ABG Compensation on the NCLEX
Acute vs chronic, full vs partial compensation, mixed disorders.
Compensation is the body's attempt to correct pH. Recognizing it distinguishes acute from chronic.
Compensation rules
Respiratory disorder → kidneys compensate (slow, days). Metabolic disorder → lungs compensate (fast, minutes-hours).
Full vs partial
Partial compensation: pH still abnormal, opposite system has shifted. Full compensation: pH normal but both CO2 and HCO3 abnormal.
Mixed disorders
Salicylate toxicity (respiratory alkalosis + metabolic acidosis). DKA + vomiting (metabolic acidosis + metabolic alkalosis). Severe COPD with renal failure.
Tic-tac-toe method
Quick interpretation: pH, CO2, HCO3 columns. Normal vs abnormal. Match abnormal to determine primary disorder.
Frequently asked
pH normal but CO2 and HCO3 abnormal?
Fully compensated. Match the side of normal pH belongs to: 7.40 is the cutoff.
Acute respiratory failure ABG?
pH down significantly with high CO2. HCO3 still close to normal, no time to compensate.
Quick check
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