Loop Diuretics

Mechanism of Action - Loop's Power Play

• Site: Thick Ascending Limb (TAL) of Loop of Henle.
• Target: $Na⁺$-$K⁺$-$2Cl⁻$ (NKCC2) cotransporter (luminal membrane).
• Action: Reversible inhibition of NKCC2.

• Key Effects:
  • Potent ("high-ceiling") diuresis; blocks reabsorption of ~25% filtered $Na⁺$.
  • ↓ Medullary hypertonicity → impairs urine concentration.
  • ↑ Renal prostaglandin synthesis ($PGE_2$) → renal vasodilation (NSAIDs blunt this).

  ⭐ Induce hypokalemic metabolic alkalosis. Unique among diuretics for causing significant calcium excretion (hypocalcemia), making them useful in hypercalcemia. 📌 Mnemonic: "LOOPs Diuretics make you LOSE $Ca^{2+}$ (and $K^{+}$, $Mg^{2+}$)".

Pharmacokinetics & Examples - The Loop Lineup

• Agents: Furosemide, Bumetanide, Torsemide, Ethacrynic acid.
• Routes: Oral (PO), Intravenous (IV).
• Onset (IV): Rapid, ~5 min; PO: 30-60 min.
• Duration:
  • Furosemide: 4-6 hrs.
  • Torsemide: Longer, 12-24 hrs.
• Excretion: Renal (tubular secretion). High protein binding (>90%).
• Relative Potency (Furosemide = 1):
  • Bumetanide: 40 (Most potent)
  • Torsemide: 2-4
• Clinical Pearls:
  • Torsemide: ↑Bioavailability, ↑Duration.
  • Ethacrynic acid: Higher risk of ototoxicity.

⭐ Ethacrynic acid is the only non-sulfonamide loop diuretic, suitable for patients with sulfonamide allergy.

Therapeutic Uses - When to Call the Loops

• Acute Pulmonary Edema (APE): Drug of choice (DOC) for rapid symptom relief.
• Edema (refractory): Associated with:
  • Congestive Heart Failure (CHF)
  • Cirrhosis (ascites)
  • Nephrotic syndrome
• Hypertension:
  • Especially with renal insufficiency or heart failure.
  • Hypertensive emergencies (e.g., with fluid overload).
• Acute Renal Failure (ARF):
  • To ↑ urine output; may convert oliguric to non-oliguric ARF.
• Hypercalcemia (Acute):
  • Promotes renal Ca²⁺ excretion (adjunct to saline hydration).
• Forced Diuresis:
  • Certain drug overdoses/poisonings.

⭐ Loop diuretics are the most potent diuretics and are first-line for managing acute pulmonary edema.

Adverse Effects & Interactions - The Downside Drip

• Electrolyte Depletion:
  • ↓K⁺ (critical!), ↓Mg²⁺, ↓Na⁺, ↓Ca²⁺ (chronic)
  • Hypochloremic metabolic alkalosis
• Volume Depletion: Dehydration, hypotension.
• Ototoxicity ⚠️: Dose-dependent; risk ↑ with rapid IV, renal failure, aminoglycosides. Ethacrynic acid highest risk.
• Metabolic:
  • Hyperuricemia (gout risk)
  • Hyperglycemia (mild)
• Allergy: Sulfa drugs (Furosemide, etc.). Use Ethacrynic acid if allergic.
• Drug Interactions:
  • Aminoglycosides: ↑ Ototoxicity
  • NSAIDs: ↓ Diuretic action
  • Digoxin: ↑ Toxicity (via ↓K⁺)
  • Lithium: ↑ Toxicity
  • Antihypertensives: Additive hypotension

⭐ Severe hypokalemia from loop diuretics potentiates digoxin toxicity, risking arrhythmias.

High‑Yield Points - ⚡ Biggest Takeaways

• Act by inhibiting the Na⁺-K⁺-2Cl⁻ cotransporter in the thick ascending limb (TAL).
• Most potent diuretics, known as "high-ceiling" diuretics.
• Key drugs: Furosemide, Bumetanide, Torsemide; Ethacrynic acid is non-sulfa.
• Primary uses: Pulmonary edema, CHF, cirrhotic ascites, nephrotic syndrome, and hypercalcemia.
• Adverse effects: Hypokalemia, hypomagnesemia, ototoxicity, hyperuricemia, sulfa allergy.
• Ethacrynic acid is the choice for patients with sulfa allergy.
• Caution with NSAIDs (reduce diuretic effect) and aminoglycosides (increased ototoxicity).