Page 349 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

340 FLUID THERAPY

TABLE 14-7 Sliding Scale for gluconeogenesis) or CO 2 and water (via mitochondrial
Potassium oxidation) in the liver. Normally, gluconeogenesis
predominates. Acetate is metabolized primarily in mus-
Supplementation cle. The alkalinizing effect of these anions is delayed
Serum mEq KCl to Maximal Fluid because of the requirement for metabolism. In one
Potassium Add to 250 mL Infusion Rate* study, equivalent doses of acetate, bicarbonate, and
(mEq/L) Fluid (mL/kg/hr) lactate had similar alkalinizing effects in anesthetized
dogs 45 minutes after infusion. 21 The effect of
<2.0 20 6 bicarbonate occurred earliest because metabolism was
2.1-2.5 15 8 not necessary.
2.6-3.0 10 12 Lactate originally was introduced for the treatment of
3.1-3.5 7 16 acidosis because of technical difficulties in preparation of
bicarbonate solutions suitable for intravenous use. 5,36
From Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, editor.
Current veterinary therapy VIII. Philadelphia: WB Saunders, 1983: 38. These technical difficulties have been overcome, but crys-
*So as not to exceed 0.5 mEq/kg/hr. talloid solutions containing lactate as a source of base
(e.g., lactated Ringer’s solution) still are widely used for
fluid therapy in clinical practice. Most patients treated
Serum sodium concentrations were similar despite
with lactate-containing replacement solutions respond
differences in the sodium concentrations of the various
well, probably as a result of ECF volume expansion and
fluids, demonstrating effective natriuresis in normal dogs
improved tissue perfusion.
receiving sodium-containing crystalloid solutions. Serum
chloride concentration increased with administration of Whether it is converted to glucose or oxidized to CO 2
the saline solutions containing 154 mEq/L chloride, and water, the metabolism of lactate consumes hydrogen
and mild metabolic acidosis developed. Serum chloride ions and has an alkalinizing effect:
concentration also increased slightly with administration Gluconeogenesis
of lactated Ringer’s solution (112 mEq/L chloride), but
þ
there was no change in acid-base balance. The increased 2CH 3 CHOHCOO þ 2H ! C 6 H 12 O 6
serum chloride concentration and alterations in acid-base
Oxidative metabolism
balance could have resulted from decreased reabsorption
of bicarbonate with sodium in the kidney during natriure-

þ
37,38 CH 3 CHOHCOO þ H þ 3O 2 ! 3CO 2 þ 3H 2 O
sis and decreased strong ion difference. Expansion
acidosis is an unlikely explanation because all fluids
administered presumably expanded the ECF volume. There has been some concern that lactate in
Anions such as acetate, gluconate, and lactate are lactated Ringer’s solution may be harmful to patients
added to crystalloid solutions as a source of base because with poor tissue perfusion and severe metabolic acidosis
their oxidative metabolism in the body yields bicarbon- (pH, <7.1 to 7.2). Administration of lactate as a salt
cannot contribute directly to metabolic acidosis.
ate. The alkalinizing effect of the metabolism of these
Rather, the ability of the liver to metabolize lactate
anions and that of citrate is as follows:
and the potentially detrimental effect of lactate on
Acetate
myocardial contractility have been debated. During
severe hypoxia, increased lactate production in gut and
þ
NaC 2 H 3 O 2 þ 2O 2 ! CO 2 þ H 2 O þ Na HCO 3
muscle and decreased hepatic extraction of lactate led
to progressive lactic acidosis. In moderate metabolic
Citrate
acidosis, administration of lactated Ringer’s solution
probably is beneficial because any tendency toward lactate
1 þ
K 3 C 6 H 5 O 7 þ 4/2O 2 ! 3CO 2 þ H 2 O þ 3K HCO 3
accumulation is likely to be offset by improved hepatic
perfusion and oxygen delivery as a result of ECF volume
Gluconate
expansion.
Newer commercially available balanced crystalloid
1 þ
NaC 6 H 11 O 7 þ 5/2O 2 ! 5CO 2 þ 5H 2 O þ Na HCO 3
solutions contain approximately twice the amount of
bicarbonate precursors when compared with lactated
Lactate Ringer’s solution. As a result, these solutions generally
are thought to be more efficient than lactated Ringer’s
þ solution in treatment of metabolic acidosis, provided that
NaC 3 H 5 O 3 þ 3O 2 ! 2CO 2 þ 2H 2 O þ Na HCO 3
metabolic conversion of the precursors to bicarbonate
Most lactate is produced in muscle and gut and occurs quickly. There is some concern that such fluids
metabolized to either glucose (via cytosolic may contribute to the development of metabolic