Mechanism of Action

Block the sodium-potassium-2 chloride (Na+/K+/2Cl−) cotransporter in the thick ascending limb of the loop of Henle — the segment that reabsorbs ~25% of filtered sodium. Shutting it down produces the most potent diuresis of any class, making loops first-line for acute decompensated heart failure, pulmonary edema, and fluid overload refractory to other agents. IV furosemide works within ~5 minutes; oral onset is ~30–60 minutes. Because the ascending limb also drives the concentrating mechanism, loops impair the medullary gradient (high-volume dilute urine) and increase calcium and magnesium excretion — the opposite of thiazides, which spare calcium.

Common Medications

furosemidePrototype

most commonly tested loop; IV onset ~5 min, oral ~30–60 min; typical starting IV dose 20–40 mg

bumetanide

torsemide

Indications

acute decompensated heart failure

pulmonary edema

rapid IV onset

fluid overload refractory to other agents

edema

from heart failure, hepatic, or renal disease

Side Effects

dehydration

expected with aggressive diuresis

orthostatic hypotension

volume depletion — manage with fluids and slow position changes, not vasodilator titration

hyponatremia

hypomagnesemia

hypocalcemia

loops increase calcium excretion

hyperuricemia

can precipitate gout

nocturia

if dosed late in the day

Contraindications & Interactions

Contraindications

anuria

no urine output to act on

sulfonamide allergy

furosemide/bumetanide/torsemide are sulfonamide derivatives

Interactions

digoxin

loop-induced hypokalemia potentiates digoxin toxicity

aminoglycosides

e.g. gentamicin — additive ototoxicity and nephrotoxicity

lithium

reduced clearance raises lithium levels

Administration & Monitoring

monitor serum potassium at least daily

target 3.5–5.0 mEq/L; critical with concurrent digoxin

hold and notify for hypokalemiaHoldK+ < 3.5 mEq/L

draw a stat level for muscle weakness, cramps, or U-waves

monitor full electrolyte panel

sodium, magnesium, calcium also wasted

give IV furosemide slowly≤ 20 mg/min

≥2 min for a 40 mg dose (≤4 mg/min for high continuous doses) to prevent ototoxicity — never a rapid bolus

assess hearing before and during therapy

especially with high-dose IV or concurrent aminoglycosides

obtain daily weight

same time, same scale, same clothing — most reliable fluid-status indicator; report loss > 1 kg/day

monitor intake and output

monitor BUN and creatinine

detect prerenal azotemia from over-diuresis

give the dose in the morning

avoids nocturia and nighttime fall risk

Patient Teaching

eat potassium-rich foods

bananas, oranges, potatoes

weigh yourself daily under consistent conditions

report a gain > 2 lb in a day or > 5 lb in a week

take the dose in the morning

to avoid nocturia

rise slowly from sitting or lying

orthostatic hypotension

report ringing in the ears or hearing changes

report muscle weakness or cramps

possible hypokalemia

Report Nowescalate immediately

profound fluid/electrolyte depletionBlack Box

furosemide FDA boxed warning — profuse diuresis can cause water and electrolyte depletion; dose must be individualized and the patient monitored closely

hypokalemia HallmarkK+ < 3.5 mEq/L

most dangerous effect — low potassium sensitizes the myocardium to dysrhythmias, especially with concurrent digoxin; signs: muscle weakness, leg cramps, flattened T-waves, U-waves, irregular beats; draw a stat potassium and notify the provider

ototoxicity

tinnitus, hearing loss — unique to loop diuretics among diuretic classes; dose-related and worse with rapid IV push; if a client reports ringing ears, slow the IV rate and notify the provider; compounded by concurrent aminoglycosides

Clinical Pearl

'Loops Lose it all' — potassium, sodium, calcium, magnesium, water, and hearing. The hypokalemia is what kills (especially with digoxin on board), and if the client on furosemide reports ringing ears, slow the IV rate and call the provider.