Overview
This lecture reviews loop diuretics, covering renal physiology, mechanisms, drug comparisons, clinical use, side effects, and considerations such as dosing and allergies.
Renal Physiology Review
- Kidneys regulate fluid, electrolyte, and acid-base balance.
- Nephrons filter blood; most sodium and water reabsorption occurs in the proximal tubule (60%) and loop of Henle (20-40%).
- Loop diuretics act on the ascending loop of Henle at the sodium-potassium-2-chloride (Na-K-2Cl) co-transporter.
Loop Diuretics Overview
- Common loop diuretics: furosemide (Lasix), bumetanide, torsemide.
- These drugs have a dose ceiling due to risk of ototoxicity at high doses.
- Oral bioavailability varies: furosemide is lowest, bumetanide and torsemide higher.
- IV administration bypasses absorption issues, especially in gut edema.
Dosing and Conversion
- Bumetanide is ~40x, torsemide ~4x more potent than furosemide.
- Oral to IV furosemide conversion is roughly 2:1 due to poor oral absorption.
- Dose adjustments are needed for renal dysfunction and over time due to diuretic resistance.
Mechanism & Resistance
- Loop diuretics inhibit Na-K-2Cl transporter, increasing excretion of sodium, water, potassium, calcium, and magnesium.
- Blocking sodium reabsorption increases sodium delivery to distal tubule, triggering compensatory sodium reabsorption and potential diuretic resistance.
- Strategies to overcome resistance: increase dose, dosing frequency, or add a thiazide diuretic—cautiously, to avoid over-diuresis and renal failure.
Side Effects & Clinical Considerations
- Electrolyte losses: hypokalemia, hypomagnesemia, hypocalcemia, and risk of QT prolongation.
- Potassium and magnesium often need to be supplemented; hypokalemia is hard to correct if magnesium is low.
- Overdiuresis risks: renal failure, contraction alkalosis, metabolic alkalosis.
- Ototoxicity is a risk at high cumulative doses.
Drug Allergies & Other Pearls
- Loop diuretics are structurally sulfonamides, but cross-reactivity with antibiotic sulfa allergies is very low.
- Ethacrynic acid lacks the sulfonamide group and is rarely used for true sulfa allergy.
- High-dose loop diuretic use is associated with worse outcomes in heart failure, partly due to side effects.
- IV administration preferred in acute decompensation due to gut edema.
Key Terms & Definitions
- Loop Diuretic — Drug inhibiting Na-K-2Cl transporter in ascending loop of Henle, causing strong diuresis.
- Bioavailability — Proportion of drug absorbed into circulation after administration.
- Diuretic Resistance — Reduced drug effectiveness due to compensatory sodium reabsorption.
- Ototoxicity — Drug-induced damage to the ear, possible with high loop diuretic doses.
- Contraction Alkalosis — Elevated pH due to excessive loss of extracellular fluid.
Action Items / Next Steps
- Review mechanisms and compare loop diuretic agents, focusing on bioavailability and potency.
- Practice dose conversion between agents.
- Read about electrolyte monitoring and supplementation for patients on loop diuretics.
- Understand precautions for patients with sulfonamide allergies.