A post-op patient's ABG shows: pH: 7.25 (↓) PaCO₂: 30 mmHg (↓) HCO₃⁻: 18 mEq/L (↓). This suggests:

The ABG shows a low pH (7.25), indicating acidemia. The PaCO₂ is low (30 mmHg), which is a respiratory compensation for a primary metabolic disturbance. The bicarbonate (HCO₃⁻) is decreased at 18 mEq/L, confirming a primary metabolic acidosis. In post-op patients, metabolic acidosis commonly results from tissue hypoperfusion, lactic acid accumulation, or renal impairment. The respiratory system compensates by hyperventilating to decrease PaCO₂ and mitigate acidemia. Thus, the findings are consistent with primary metabolic acidosis with respiratory compensation.

Reference: Clinical Anesthesia, Barash, 7th Edition.