Page 254 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Introduction to Acid-Base Disorders 245

THE NONTRADITIONAL anions in the plasma of normal dogs and cats are
APPROACH TO ACID-BASE presented in Table 9-7.
The weak anions in ECF are HCO 3 , plasma proteins,
EVALUATION and phosphate. Of these, plasma proteins and phosphate
constitute the independent variable A tot , whereas
The traditional approach to acid-base evaluation focuses
is a dependent variable. Hypoproteinemia has

on the relationship between pH, HCO 3 , and PCO 2 as HCO 3
been shown to be associated with metabolic alkalosis in
described by the Henderson-Hasselbalch equation. In
critically ill human patients in whom a decrease in serum
this approach, pH is shown to be a function of HCO 3
albumin concentration of 1 g/dL caused an increase in
concentration and PCO 2 .The PCO 2 is viewed as the respi- 44
standard BE of þ3.7 mEq/L. Serum phosphorus con-
ratory component and is determined by alveolar ventila-
tion, whereas the HCO 3 concentration is considered the centration (normally approximately 2 mEq/L) cannot

metabolic (or nonrespiratory) component and is decrease enough to cause alkalosis, but hyperphos-
regulated by the kidneys. This approach may lead to phatemia in patients with renal failure can make a substan-
the impression that PCO 2 and HCO 3 are independent tial contribution to A tot and metabolic acidosis. The

variables. In reality, only PCO 2 is independent. When a nontraditional approach to acid-base evaluation is consid-
ered in detail in Chapter 13.
primary increase in PCO 2 occurs, proteins (notably hemo-
globin) buffer the hydrogen ions that are produced by dis-
THE CONCEPT OF EXTERNAL
sociation of H 2 CO 3 , and the HCO 3 concentration
increases secondarily. Furthermore, an understanding of HYDROGEN ION BALANCE
the effects of changes in other electrolytes (e.g., Na ,
þ
K ,Cl ) and plasma proteins on acid-base balance is not External balance for hydrogen ions is maintained by renal

þ
facilitated by the traditional approach. The nontraditional excretion of a number of hydrogen ions equal to that con-
approach allows the clinician to better understand the sumed in the diet and produced each day by metabolic
complexity of the acid-base disturbances in some patients. processes. The majority of hydrogen ions originate from
Stewart formulated a model of acid-base chemistry in metabolic processes, and little fixed acid originates as such
biologic systems governed by three physical laws: (1) from the diet. A small amount of base is lost each day from
maintenance of electroneutrality; (2) satisfaction of disso- the gastrointestinal tract (primarily as organic anions),
ciation equilibria for incompletely dissociated solutes; and this is equivalent to a gain of fixed acid. These
and (3) conservation of mass. 63,64 The equations that sat- processes result in a net daily gain of 50 to 100 mEq of
isfy these laws were solved simultaneously to identify hydrogen ions. Bicarbonate ions that have been titrated
þ
variables that control [H ]. Independent variables are by these hydrogen ions must be regenerated. The kidneys
þ
those that may be altered from outside the system, are the only regulated route for H loss from the body.
whereas dependent variables are internal to the system Metabolic processes that convert cationic compounds
and change only in response to changes in independent to neutral products generate hydrogen ions, whereas
variables. Simultaneous solution of Stewart’s equations those that convert anionic compounds to neutral
identified three independent variables: strong ion differ- products consume hydrogen ions. 15,21,73 The main
sources of acid are oxidation of the sulfur-containing
ence (SID), the total concentration of weak acid (HA þ
A )or[A tot ], and PCO 2 . (e.g., cysteine, methionine) and cationic (e.g., lysine,

The SID changes if the difference between the sum of arginine) amino acids and hydrolysis of organic phos-
strong cations and the sum of strong anions changes. Ions phate diesters, such as phospholipids and nucleic acids.
are considered strong if they are almost completely Oxidation of the sulfur-containing amino acids is the
dissociated at the pH of body fluids. The strong cations major source of acid produced each day:
consist of sodium, potassium, calcium, and magnesium.
Of these, only Na is present at high enough concentra- C 5 H 11 O 2 NSðmethionineÞþ 7 / 2O 2 !
þ
1
tion in ECF that a change in its concentration is likely to 1 / 2CH 4 ON 2 ðureaÞþ4 / 2CO 2 þ3 / 2H 2 OþSO 2 þ
1
1
4 þ2H
have a substantial effect on SID. The strong anions con-
1
C 3 H 7 O 2 NSðcysteineÞþ5 / 2O 2 !
sist of chloride and several other anions that are not rou-
1
1
tinely measured clinically, and they collectively are 1 / 2CH 4 ON 2 ðureaÞþ2 / 2CO 2 þ1 / 2H 2 OþSO 2 þ2H þ
4
referred to as unmeasured strong anions (e.g., lactate,
acetoacetate, b-hydroxybutyrate, sulfate). Chloride and The main sources of base are metabolism of anionic
some unmeasured strong anions can be sufficiently amino acids (e.g., glutamate, aspartate) and the oxidation
altered in certain disease states to have a substantial effect or use for gluconeogenesis of other organic anions (e.g.,
on SID. The average concentrations of all cations and lactate, citrate).