Page 252 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Introduction to Acid-Base Disorders 243

The clinician should first consider the patient’s blood disease or sepsis. The original interpretation of the blood
pH. Evaluation of pH often provides the answer to the gas data must be questioned if the acid-base disturbance
question of whether an acid-base disturbance is present. does not fit the patient’s history, clinical findings, and
If the pH is outside of the normal range, an acid-base dis- other laboratory data. Diagnostic difficulties are most
turbance is present. If the pH is within the normal range, likely in mild acid-base disturbances with blood gas
an acid-base disturbance may or may not be present. If the results still within the normal range, in mixed
patient is acidemic and plasma HCO 3 concentration is disturbances with counterbalancing components that

decreased, metabolic acidosis is present. If the patient result in a pH within the normal range, and in acute, rap-
is acidemic and PCO 2 is increased, respiratory acidosis is idly changing disorders without adequate time for

present. If the patient is alkalemic and plasma HCO 3 achievement of a compensated steady state.
concentration is increased, metabolic alkalosis is present.
If the patient is alkalemic and PCO 2 is decreased, respira- ANION GAP
tory alkalosis is present. These relationships are
summarized in Table 9-4. However, complicating acid- The major cations of ECF are sodium, potassium, cal-
base disturbances that would alter pH in the same direc- cium, and magnesium; the major anions are chloride,
tion as the primary disturbance cannot be ruled out at this bicarbonate, plasma proteins, organic acid anions
point in the evaluation. (including lactate), phosphate, and sulfate. The approxi-
The next step is to calculate the expected compensa- mate charge contributions of these ions in dogs and cats
tory response in the opposing component of the system are listed in Table 9-7. Automated clinical chemistry
(e.g., respiratory alkalosis as compensation for metabolic analyzers provide values for serum sodium, potassium,
acidosis, metabolic alkalosis as compensation for respira- chloride, and total CO 2 concentrations. Thus, the sum
tory acidosis) using the rules of thumb listed in Table 9-5. of the concentrations of commonly measured cations
If the patient’s secondary or adaptive response in the exceeds the sum of the concentrations of commonly
compensating component of the system falls within the measured anions, and the difference has been called the
expected range, a simple acid-base disturbance is proba- anion gap: 18,47
bly present. If the adaptive response falls outside the
expected range, a mixed disorder may be present (see ðNa þ K Þ ðCl þ HCO Þ

þ
þ
3
Chapter 12).
Considering the magnitude of change in pH can help The serum concentration of potassium varies little, and its
in assessment of mixed disorders. This can be seen by charge contribution is small compared with that of
consideration of the Henderson equation: sodium. Therefore, the anion gap often is defined as:
þ
24PCO 2 Na ðCl þ HCO Þ
þ 3
½H Š¼

½HCO Š
3
From several reported studies, the normal anion gap cal-
þ

The effect on extracellular pH of a mixed disorder is culated as (Na þ K ) (Cl þ HCO 3 ) is approximately
þ
minimized if the disorders change PCO 2 and HCO 3 in

the same direction (e.g., respiratory acidosis and meta-
bolic alkalosis) and is maximized if the disorders change TABLE 9-7 Approximate
PCO 2 and HCO 3 in opposite directions (e.g., respiratory Concentrations of

acidosis and metabolic acidosis). In the former instance, Cations and Anions in
blood pH may remain within the normal range, whereas Plasma in Normal Dogs
in the latter instance, blood pH is markedly abnormal. and Cats (mEq/L)
Mixed acid-base disorders are discussed in detail in
Chapter 12. Cations Dog Cat Anions Dog Cat
Once the clinician has classified the disturbance as sim-
ple or mixed and has defined the type of disturbance(s) Sodium 145 155 Chloride 110 120
present, an attempt should be made to determine Potassium 4 4 Bicarbonate 21 21
Calcium 5 5 Phosphate 2 2
whether the acid-base disturbance(s) is (are) compatible
Magnesium 2 2 Sulfate 2 2
with the patient’s history and clinical findings. Examples
Trace elements 1 1 Lactate 2 2
include metabolic acidosis in renal failure, acute diarrhea, Other organic 4 6
ethylene glycol ingestion, or diabetic ketoacidosis; respi- acids
ratory acidosis in advanced pulmonary disease; metabolic Protein 16 14
alkalosis in vomiting of stomach contents or loop diuretic Total: 157 167 157 167
administration; and respiratory alkalosis in pulmonary