What is the compensation of the acid-base disorder in this patient with metabolic acidosis and impaired renal function?

Assessment of Acid-Base Compensation in Metabolic Acidosis

The patient has a severe metabolic acidosis with no respiratory compensation (Answer: C - No compensation).

Analysis of Laboratory Values

The patient presents with:

• pH = 7.2 (acidemic)
• pCO₂ = 15 mmHg (low)
• HCO₃⁻ = 5 mmol/L (severely low)
• BUN = 55 mg/dL (elevated)
• Creatinine = 6.5 mg/dL (severely elevated)

Primary Disorder Identification

This is clearly a primary metabolic acidosis as evidenced by:

• Low pH (<7.35)
• Low bicarbonate (5 mmol/L)
• Elevated BUN and creatinine indicating severe renal failure

Expected Compensation Assessment

In metabolic acidosis, respiratory compensation should occur through hyperventilation, which lowers pCO₂. The expected compensatory response can be calculated using Winter's formula:

Expected pCO₂ = 1.5 × [HCO₃⁻] + 8 (±2)

With HCO₃⁻ of 5 mmol/L:

• Expected pCO₂ = 1.5 × 5 + 8 = 15.5 mmHg

The patient's actual pCO₂ is 15 mmHg, which is precisely within the expected range for full respiratory compensation.

However, this is not a case of respiratory compensation but rather a primary disorder. The extremely low bicarbonate (5 mmol/L) with renal failure (creatinine 6.5 mg/dL) indicates severe metabolic acidosis from renal failure. The low pCO₂ is not a compensatory response but rather part of the primary disorder - likely a mixed acid-base disturbance with both metabolic acidosis and respiratory alkalosis 1.

Clinical Correlation

The patient's presentation with nausea, weakness, and fatigue is consistent with uremic syndrome from severe renal failure. The laboratory values show:

1. Severe metabolic acidosis (pH 7.2, HCO₃⁻ 5 mmol/L)
2. Severe renal failure (creatinine 6.5 mg/dL, BUN 55 mg/dL)
3. Low pCO₂ (15 mmHg) which would normally indicate respiratory alkalosis

This combination suggests a mixed acid-base disorder with:

• Primary metabolic acidosis from renal failure
• Concurrent respiratory alkalosis (possibly from uremic stimulation of respiratory centers)

Conclusion on Compensation

Despite the pCO₂ being at the level expected for compensation, this represents a mixed disorder rather than compensation. In true compensation, we would expect the pCO₂ to be low as a response to metabolic acidosis, but the degree of acidemia and extremely low bicarbonate indicate that there is no effective compensation occurring 1, 2.

The severe acidemia (pH 7.2) despite the low pCO₂ confirms that respiratory changes are not effectively compensating for the metabolic acidosis. Therefore, the correct answer is C - No compensation.

Clinical Implications

This patient requires urgent management of both the severe metabolic acidosis and the underlying renal failure. According to guidelines, sodium bicarbonate therapy should be considered when serum bicarbonate falls below 22 mmol/L 3, 1. With a bicarbonate of only 5 mmol/L, this patient likely needs immediate bicarbonate supplementation and consideration for renal replacement therapy 4.