Page 456 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

444 FLUID THERAPY

losses are virtually always the precipitating cause of the plasma to the gut lumen. In osmotic diarrhea, the con-
metabolic alkalosis. 40 However, the presence of centration of sodium in the stool may remain below that
hypochloremic metabolic alkalosis is not always an indica- of plasma, leading to water loss in excess of sodium, dehy-
tion of proximal GIT obstruction, and it may develop just dration, and hypernatremia, especially when water intake
as often in distal GIT obstruction. 11 Metabolic alkalosis is is inadequate. 38 This finding has been observed in dogs
self-perpetuating because of increased renal reabsorption and people with hepatic encephalopathy (HE) treated
of NaHCO 3 in the presence of volume, chloride, and with lactulose. 88 Intestinal absorption may also be
potassium depletion. 102 These metabolic disturbances affected by diseases that cause increased venous pressure,
arise as a result of preferential conservation of volume lymphatic pressure, decreased interstitial osmotic pres-
at the expense of extracellular pH. The renal reabsorption sure (hypoalbuminemia), and increased epithelial perme-

of almost all filtered HCO 3 and the exchange of sodium ability. Disorders of intestinal motility result in decreased
for hydrogen ions in the distal tubule promote an acidic intestinal fluid absorption because of decreased contact
urine pH despite extracellular alkalemia (so-called para- time between luminal contents and the enterocytes.
doxical aciduria). 102,129 An understanding of these pathomechanisms is useful
Metabolic alkalosis in patients with gastrointestinal in identifying the pathophysiology and establishing
signs is not invariably associated with outflow obstruction appropriate treatment of patients with diarrhea, but diar-
and has been encountered in dogs with parvovirus enter- rhea caused by only one mechanism is rare in clinical prac-
itis and acute pancreatitis. 48 Diseases characterized by tice. For example, a dog with inflammatory bowel disease
hypersecretion of acid, such as gastrinoma, may also be may have decreased intestinal absorption caused by
associated with metabolic alkalosis and aciduria. Basal decreased surface area, increased mucosal permeability,
gastric acid secretion in two dogs with gastrin-producing increased intraluminal osmotic forces, and decreased
tumors (1.7 and 2.7 mmol/hr/kg 0.75 HCl) was maximal interstitial osmotic forces coupled with rapid transit of
in the unstimulated state. 115 In this situation, intestinal contents and increased intestinal secretion.
hypochloremia, hypokalemia, metabolic alkalosis, and
dehydration are probably caused by gastric hypersecre- Fluid and Electrolyte Abnormalities in
tion of acid. 115 Diarrhea
The fluid and electrolyte abnormalities associated with
DIARRHEA diarrhea include volume depletion, hyponatremia or
The pathomechanisms in patients with diarrhea include hypernatremia, hypokalemia, and metabolic acido-
increased intestinal secretion, decreased intestinal absorp- sis. 23,82,127 The metabolic acidosis that develops is
tion, rapid transit of intestinal contents, and mesenteric, characterized by hyperchloremia and a normal anion
vascular, or lymphatic disease. 91 gap caused by loss of diarrheal fluid with relatively low
Secretory agents include neuropeptides of the chloride and high bicarbonate concentrations. Serious
enteric system (found in neuroendocrine tumors), electrolyte and acid-base abnormalities are relatively
cholinergic agonists, gastrointestinal hormones, bacterial uncommon in patients with diarrhea as a sole complaint.
enterotoxins, deconjugated bile acids, and hydroxy fatty When diarrhea is severe and protracted or is accompanied
acids. 91 Secretory diarrhea results when prosecretory by vomiting, acid-base and electrolyte disturbances are
stimulation overwhelms absorptive forces. Secretory diar- more likely, but it is difficult to predict which
rhea is manifested by increased colonic secretion of abnormalities will be present. For example, decreased
sodium-rich fluid. Volume depletion resulting from total CO 2 concentrations were identified in less than
sodium and water loss stimulates antidiuretic hormone 17% of 134 dogs with parvovirus enteritis in one study, 55
release, which in turn stimulates water retention by the whereas metabolic alkalosis and hypochloremia were
kidneys and dilutional hyponatremia. more common than metabolic acidosis in another study
48
Decreased intestinal absorption may result from of dogs with parvovirus enteritis. Another study of
decreased intestinal surface area as a consequence of dam- 22 dogs infected with parvovirus identified lower plasma
age by infectious agents (e.g., parvovirus), cellular infil- concentrations of sodium, potassium, chloride, and bicar-
tration, or surgery. Damage to the intestinal epithelial bonate than controls. Serum L-lactate concentrations
barrier may also increase intestinal permeability, were increased in some puppies with canine parvoviral
disrupting paracellular and transcellular absorptive enteritis, but most affected puppies developed only mild
pathways. An increase in the osmolality of intestinal compensated metabolic acidosis. 87 Antiinflammatory
contents may also decrease absorption. Specific causes and immunosuppressive dosages of corticosteroids can
of osmotic diarrhea include overeating, sudden dietary result in type B hyperlactatemia (secondary to increased
change, osmotic laxative ingestion, maldigestion, or mal- gluconeogenesis from protein). 12 Therefore, caution
absorption. Absorption of water and electrolytes is must be taken when interpreting blood lactate
retarded by accumulation of nonabsorbable solutes in concentrations in dogs with hyperadrenocorticism, in
the gut lumen, and there is net water movement from those receiving corticosteroids, or in those affected by