In an elderly postoperative colectomy patient with dehydration and postural hypotension, whose arterial blood gas shows pH 7.8, bicarbonate 48 mmol/L, PCO₂ 44 mm Hg, and low PO₂, what is the correct interpretation of the ABG?

Uncompensated Metabolic Alkalosis

This ABG demonstrates uncompensated metabolic alkalosis (Answer A), characterized by markedly elevated pH (7.8) and bicarbonate (48 mmol/L) with inadequate respiratory compensation, in the clinical context of post-colectomy dehydration with volume depletion. 1

ABG Interpretation Framework

Primary Acid-Base Disturbance

• pH 7.8 indicates severe alkalemia (normal 7.35-7.45), confirming a primary alkalotic process 1
• HCO₃⁻ 48 mmol/L is markedly elevated (normal 22-26 mEq/L), identifying the metabolic component as the primary disturbance 1
• PCO₂ 44 mmHg is within normal range (normal 35-45 mmHg), demonstrating the absence of meaningful respiratory compensation 1

Assessment of Compensation Status

The key distinguishing feature is that PCO₂ has NOT risen appropriately to compensate for the severe metabolic alkalosis: 1

• In metabolic alkalosis, expected respiratory compensation involves hypoventilation to retain CO₂ and lower pH 1
• With HCO₃⁻ of 48 mmol/L, adequate compensation would require PCO₂ to rise substantially above 44 mmHg 1
• The PCO₂ remaining at 44 mmHg (essentially normal) while pH is 7.8 defines this as uncompensated 1

Clinical Context Supporting Metabolic Alkalosis

Post-colectomy patients commonly develop metabolic alkalosis through multiple mechanisms: 2

• Volume depletion from dehydration drives contraction alkalosis as kidneys retain bicarbonate to preserve volume 3
• Gastric fluid losses (nasogastric suction, vomiting) result in hydrogen ion and chloride depletion 3
• Postural hypotension confirms significant volume depletion, perpetuating renal bicarbonate retention 2

Why Other Options Are Incorrect

Option B (Compensated metabolic & respiratory alkalosis) is wrong because:

• PCO₂ 44 mmHg is normal, not low—there is no concurrent respiratory alkalosis 1
• The disorder is uncompensated, not compensated, as evidenced by the severely abnormal pH 1

Option C (Respiratory alkalosis with metabolic compensation) is wrong because:

• PCO₂ is normal (44 mmHg), not low—ruling out primary respiratory alkalosis 1
• The elevated bicarbonate is the primary problem, not a compensatory response 1

Option D (Metabolic alkalosis with respiratory compensation) is wrong because:

• While the primary disorder is correctly identified as metabolic alkalosis, compensation is absent 1
• True compensation would show PCO₂ elevated well above 44 mmHg and pH closer to normal range 1

Critical Clinical Considerations

The low PO₂ requires immediate attention despite the alkalosis: 4

• PO₂ <60 mmHg represents life-threatening hypoxemia demanding urgent oxygen supplementation 4
• Target SpO₂ 94-98% in this patient without COPD risk factors 4
• Repeat ABG within 30-60 minutes after initiating oxygen therapy 4

Severe alkalemia (pH 7.8) carries significant mortality risk: 5

• Mortality increases progressively as pH rises, reaching 48.5% when pH exceeds 7.60 5
• Mixed respiratory and metabolic alkalosis has particularly poor prognosis (44.2% mortality), though not present in this case 5

Management priorities include: 3

• Volume resuscitation with normal saline to correct contraction alkalosis 3
• Correction of electrolyte abnormalities, particularly chloride and potassium depletion 3
• Treatment of underlying surgical complications if present 2