Page 285 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

276 ACID-BASE DISORDERS

Gastric Fluid Loss can result in marked increases in plasma HCO 3 concen-
The H þ and Na þ concentrations of gastric fluid are tration, and chronic vomiting of stomach contents is the
þ
inversely related to one another, whereas the K concen- most common cause of metabolic alkalosis in small animal
tration is relatively stable (approximately 10 mEq/L). practice.

The Cl concentration is very high (approximately 150 In studies of gastric alkalosis, experimental subjects are
mEq/L) and remains remarkably constant even when rendered sodium avid by feeding a low-salt diet. Gastric
hypochloremia develops. Subtracting the sum of the fluid is then continuously removed by nasogastric suc-
Na þ and K þ concentrations of gastric fluid from the tion, and fluid and electrolyte losses other than HCl are
Cl concentration yields an approximation of the H þ quantitatively replaced. 131,132,172 The effects of repeated
concentration. The composition of gastric fluid is com- gastric drainage over 3 days on plasma HCO 3 and
pared with that of other body fluids in Figure 10-10. chloride concentrations and on potassium, sodium, and
When a dog or cat vomits stomach contents, water is lost chloride balance in experimental dogs are shown in
along with large amounts of HCl and small amounts of Figure 10-12. Note that the resulting metabolic alkalosis
potassium and sodium. is corrected by provision of NaCl despite a progressively
The H þ produced during gastric acid secretion negative potassium balance. In the clinical setting, persis-
originates from the dissociation of carbonic acid; thus tent vomiting of stomach contents leads to fluid and
þ

an equal number of HCO 3 ions are generated in ECF. electrolyte losses (H þ and Cl > Na þ and K ), and
In the normal animal, gastric acid secretion does not dis- anorexia prevents adequate dietary intake of electrolytes.
turb acid-base balance because the increase in ECF In patients with pyloric obstruction, gastrin secretion is

HCO 3 concentration that accompanies parietal cell enhanced, and gastric acid secretion is stimulated further.
þ

H secretion is balanced by pancreatic HCO 3 secretion Overproduction of gastrin by a gastrin-secreting tumor

in the duodenum, and Cl secreted into the stomach is may also stimulate gastric acid secretion. In one dog with
recaptured lower in the gastrointestinal tract. When gastrinoma, severe metabolic alkalosis and hypokalemia

stomach contents are lost, H and Cl are removed from were associated with a history of chronic vomiting. 221
þ
this system, and HCO 3 secreted into the duodenum by Renal avidity for sodium and defense of the ECFV

þ
the pancreas is no longer titrated by gastric H but is occur because of ongoing fluid and electrolyte losses in
reabsorbed farther down in the gastrointestinal tract in the vomiting animal or intake of a low-salt diet and naso-
place of Cl . The normal relationship between gastric gastric suction in the experimental setting. To maintain

and pancreatic secretions in the gastrointestinal tract is ECFV, the kidneys must reabsorb sodium by all available
shown in Figure 10-11. Continued loss of gastric fluid mechanisms. Because of ongoing loss of gastric HCl and
Gastric Pancreatic
Plasma ISF RBC ICF secretion secretion
200
+ –
Na HCO 3

150 –
– + HCO
HCO Na 3
3 –
HCO A –
3
–
A A –
mEq/L 100 K + HCO – 3
+ H + +
+ + K – Na
Na Na Cl
––
Cl – Cl – Cl – HPO 4
50
–
Cl
–
Cl
+
Na
K +
0
–9
–9
+
–9
–9
–3
–9
[H ] 40 (10 ) 36 (10 ) 54 (10 ) 100 (10 ) 120 (10 ) 10 (10 )
–
–3
–3
–3
–3
–3
[HCO 3 ] 24 (10 ) 32 (10 ) 18 (10 ) 12 (10 ) 0 100 (10 )
Figure 10-10 Comparison of electrolyte composition of gastric juice to other body fluids using
Gamblegrams. Strong ions are crosshatched. (From Jones NL. Blood gases and acid-base physiology, 2nd ed.
New York: Thieme Medical, 1987: 133.)