What is the interpretation and recommended management of an arterial blood gas with pH 7.30, arterial carbon dioxide tension 30 mm Hg, and arterial oxygen tension 70 mm Hg?

Metabolic Acidosis with Respiratory Compensation

This arterial blood gas demonstrates metabolic acidosis (pH 7.30, low) with appropriate respiratory compensation (PaCO2 30 mmHg, low) and hypoxemia (PaO2 70 mmHg), requiring immediate investigation of the underlying cause, optimization of oxygenation, and preservation of the patient's compensatory hyperventilation—do not administer bicarbonate when pH is ≥7.15. 1

ABG Interpretation

Acid-Base Status:

• The pH of 7.30 indicates acidemia (normal 7.35-7.45) 1
• The PaCO2 of 30 mmHg is low (normal 35-45 mmHg), indicating hyperventilation as a compensatory mechanism 1
• Using Winter's formula to assess appropriateness of compensation: Expected PaCO2 = 1.5 × [HCO3⁻] + 8 (±2). With an estimated HCO3⁻ of approximately 14-15 mEq/L (calculated from Henderson-Hasselbalch), the expected PaCO2 would be ~29-31 mmHg, confirming this is appropriate respiratory compensation for metabolic acidosis 1

Oxygenation Status:

• PaO2 of 70 mmHg indicates hypoxemia requiring supplemental oxygen 1

Immediate Management Priorities

Do NOT Administer Sodium Bicarbonate:

• Bicarbonate is contraindicated when pH ≥7.15, as per Surviving Sepsis Campaign guidelines 1
• The patient is successfully compensating through hyperventilation; bicarbonate administration can worsen intracellular acidosis and suppress the necessary respiratory drive 1

Oxygen Therapy:

• Provide supplemental oxygen to correct hypoxemia while maintaining adequate ventilation 1
• If COPD is suspected, use controlled oxygen therapy (28% Venturi mask or 2 L/min nasal cannulae) targeting saturation 88-92% to avoid suppressing respiratory drive 2
• For non-COPD patients, titrate oxygen to achieve adequate oxygenation 1

Preserve Respiratory Compensation:

• Do not suppress the patient's hyperventilation—this is a critical physiologic response maintaining pH near normal 1
• Avoid sedation or interventions that may impair ventilatory effort, as fatigue can precipitate rapid pH decline 1

Diagnostic Workup

Calculate Anion Gap Immediately:

• Anion Gap = Na⁺ - (Cl⁻ + HCO3⁻), normal 8-12 mEq/L 1

High Anion Gap (>12 mEq/L) Etiologies:

• Lactic acidosis from sepsis, shock, or tissue hypoperfusion—measure serum lactate immediately 1
• Diabetic ketoacidosis—check serum glucose, β-hydroxybutyrate, and urine ketones 1
• Renal failure—check creatinine and BUN 1
• Toxic ingestions (methanol, ethylene glycol, salicylates)—obtain toxicology screen if suspected 1

Normal Anion Gap (8-12 mEq/L) Etiologies:

• Diarrhea or GI bicarbonate losses 1
• Renal tubular acidosis 1

Specific Treatment Based on Etiology

For Sepsis/Shock:

• Restore adequate circulation and tissue perfusion with aggressive fluid resuscitation 1
• Administer early appropriate antibiotics 1
• Serial lactate measurements to guide resuscitation 1

For Diabetic Ketoacidosis:

• Initiate insulin therapy promptly 1
• Correct volume depletion with isotonic saline 1

For Acute Renal Failure:

• Consider renal replacement therapy if severe 1

Monitoring Protocol

Arterial Blood Gases:

• Recheck ABGs every 2-4 hours to verify compensation remains adequate and detect evolution of mixed disorders 1
• More frequent monitoring if clinical deterioration occurs 3

Serum Electrolytes:

• Monitor potassium and calcium every 2-4 hours, as alkalemia from respiratory compensation can drive potassium intracellularly 1

Serial Lactate:

• If lactic acidosis suspected, measure lactate serially to assess response to therapy 1

Critical Pitfalls to Avoid

• Never give bicarbonate when pH ≥7.15—this worsens intracellular acidosis and is not indicated 1
• Do not suppress respiratory compensation with sedation or inadequate ventilatory support 1
• Do not assume the disorder is benign because pH is near normal—the underlying metabolic acidosis requires urgent investigation 1
• Watch for respiratory fatigue—if the patient cannot maintain hyperventilation, pH will rapidly decline and mechanical ventilation may be needed 1
• Investigate for mixed disorders if PaCO2 is lower than predicted by Winter's formula, suggesting concurrent primary respiratory alkalosis 1