MANAGEMENT OF ACUTE DIARRHEA

General Considerations

Clinically, most acute medical diseases of the small intestine in dogs and cats are characterized by watery diarrhea generally passed two to four times daily.

Vomiting results from vagal afferent input into the emetic center and is present in many cases. Depending on the cause, abdominal pain and signs of systemic illness (fever, leuko- gram abnormalities) can occur. Rarely can the exact cause of acute small intestinal disease be determined by the results of patient history and physical examination findings. However, history and signalment can help aid in ranking differential diagnoses. For example, viral enteritis and diarrhea due to parasitism is most common in puppies and kittens from crowded environments. Diagnostic testing is usually necessary to make a definitive diagnosis. In most cases the therapeutic plan for many patients with acute medical diseases of the small intestine and severe diarrhea is initially handled in a similar manner.

Fluid and Electrolyte Imbalances

In cases with acute GI disturbances, of immediate concern is correction of fluid and electrolyte imbalances. In most instances patients are presumed to have lost sufficient isotonic fluids to become dehydrated. Restoration of normal circulating fluid volume is an immediate priority, both to prevent renal functional impairment and to minimize further GI injury. Even if clinical evidence of dehydration is not apparent, patients are assumed to be at least 5% dehydrated. Fluid administration is calculated on the basis of the formula in Box 6-3.

In dehydrated patients fluids should always be administered intravenously. If dehydration is so

Fluid Calculations for Use With Acute Gastrointestinal Diseases

Replacement of Losses

Dehydration (%) ? body weight (kg) ? 10 = ml fluid to be administered over 18 to 24 hours

Maintenance Needs

30 ml/lb/day

Ongoing Losses

Estimate continuing losses via vomiting and/or diarrhea (overestimate if in doubt) and replace every 8 hours

Sample Calculation

25-kg dog, 5% dehydrated, losing 200 ml of stool every 8 hours

Replacement of losses: 1250 ml

Maintenance needs: 1500 ml

Ongoing losses: 600 ml

Total volume to be administered over 24 hours:

3350 ml, or 1117 ml every 8 hours severe that an intravenous catheter cannot be placed, fluids should be given initially using intraosseous administration.

Subcutaneous administration of fluids has unpredictable absorption and is too slow to be useful in replacing large-volume deficits in hypovolemic, hypotensive patients. Jugular catheters are preferred over peripheral catheters in most cases; blood samples can be drawn, the catheters can be maintained for up to 5 days, the catheters are rarely affected by position changes, there is reduced risk for phlebitis, and there is the potential to measure central venous pressure to aid in monitoring fluid therapy.

Fluids are initially administered rapidly (approximately 5 to 10 ml/lb over 30 to 60 minutes, then the infusion rate is slowed) in patients with clinical hypovolemic shock (weak femoral pulse pressure, tachycardia, delayed capillary refill time, tacky mucous membranes). For more prolonged fluid administration, fluid needs for 24 hours are calculated, and the total volume is divided into three equal amounts, each to be administered every 8 hours. An infusion pump is optimal for constant-rate fluid infusion to patients of any size to avoid overhydration.

The fluid of choice for initial volume replacement is either a buffered crystalloid solution such as lactated Ringer's solution or Normosol-R or 0.9% saline solution. Both supply sodium and chloride in adequate amounts. Also, lactated Ringer's solution and Normosol-R are mildly alkalinizing (the buffer supplies bicarbonate) and may be beneficial in patients with metabolic acidosis, especially patients with severe diarrhea. The buffer sources in Normosol-R are acetate and gluconate. There has been a trend toward using gluconate and acetate because they do not require hepatic metabolism and they do not contribute to lactate levels. Administration of hypertonic solutions (e.g., 5% dextrose in lactated Ringer's solution) has been advocated because hypertonic fluids, by shifting water from the intracellular to the extracellular compartment, may expand the extravascular fluid compartment more than do isotonic fluids.

Fluids without high sodium concentrations (e.g., 2.5% dextrose in water or 2.5% dextrose in 0.45% saline) should be avoided because of the failure to replenish the sodium deficit, which may decrease circulating fluid volume.

Adequacy of fluid replacement therapy can be easily and accurately evaluated by serial measurement of body weight relative to body weight on entry. Body weight should increase over the first 24 hours by at least the calculated fluid deficit (dehydration); 500 ml of water weighs 1 lb. Fluid administration rate should be increased if body weight begins to drop and should be decreased if body weight increases excessively (more than 5% to 10% of calculated normal weight). Other parameters for assessing adequacy of fluid replacement therapy include serial measurement of hematocrit and plasma protein concentration, estimation of pulse pressure, capillary refill time, and mucous membrane texture.

Early potassium supplementation is critical for optimum management of dogs and cats with severe gastroenteritis and volume depletion, because depletion of body potassium stores develops rapidly. Routine potassium supplementation should begin before hypokalemia is detected because serum potassium content represents only a small fraction of total body potassium stores. Adverse consequences of hypokalemia are numerous and include decreased GI motility, decreased cardiac output, hypotension, skeletal muscle weakness, and general malaise and inappetence. Cats seem particularly likely to develop hypokalemia during periods of GI fluid loss and fluid replacement therapy, and they should be supplemented early and aggressively. Empirical supplementation of potassium is begun at 20 to 40 mEq/L of fluids administered; cats should receive at least 40 mEq/L. Daily measurement of serum potassium is recommended to facilitate maintenance of normal concentration. If at all possible, serum potassium concentration should be determined before initiating supplementation.

For example, although patients with adrenocortical insufficiency and advanced oliguric renal failure often develop GI disease, they should not receive potassium because they are often hyperkalemic. These two important disorders may be detected, and inappropriate potassium supplementation avoided, by determination of serum electrolyte and urea nitrogen levels. A rapid and reasonably reliable screening test for identifying life-threatening hyperkalemia early is an electrocardiogram. Typical early electrocardiographic changes include tall, peaked T waves, loss of P waves, and wide QRS complexes.

Glucose administration is particularly beneficial in treating septic patients, especially dogs with par- voviral enteritis and severe diarrhea. Sepsis results in a number of disturbances in glucose metabolism that are often manifested clinically by development of hypoglycemia. Hypoglycemia also develops more readily in young patients because of their small hepatic glucose reserves. In these patients, glucose administration (typically as a 5% solution) may produce a dramatic response. The caloric content of a 5% dextrose solution is far less than the patient's requirements but nonetheless seems to be helpful in alleviating some of the adverse effects of sepsis. Glucose supplementation is begun either at the outset or early in the course of fluid therapy in patients with severe enteritis. It is recommended that glucose levels be evaluated every 8 to 12 hours during the period of intensive therapy. Blood glucose levels in normal patients on a 5% dextrose drip should range from approximately 130 to 180 mg/dl. A patient on a 5% dextrose infusion with a low-normal glucose level is probably significantly hypoglycemic and may be septic. Additional glucose (e.g., 7.5% infusion or intravenous bolus [0.5 ml of 25% dextrose per pound]) and other treatments for sepsis (see later section in this chapter on acute viral enteritis) may be indicated. Bicarbonate administration is rarely indicated and in most instances is contraindicated by the added risks of iatrogenic hypernatremia, hyperosmolality, and alkalosis.

Metabolic acidosis usually corrects rapidly after volume deficit replacement.

Nonspecific Treatment of Diarrhea

Many episodes of acute diarrhea are of viral or bacterial origin, are self-limiting, and generally do not require specific therapy. Patients should not receive solid foods for at least 24 to 48 hours but should still be allowed access to liquids, because intestinal absorptive functions are usually intact. Electrolyte-containing solutions may be useful as sources for oral fluid replacement in patients without severe GI disease and large fluid losses. Suitable electrolyte replacement solutions include Enterolyte or Gatorade. Alternatively, a replacement solution can be formulated using guidelines developed by the World Health Organization: each liter of replacement solution should contain 120 mEq sodium, 25 mEq potassium, 48 mEq sodium bicarbonate, and 1.1 g glucose.

Antidiarrheal agents are occasionally indicated for the treatment of idiopathic, acute diarrhea of nonbacterial origin (fecal cytologic findings are noninflammatory; see earlier section on diagnostic considerations), especially diarrhea caused by dietary changes. Narcotics, generally considered the most effective antidiarrheal agents, act by increasing segmental contractions of the small and large intestine. Recommended narcotics for short-term treatment of acute diarrhea (Table 6-1) include paregoric, diphenoxylate (Lomotil), or loperamide (Imodium). Diphenoxylate is contraindicated in patients with severe underlying hepatic disease. Neither diphenoxylate nor loperamide should be used in patients with viral enteritis, because delayed intestinal motility may predispose to the development of sepsis. In addition, these agents have been shown to prolong illness in humans with salmonellosis, shigellosis, and campylobacteriosis by interfering with normal immune clearance mechanisms.

Salicylate-containing drugs, such as bismuth subsalicylate (Pepto-Bismol), may be beneficial for treatment of prostaglandin-mediated diarrhea.

Intestinal adsorbents such as kaolin pectate are generally of limited usefulness and must be administered in high doses.

Antiemetic medication may be indicated for treatment of patients with persistent vomiting. However, these agents should not be administered to patients that are vomiting as a result of intestinal obstruction or before completing an adequate diagnostic work-up. Injectable drugs are usually needed, and their use is therefore restricted to inhospital patients.

Chlorpromazine (Thorazine) is preferred as the initial agent because it has a wide safety margin and is a potent antiemetic, acting on the emetic center, chemoreceptor trigger zone, and peripheral chemoreceptors (see Table 6-1). In addition, chlorpromazine is thought to function as a calcium channel antagonist, thereby decreasing cyclic adenosine monophosphate concentration in intestinal epithelial cells. The result is decreased intestinal epithelial cell secretion, especially when excess secretion is mediated by enterotoxins. Chlorpromazine is also excellent for alleviating some of the discomfort caused by nausea. Chlorpromazine may precipitate hypotension in dehydrated patients and should therefore not be given before fluid replacement in volume-depleted patients.

Metoclopramide (Reglan) given subcutaneously or as a constant intravenous infusion (see Table 6-1) exerts antiemetic activity in the chemoreceptor trigger zone of the dog and increases gastric emptying in dogs and cats. Adverse effects from metoclopramide in dogs and cats are uncommon, consisting largely of excessive excitement. Severe,

UABLEOU Drugs Commonly Used in the Management of Acute
Gastrointestinal Diseases
Druq	Reqimen
Amikacin	7-9 mg/lb IV one time a day
Ampicillin	7-9 mg/lb IV three to four times a day
Bismuth subsalicylate	0.5-1 mg/lb PO three to four times a day
Cefazolin	9-15 mg/lb IV three times a day
Cetoxitin	9-15 mg/lb IV three times a day
Chlorpromazine	0.1-0.3 mg/lb IM three to four times a day
Diphenoxylate^*	0.028 mg/lb PO three times a day
Enrofloxacin^t	1.1-3 mg/lb IV two times a day
Flunixin meglumine*	0.45 mg/lb IV once
Hetastarch	10-20 ml/lb IV over 24 hours
Imipenem^t	2.2-4.5 mg/lb IV three times a day
Loperamide*	0.04 mg/lb PO three times a day
Metoclopramide	Constant infusion; 0.02-0.04 mg/lb/hr IV 0.1-0.3 mg/lb SQ three to four times a day
Metronidazole^t	5-7 mg/lb IV two to three times a day
Ondansetron	0.05-0.1 mg/lb slow IV two to three times a day
Paregoric*	1-2 ml PO four times a day or as needed
Plasma (immunotherapy)	0.45 ml/lb IV, SQ, IM
Plasma (oncotic pressure)	2.3-5 ml/lb IV

IV, Intravenously; PO, orally; IM, intramuscularly; SQ, subcutaneously. *Dogs only.

^Extralabel use, give as slow bolus.

^tAdminister over 20 to 30 minutes.

protracted vomiting that does not respond well to either chlorpromazine or metoclopramide should prompt consideration of possible intestinal obstruction or pancreatitis and additional diagnostic evaluation before continuing prolonged antiemetic therapy.

Ondansetron (Zofran) is a potent antiemetic drug that is frequently effective in reducing severe and frequent vomiting. It has been used in human cancer patients undergoing therapy with cisplatin, a drug that frequently causes nausea and severe vomiting. Ondansetron acts as a selective antagonist of serotonin 5HT3 receptors (a principal mediator of the emetic reflex). It is also effective in decreasing the frequency of vomiting in patients with severe parvoviral enteritis and should be used when chlorpromazine and metoclopramide do not provide adequate control. As the nausea and vomiting are controlled, a state of increased comfort seems to prevail. At this time the primary limitation for ondansetron is expense. It is strongly recommended, however, that all hospitals that treat dogs and cats stock at least one bottle of ondansetron so that it will be readily available for use in patients that have intractable vomiting.

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Source: Tams T.. Handbook of Small Animal Gastroenterology. Saunders,2003. — 496 p.. 2003

MANAGEMENT OF ACUTE DIARRHEA

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