Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

V. Physiology of Acid-Base Balance
541
Vomiting in a dog; heavy sweat loss in an endurance
horse; displaced abomasum in a cow; and the administration
of the loop diuretic, furosemide, in a cat result in
similar acid-base disturbances. In each circumstance, a disproportionate
loss of chloride relative to sodium results in
a hypochloremia and an increase in [SID]. Correction of
the alkalosis is brought about by the provision of chloride,
generally as sodium chloride or potassium chloride, which
results in a decrease in [SID] and thus a return of the dependent
variables, bicarbonate and pH (hydrogen ion), toward
normal. A metabolic acidosis with a large base deficit is
generally treated with sodium bicarbonate. In the traditional
approach, the calculated bicarbonate requirement is
administered as sodium bicarbonate to replace the bicarbonate
deficit, whereas in the strong ion approach, the sodium
bicarbonate is administered to provide the strong cation,
sodium, without a strong anion. Other metabolizable anions
could be substituted for bicarbonate and achieve a similar
effect. In practice, both approaches work, but the rationale is
substantially different.
Calculation of SID is simple and provides useful insight
in patients with metabolic acid-base disturbances. Factors
that influence SID range from changes in free water, to
sodium-chloride imbalances that result from excessive losses
or disproportionate retention of sodium or chloride, to the
accumulation of strong organic anions. Organic acidosis can
be produced by the accumulation of exogenous as well as
endogenous organic anions. Examples of exogenous anions
include salicylate, glycolate and formate associated with the
ingestion of aspirin, ethylene glycol, and methanol, respectively.
Many of these endogenous and exogenous organic
anions are not routinely monitored in the diagnostic laboratory.
This situation can create problems when calculating
the SID because the presence of these unmeasured strong
anions may not be appreciated. Although the anion gap can
be helpful, it does not always accurately predict the presence
of these compounds. More sophisticated mathematical
methods have been suggested as a means for the detection
of unmeasured anions the strong ion gap ( Constable et al .,
1998 ; Stewart, 1981 ) and, more recently, the simplified
strong ion gap ( Constable and Stampfli, 2005 ). In animals
with major changes in protein or albumin concentration,
the primary concern must be a thorough investigation of the
cause of the increase or decrease in protein. The acid-base
consequences of change in protein and albumin concentration
tend to be modest but are a potential source of confusion
when evaluating acid-base data.
The traditional approach and the nontraditional or
strong ion approach to acid-base balance each has its supporters,
and discussion over the benefits and limitations of
either approach has at times been strident. However, this
need not be an either-or situation. Both approaches have
proven useful to address practical problems in both research
and medical settings. The traditional approach based on
the Henderson-Hasselbalch equation is simpler, more user
friendly, and more widely accepted. Bicarbonate concentration
estimates the severity of the acid-base disorder, but
the strong ion approach may provide a better understanding
as to why the bicarbonate is changing because it integrates
acid-base and electrolyte disorders ( de Morais, 2005 ). The
strong ion approach has been gaining acceptance from the
critical care community, which finds it useful in the analysis
of the complex fluid, electrolyte, and acid-base problems
presented to intensive care units. A number of computer
programs adaptable to hand-held electronic devices have
been developed and take some of the mathematical fear out
of using the strong ion approach.
J . Dietary Factors in Acid-Base Balance
Dietary factors, particularly the dietary cation-anion balance
(DCAB), have been extensively studied in cattle,
swine, poultry, and horses. The calculation of DCAB of a
dry feed ration is remarkably similar to the calculation SID
of body fluids. The DCAB in mEq is generally represented
as (sodium potassium) (chloride sulfate) per kg of
dry matter of the diet. Diets with a high DCAB, such as
alfalfa hay, have an alkalinizing effect and are an important
factor in the alkaline urine of most herbivores. High
grain rations tend to have a lower DCAB. Manipulation
of the DCAB has been employed to enhance milk yield in
dairy cattle, to reduce the incidence and severity of gastric
ulceration in swine, to decrease the incidence of milk fever
in cattle, and to alter the urine pH and calcium balance in
horses. The addition of sodium bicarbonate to the ration
of lactating dairy cattle to raise the DCAB from 100 to
200 mEq/kg diet DM resulted in an increase of milk production
of over 8%, which was due, in part, to more effective
ruminal digestion ( Block, 1984 ). On the other hand,
supplementation of the diet of late-term cattle with calcium
chloride or ammonium chloride so as to lower the DCAB
and have an acidifying effect has been shown to reduce the
incidence of milk fever by enhancing the mobilization of
calcium from the bone ( Block, 1984 ). As the application
of these dietary practices becomes more widespread, it is
essential that we appreciate the implications of dietary factors
and electrolyte supplementation on mineral metabolism
and acid-base balance ( Fredeen et al ., 1988 ).
Sodium bicarbonate supplementation has been used
as a prerace ergogenic aid in racehorses. Relatively large
doses, 500 g or more of sodium bicarbonate, often mixed
with sugar and water and referred to as “ milk shakes, ” have
been given via nasogastric tube and result in a marked metabolic
alkalosis. Although experimental studies have often
failed to detect a measurable performance benefit from
sodium bicarbonate supplementation, practical experience
suggests that some horses, particularly standardbreds,
show marked improvement in race times. Administration
of any substance with intent to alter the performance is
illegal in most racing jurisdictions. In many racing states,