Mixed acid-base disorders

Mixed acid-base disturbances are combinations of two or more primary acid-base disturbances. Mixed acid-base disturbances may be suspected on the basis of findings obtained from the medical history, physical examination, serum electrolytes and chemistries, and anion gap. The history, physical examination, and serum biochemical profile may reveal disease processes commonly associated with acid-base disturbances. Changes in serum total CO2, serum potassium and chloride concentrations, or increased anion gap may provide clues to the existence of acid-base disorders. Blood gas analysis is usually required to confirm mixed acid-base disorders. To identify mixed acid-base disorders, blood gas analysis is used to identify primary acid-base disturbance and determine if an appropriate compensatory response has developed. Inappropriate compensatory responses (inadequate or excessive) are evidence of a mixed respiratory and metabolic disorder. The anion gap is also of value in detecting mixed acid-base disturbances. In high anion gap metabolic acidosis, the change in the anion gap should approximate the change in serum bicarbonate. Absence of this relationship should prompt consideration of a mixed metabolic acid-base disorder. Finding an elevated anion gap, regardless of serum bicarbonate concentration, suggests metabolic acidosis. In some instances, elevated anion gap is the only evidence of metabolic acidosis. In patients with hyperchloremic metabolic acidosis, increases in the serum chloride concentration should approximate the reduction in the serum bicarbonate concentration. Significant alterations from this relationship also indicate that a mixed metabolic disorder may be present. In treatment of mixed acid-base disorders, careful consideration should be given to the potential impact of therapeutically altering one acid-base disorder without correcting others.