DIAGNOSIS OF ACID BASE DISORDERS
of acid–base status from analysis of blood gases requires a systematic
approach. A rec-ommended approach follows (Figure 50–6):
Examine arterial pH: Is acidemia or alkalemia present?
Examine Paco2: Is the change in Paco2 consistent
with a respiratory component?
If the change in Paco2 does not
explain the change in arterial pH, does the change in [HCO3−] indicate a metabolic
Make a tentative diagnosis (see Table 50–1).
Compare the change in [HCO3−] with the change in
Paco 2. Does a compensatory response exist (Table 50–7)? Because arterial pH
is related to the ratio of Pa co2 to [HCO3−], both respiratory and
renal compensatory mechanisms are always such that Paco2and [HCO3−] change in the same
A change in opposite directions implies a mixed acid–base disorder.
If the compensatory response is more or less than expected, by
definition, a mixed acid–base disorder exists.
Calculate the plasma anion gap in the case of metabolic acidosis.
Measure urinary chloride concentration in the case of metabolic
alternative approach that is rapid, but per-haps less precise, is to correlate
changes in pH with changes in CO2 or HCO3. For a
respiratory dis-turbance, every 10 mm Hg change in CO2 should change
arterial pH by approximately 0.08 U in the opposite direction. During metabolic
disturbances, every 6 mEq change in HCO 3 also changes arterial pH
by 0.1 in the same direction. If the change in pH exceeds or is less than
predicated, a mixed acid–base disorder is likely to be present.