic diarrhea caused by medications. Sorbitol is frequently and inadvertently given to patients in the ICU as a means of preparing many medications for delivery via feeding tubes and is an often overlooked culprit causing diarrhea.5 Other osmotic agents include Golytely and magnesium-containing medications.
b Incomplete digestion and malabsorption. The incidence of malabsorption in the ICU is unknown. However, there are many instances where malabsorption should be considered as a cause of diarrhea in the critically ill patient. These include:
i Incomplete protein digestion (azotorrhea). Protein digestion occurs mainly in the stomach by pepsin (only activated at low pH) and hydrochloric acid. In the ICU, virtually all patients receive medications to raise intragastric pH, such as histamine receptor type 2 (H2) blockers or proton pump inhibitors.6,7 In addition, feeding tubes frequently “bypass” the stomach, eliminating both gastric acid and gastric proteolytic digestion.
ii Undigested carbohydrates. In addition to sorbitol (see earlier discussion), excessive glucose, lactose, or fructose in tube-feeding formulas can overwhelm the absorptive capacity of the small bowel, causing an osmotic influx into the gut lumen.8
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