What is the acid‑base status of a patient with a pCO₂ of 5.7 kPa, pH of 7.473, pO₂ of 7.84 kPa, and bicarbonate (HCO₃⁻) of 30.7 mmol/L?

Acid-Base Status Interpretation

This patient has a metabolic alkalosis with partial respiratory compensation. The elevated pH (7.473), high bicarbonate (30.7 mmol/L), and mildly elevated pCO₂ (5.7 kPa ≈ 43 mmHg) indicate a primary metabolic alkalosis with the respiratory system attempting to compensate by retaining CO₂. 1

Detailed Analysis

Primary Disorder Identification

• pH 7.473 is alkalemic (normal range 7.35–7.45), confirming alkalosis rather than acidosis 1
• Bicarbonate 30.7 mmol/L is elevated above the normal range of 22–26 mmol/L, indicating a metabolic component 1
• pCO₂ 5.7 kPa (≈43 mmHg) is at the upper limit of normal or mildly elevated (normal 4.7–6.0 kPa or 35–45 mmHg), representing respiratory compensation 1
• pO₂ 7.84 kPa (≈59 mmHg) is low, suggesting hypoxemia that may be contributing to the clinical picture 2

Distinguishing Primary from Compensatory Processes

The elevated bicarbonate is the primary abnormality, not a compensatory response to chronic respiratory acidosis, because: 1

• The pH is frankly alkalemic (>7.45), not normal or acidemic
• In compensated chronic respiratory acidosis, the pH would be normal (7.35–7.45) despite elevated bicarbonate and pCO₂ 1
• The pCO₂ elevation is modest and proportionate to the degree of metabolic alkalosis, consistent with appropriate respiratory compensation 1

Expected Compensation Assessment

Using Winter's formula to assess adequacy of respiratory compensation: 3

• Expected pCO₂ = 1.5 × HCO₃ + 8 = 1.5 × 30.7 + 8 = 54 mmHg (≈7.2 kPa)
• Actual pCO₂ is 43 mmHg (5.7 kPa), which is lower than expected
• This suggests either incomplete respiratory compensation or a concurrent respiratory alkalosis component 3

Common Causes to Investigate

Volume Depletion and Contraction Alkalosis

• Loop diuretic use causing chloride, sodium, and water losses with compensatory bicarbonate retention 1
• Assess for orthostatic hypotension, decreased skin turgor, elevated BUN/creatinine ratio 1
• Check serum chloride (typically low <95 mEq/L in contraction alkalosis) 1

Chronic Respiratory Conditions

• COPD, obesity hypoventilation syndrome, or neuromuscular disease could contribute to baseline CO₂ retention 1
• The low pO₂ (7.84 kPa) suggests underlying respiratory pathology 2
• However, the alkalemic pH rules out primary chronic respiratory acidosis 1

Gastrointestinal Losses

• Vomiting or nasogastric suction causing loss of gastric HCl 1
• Evaluate for nausea, vomiting, or NG tube presence

Clinical Management Approach

Immediate Assessment

1. Obtain complete electrolyte panel including sodium, potassium, chloride, and calculate anion gap 1
2. Assess volume status through physical examination (orthostatic vitals, skin turgor, mucous membranes) 1
3. Review medication list for loop diuretics, thiazides, or other alkalosis-inducing agents 1
4. Evaluate respiratory status given the hypoxemia (pO₂ 7.84 kPa) 2

Treatment Strategy

Do not attempt to "correct" the elevated bicarbonate directly with acidifying agents, as the metabolic alkalosis is protective and maintaining a near-normal pH: 1

• For diuretic-induced alkalosis: Reduce or temporarily hold diuretics if bicarbonate rises significantly above 30 mmol/L and patient is volume depleted 1
• Replete chloride and volume with normal saline (0.9% NaCl) to restore volume and provide chloride 1
• Address hypoxemia: Target oxygen saturation of 88–92% if chronic hypercapnia is present, or >92% if no chronic respiratory disease 2, 1

Monitoring Parameters

• Serial blood gases if bicarbonate continues to rise above 35 mmol/L or if respiratory symptoms develop 1
• Serum electrolytes every 2–4 hours during active fluid resuscitation 1
• Urine output and renal function to assess response to volume repletion 1

Critical Pitfalls to Avoid

• Do not give bicarbonate therapy – this patient has alkalosis, not acidosis; bicarbonate would worsen the condition 1, 4
• Do not target "normal" oxygen saturation (94–98%) if chronic hypercapnia is present, as this can suppress hypoxic drive 2, 1
• Do not overlook the hypoxemia (pO₂ 7.84 kPa) – investigate and treat the underlying cause 2
• Do not assume chronic respiratory acidosis based solely on elevated bicarbonate; the alkalemic pH excludes this diagnosis 1