In salicylate poisoning, which acid-base disturbance occurs first, followed by a mixed picture?

Explanation

## Acid-Base Disturbances in Salicylate Poisoning ### Biphasic Acid-Base Pattern Salicylate poisoning produces a characteristic **two-stage acid-base disturbance**: | Stage | Mechanism | pH | PaCO₂ | HCO₃⁻ | Clinical Features | |-------|-----------|-----|-------|-------|-------------------| | **Early (1st)** | Direct respiratory center stimulation by salicylate | ↑ (alkalemia) | ↓ | Normal/↓ | Tachypnea, hyperventilation, respiratory alkalosis | | **Late (2nd)** | Metabolic acidosis from uncoupling + salicylate accumulation | ↓ (acidemia) | ↓ | ↓ | Mixed respiratory alkalosis + metabolic acidosis; severe acidemia | ### Early Phase: Respiratory Alkalosis **Key Point:** Salicylate directly stimulates the respiratory center in the medulla, causing hyperventilation and CO₂ loss → **respiratory alkalosis**. - Occurs within 1–2 hours of ingestion - Tachypnea is a hallmark clinical sign - Blood pH is elevated ### Late Phase: Mixed Acid-Base Disturbance **High-Yield:** As salicylate accumulates, **metabolic acidosis** develops due to: 1. Uncoupling of oxidative phosphorylation → ↑ O₂ consumption, ↑ heat, ↑ metabolic rate 2. Accumulation of organic acids (lactate, ketones) 3. Salicylate itself acts as an acid **Clinical Pearl:** The **mixed disturbance** (respiratory alkalosis + metabolic acidosis) is pathognomonic for salicylate poisoning and is the most dangerous phase: - Respiratory alkalosis masks the severity of metabolic acidosis - pH may appear near-normal despite severe underlying acidosis - Risk of pulmonary edema and altered mental status ### Mnemonic **SALICYLATE = Stimulates Alveolar Hyperventilation, Later Induces Cellular Acidosis, Yet Appears Lethal Acidotic Toxicity Emergently** Or simply: **"Blow first, then acid"** — respiratory alkalosis first, metabolic acidosis follows. [cite:Harrison 21e Ch 439]