Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (609 page)

   Determination of electrolytes, pH, and blood gases should be performed on blood specimens obtained simultaneously, since the acid–base situation may be very labile (Table
13-3
).

   Repeated determinations are often indicated because of the development of complications, the effect of therapy, and other factors.

   Acid–base disorders are often mixed rather than in the pure form. These mixed disorders may represent simultaneously occurring diseases, complications superimposed on the primary condition, or the effect of treatment.

   Changes in chronic forms may be notably different from those in the acute forms.

   For judging hypoxemia, it is also necessary to know the patient’s Hb or Hct and whether the patient was breathing room air or oxygen when the specimen was drawn.

   ABGs cannot be interpreted without clinical information about the patient.

   Renal compensation for a respiratory disturbance is slower (3–7 days) but more successful than respiratory compensation for a metabolic disturbance, but it cannot completely compensate for arterial CO
₂
pressure (PaCO
₂
) >65 mm Hg, unless another stimulus for HCO
₃
retention is present. The respiratory mechanism responds quickly but can only eliminate sufficient CO
₂
to balance the mildest metabolic acidosis (Table
13-4
).

   A normal pH does not ensure the absence of an acid–base disturbance if the pCO
₂
is not known.

   An abnormal HCO
₃
indicates a metabolic rather than a respiratory problem (Table
13-5
; Figures 13-2 and 13-3).

   Decreased HCO
₃
⁻
indicates metabolic acidosis.

   Increased HCO
₃
⁻
indicates metabolic alkalosis.

   Respiratory acidosis is associated with a pCO
₂
>45 mm Hg.