Page 317 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

308 ACID-BASE DISORDERS

or hyperphosphatemia, however, AG should be corrected
for the presence of hyperphosphatemia [AG phosph-adjusted
AG TP-adjusted ¼ AG þ 2:5 ð6:37 ½TPŠÞ ¼ AG þ (2.52 0.58 [Phosph])] before calculating
SIG . The SIG simplified has not been adequately tested
simplified
where [alb] is albumin concentration in g/dL and [TP] is in dogs and cats, but its derivation is sound, and it is supe-
total protein concentration in g/dL. rior to the AG for detecting increases in unmeasured
Although the contribution of serum phosphate con- strong anions in horses. 11
centration to the AG is negligible in normal dogs and A stepwise approach should be followed in all patients
cats, hyperphosphatemia also can increase the AG in with suspected mixed acid-base disorders (Figure 12-1):
the absence of an increase in strong unmeasured anions.
1. Perform electrolyte and blood gas analysis.
The AG can be adjusted for an increase in phosphate
2. Determine the pH and the nature of the primary
concentration by expressing phosphate in mEq/L
disorder.
(see Chapter 7) and assuming plasma pH to be 7.4 as:
3. Calculate the expected compensation: Is it a simple or
mixed disorder?
AG alb-phosph-adjusted ¼ AG þ 4:2 ð3:77 ½albŠÞ
4. Calculate the chloride contribution to metabolic
þð2:52 0:58 ½PhosphŠÞ
þ
disorder ([Cl ]gap, [Cl ]/[Na ] ratio, [Cl ]
þ
AG TP-phosph-adjusted ¼ AG þ 0:25 ð6:37 ½TPŠÞ [Na ]; see Table 12-3).
þð2:52 0:58 ½PhosphŠÞ 5. Estimate the concentration of the unmeasured strong
anions (AG, BE algorithm, or SIG simplified ).
where [Phosph] is the concentration of phosphorus in 6. Compare the chloride contribution with the presence
milligrams per deciliter. of unmeasured strong anions: Is there a mixed
The base excess algorithm is another method to esti- metabolic disorder? (Table 12-4)
mate unmeasured strong ions that has been adapted for 7. Consider other laboratory data (e.g., creatinine,
14 17,30,59 glucose, and so on).
use in dogs and cats and applied in clinical cases.
Itaccountsfirstfor theeffectsofchangesinfreewater,chlo-
ride, protein, and phosphate concentrations in the BE. Any
remaining BE is attributed to the presence of unmeasured
strong anions. Formulas to use with the BE algorithm are
presented in Chapter 13 (see Box 13-4). Values less than
5 mmol/L are suggestive of an increase in unmeasured
strong anions. 14 The BE algorithm is a useful clinical tool
despite a few shortcomings. There are theoretical
limitations in extrapolating traditional BE calculations for
use in dogs and cats. In addition, protein influence on BE
is estimated based on data for human albumin, which
behaves differently than canine 12 and feline albumin. 36
The strong ion gap is the difference between all
unmeasured strong anion charges and all unmeasured
strong cation charges. 11 The SIG has been simplified
(SIG simplified ) to be estimated based on [A tot ], the total
concentration of nonvolatile weak acids in plasma (see
Chapter 13). 11 Albumin is used to estimate [A tot ]in
the SIG simplified because albumin is the most important
buffer in plasma. Assuming a plasma pH of 7.4,
12
SIG simplified can be calculated in dogs as :
SIG simplified ¼½albŠ 4:9 AG

In cats, at a plasma pH of 7.35, SIG simplified is estimated
36
as :
SIG simplified ¼½albŠ 4:58 AG þ 9

Increase in unmeasured strong anions is suspected when- Figure 12-1 Algorithm for evaluation of acid-base status in
ever SIG simplified is less than 5 mEq/L. In patients with patients with suspected mixed acid-base disorders.