DIAGNOSIS OF ACID BASE DISORDERS
Interpretation 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 component?
· 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 direction.
· 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 alkalosis.
An 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.
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