loop diuretic
A loop diuretic is the heavy-duty pump for getting fluid out fast. Where a thiazide leaves a tap dripping, a loop diuretic opens the floodgates. It targets the part of the kidney that does the heaviest lifting in salt reabsorption, so blocking it produces a large, rapid loss of salt and water, which is why these drugs can rescue someone drowning in their own fluid.
Loop diuretics act on the thick ascending limb of the loop of Henle, where they inhibit the sodium-potassium-chloride (Na-K-2Cl) cotransporter. Because this segment normally reabsorbs a large fraction of filtered sodium, blocking it produces a powerful diuresis that works even when kidney function is reduced. Furosemide is the classic example, along with bumetanide and torasemide; they are first choice for the pulmonary congestion of acute heart failure and for fluid overload in renal and liver disease.
Their potency cuts both ways. They can cause rapid dehydration, low potassium, low sodium, low magnesium and a metabolic alkalosis, and high-dose intravenous use can damage hearing (ototoxicity), especially with rapid injection or alongside aminoglycosides. They also flush out calcium, useful in treating high blood calcium but a long-term bone concern. Dosing is titrated to the patient's fluid status and kidney function.
A patient arrives breathless with fluid on the lungs; intravenous furosemide, a loop diuretic, produces a brisk diuresis within an hour and the breathlessness eases.
Intravenous loop diuretics act within minutes to an hour, the emergency tool for pulmonary oedema.
Loop diuretics are called high-ceiling diuretics because increasing the dose keeps increasing the response, unlike thiazides whose effect plateaus.