Pharmacology · Topic 71 of 110
Loop Diuretics
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Loop Diuretics
(Furosemide, bumetanide) The strongest diuretics: sodium and water leave, and potassium falls with them.
Zooming into the kidney onto one nephron. In the loop of Henle a pump sends sodium back into the blood, and water follows the sodium. Furosemide, a loop diuretic, blocks the pump, so the sodium and the water stay in the tubule and leave in the urine: the urine output climbs and the extra fluid comes off.
Used for · don't give
Used for
Don't give / use caution
How it works
Loops block the Na⁺/K⁺/2Cl⁻ pump in the loop of Henle — the most potent diuretic class
Why potassium falls
"Loops lose it all": potassium, sodium, magnesium and calcium leave too
The same nephron, the loop pump still blocked by furosemide: sodium and water keep leaving in the urine, and potassium rides out with them. The blood potassium falls below the normal 3.5 to 5.0, which sets up dysrhythmias.
Side effects
Low potassium is the most dangerous effect
Low K⁺ on the monitor
A live monitor strip of hypokalemia: flattened T waves, a U wave (an extra hump after each T) and a dipping ST segment.
Too much fluid off
Lying down, the blood pressure is 118/76 and the pulse 88. The patient sits, then stands: within 3 minutes the blood pressure falls to 94/64, a systolic drop of 24, and the pulse climbs to 112. This is orthostatic hypotension, a sign of low volume and a fall risk: dangle the legs and rise slowly.
Labs & monitoring
Check potassium before giving — normal 3.5–5.0 mEq/L
Red flags
Giving it
Check K⁺ first — push IV slowly, over 1–2 minutes
IV works within about 5 minutes; by mouth within 1 hour.
A fast push can damage hearingTake it in the morning so the diuresis doesn’t cause nocturia (getting up at night to urinate).
Eat potassium-rich foods (bananas, oranges, potatoes), or take the supplement if prescribed.
More detail
Nursing priorities
Loop or K⁺-sparing?
| Loop | K⁺-sparing | |
|---|---|---|
| Examples | Furosemide, bumetanide, torsemide | Spironolactone, eplerenone, amiloride, triamterene |
| Works in | The loop of Henle | The late distal tubule and collecting duct |
| Strength | The most potent diuretic class | Weak alone — paired with a loop or thiazide |
| Potassium | Wastes K⁺: hypokalemia | Keeps K⁺: hyperkalemia |
| Hold if | K⁺ under 3.5 · hold + notify | K⁺ above 5.0 · hold + notify |
| ECG | Flat T waves, U waves | Tall, peaked T waves |
| Food | Eat potassium-rich foods | No extra potassium foods, no salt substitutes |
| Its own effect | Ototoxicity (ringing ears, hearing loss) | Gynecomastia (spironolactone) |
Teach your patient
Loops lose it all: check K⁺ before every dose, and hold if it is under 3.5. Push IV slowly, over 1–2 minutes — and weigh daily.
Sources · Furosemide injection label (DailyMed) · Furosemide tablets label (DailyMed) · StatPearls: Loop Diuretics (NCBI Bookshelf) · StatPearls: Therapeutic Uses of Diuretic Agents (NCBI Bookshelf) · OpenStax Pharmacology for Nurses 34.2: Loop Diuretics · OpenStax Pharmacology for Nurses 19.5: Diuretics (heart failure) · OpenStax Medical-Surgical Nursing 10.3: Electrolyte Imbalance · OpenStax Clinical Nursing Skills 19.2: Nursing Assessment for Fluid and Electrolytes · Films: the NurseSavvy learning-flow visuals. Follow your facility’s protocol.
Loop Diuretics
Furosemide can drop potassium fast enough to trigger a fatal dysrhythmia — but ototoxicity is the adverse effect students forget until the NCLEX reminds them.
Loop diuretics (furosemide, bumetanide, torsemide) produce aggressive diuresis that depletes electrolytes along with fluid. The most dangerous adverse effect is hypokalemia, because low potassium sensitizes the myocardium to dysrhythmias — especially when the patient is also on digoxin. Monitor potassium before and during therapy; the target is 3.5–5.0 mEq/L. Beyond potassium, loops waste sodium, magnesium, and calcium, so a full electrolyte panel matters. Ototoxicity — tinnitus, hearing loss — is unique to loop diuretics among diuretic classes and is dose-related; risk spikes with rapid IV push (furosemide IV should be given no faster than 10 mg/min in most protocols). Dehydration and orthostatic hypotension are expected with aggressive diuresis. Weigh the patient daily (same time, same scale, same clothing) and report weight loss greater than 1 kg/day. Monitor I&O, BUN/creatinine for prerenal injury, and uric acid because loops can precipitate gout. Teach the patient to rise slowly, eat potassium-rich foods (bananas, oranges, potatoes), and take the dose in the morning to avoid nocturia.
Key Distinctions
Don't confuse loop diuretic hypokalemia with potassium-sparing diuretic hyperkalemia — they cause opposite potassium shifts and require opposite dietary teaching. Students often attribute ototoxicity to all diuretics, but it is specific to loop diuretics. Orthostatic hypotension from loops reflects volume depletion, not a vasodilatory mechanism — the intervention is fluid and position management, not dose titration of a vasodilator.
Clinical Pearl
'Loops Lose it all' — potassium, sodium, calcium, magnesium, and hearing. If the patient on furosemide reports ringing ears, slow the IV rate and notify the provider immediately.
MOA & Use
Loop diuretics (furosemide, bumetanide, torsemide) block the sodium-potassium-2 chloride (Na+/K+/2Cl−) cotransporter in the thick ascending limb of the loop of Henle — the segment responsible for reabsorbing roughly 25% of filtered sodium. By shutting this transporter down, loops produce the most potent diuresis of any diuretic class. This makes them first-line for acute decompensated heart failure, pulmonary edema, and fluid overload refractory to other agents. IV furosemide begins working within 5 minutes; oral onset is about 30–60 minutes. The typical starting IV dose is 20–40 mg. Because the ascending limb also drives the kidney's concentrating mechanism, loops impair the medullary gradient, producing dilute urine in high volume. They also increase calcium and magnesium excretion — a distinction from thiazides, which retain calcium. Furosemide is the most commonly tested loop diuretic on NCLEX.
Key Distinctions
Don't confuse loops with thiazides: loops waste calcium while thiazides spare it — this matters for a patient with osteoporosis. Students mix up site of action: loops act on the ascending limb of the loop of Henle, not the distal tubule (that's thiazides). Unlike potassium-sparing diuretics, loops cause significant potassium and magnesium loss, not retention.
Clinical Pearl
Think "Loops Lose it all" — sodium, potassium, calcium, magnesium, and water. If it's an electrolyte, a loop diuretic is probably wasting it.
Knowledge Check
3 quick questions on the must-knows for this topic.
What is the most dangerous electrolyte effect of loop diuretics?
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