A 58-year-old man with diabetes mellitus presents to the emergency department with acute kidney injury following a contrast-enhanced coronary angiography performed 48 hours ago. Serum creatinine has risen from 1.8 mg/dL to 4.2 mg/dL. Urinalysis shows muddy brown casts and fractional excretion of sodium (FENa) is 2.1%. He is euvolemic on clinical examination. What is the drug of choice to prevent progression of contrast-induced acute tubular necrosis?

## Contrast-Induced Acute Tubular Necrosis (CI-ATN) ### Pathophysiology Contrast-induced nephropathy causes direct tubular toxicity and renal vasoconstriction, leading to ATN. The mechanism involves: 1. Osmotic load and tubular obstruction 2. Reactive oxygen species (ROS) generation 3. Renal medullary hypoxia 4. Direct tubular epithelial injury ### Pharmacological Prevention & Management **Key Point:** N-acetylcysteine (NAC) is the drug of choice for prevention and early management of contrast-induced ATN because it is a potent antioxidant that scavenges free radicals and reduces oxidative stress. **High-Yield:** NAC works through multiple mechanisms: - Antioxidant: reduces ROS-mediated tubular injury - Vasodilator: improves renal blood flow - Reduces contrast viscosity - Restores glutathione stores in tubular epithelium ### Evidence & Dosing | Parameter | NAC Protocol | |-----------|-------------| | **Dose** | 600 mg orally twice daily (or 150 mg/kg IV loading, then 50 mg/kg maintenance) | | **Timing** | Start 24 hours before contrast, continue 48 hours after | | **Route** | Oral preferred (cost-effective); IV for high-risk patients | | **Evidence** | Multiple RCTs show 50% reduction in ATN incidence when combined with hydration | **Clinical Pearl:** The combination of **isotonic saline hydration + NAC** is superior to either agent alone. Hydration remains the cornerstone; NAC is the pharmacological adjunct. ### Why Other Agents Fail | Drug | Why NOT Used | Mechanism of Failure | |------|--------------|---------------------| | **Furosemide** | Increases tubular obstruction | Loop diuretics worsen medullary hypoxia and concentrate contrast in tubules | | **Dopamine** | No renal protection | Low-dose dopamine does not prevent ATN; increases systemic effects | | **Mannitol** | Osmotic load worsens injury | Increases tubular fluid osmolarity; may precipitate in dehydrated tubules | **Warning:** Diuretics (furosemide, mannitol) and dopamine are contraindicated in contrast-induced ATN because they exacerbate tubular obstruction and medullary hypoxia.