A 52-year-old man with chronic kidney disease stage 4 presents with fatigue and dyspnea. Arterial blood gas shows pH 7.28, PaCO₂ 32 mmHg, HCO₃⁻ 14 mEq/L, and PaO₂ 88 mmHg. What is the most common cause of this acid-base disturbance in CKD patients?

Explanation

## Acid-Base Disturbance in CKD **Key Point:** The most common acid-base disorder in chronic kidney disease is **metabolic acidosis**, specifically due to retention of unmeasured anions (sulfate, phosphate, organic acids) and impaired renal ammonia excretion. ### Mechanism of CKD-Related Metabolic Acidosis 1. **Reduced GFR** → decreased filtration of fixed acids (sulfate, phosphate) 2. **Impaired ammonia excretion** → ammonia (NH₃) is a major renal buffer; reduced proximal tubular synthesis and secretion impairs acid excretion 3. **Loss of functional nephrons** → decreased capacity for titratable acid excretion 4. **Accumulation of unmeasured anions** → creates a high anion gap metabolic acidosis (typically AG 12–16 mEq/L in CKD) ### ABG Interpretation in This Case | Parameter | Value | Interpretation | |-----------|-------|----------------| | pH | 7.28 | Acidemia | | HCO₃⁻ | 14 mEq/L | Low (normal 22–26) | | PaCO₂ | 32 mmHg | Low (respiratory compensation) | | Anion Gap | ~16 | High (unmeasured anions) | **Clinical Pearl:** The low PaCO₂ (32 mmHg) indicates appropriate respiratory compensation via hyperventilation—this is a **primary metabolic acidosis with appropriate respiratory response**, not a primary respiratory disorder. **High-Yield:** CKD metabolic acidosis is **NOT due to bicarbonate wasting in urine** (that occurs in renal tubular acidosis); it is due to **inability to excrete acid** (retention mechanism). ### Why Respiratory Acidosis Is Less Common Respiratory acidosis (elevated PaCO₂) is NOT the most common primary disturbance in CKD. Pulmonary edema and aspiration are acute complications, not the chronic metabolic signature of CKD. [cite:Harrison 21e Ch 278]