Loop diuretics are diuretics that act on the ascending loop of Henle in the kidney. They are primarily used in medicine to treat hypertension and edema often due to congestive heart failure or renal insufficiency.
Mechanism of action
Loop diuretics act on the Na+/K+/Cl- cotransporter in the ascending loop of Henle to inhibit sodium and chloride reabsorption. Because magnesium and calcium reabsorption in the thick ascending loop is dependent on sodium and chloride concentrations, loop diuretics also inhibit their reabsorption. This raises the osmotic pressure inside the loop, driving more water into the filtrate causing increased urine volume.
Loop diuretics also cause vasodilation of the veins and of the kidney's blood vessels, mechanically causing a decrease in blood pressure.
The collective effects of increased urine production and vasodilation decrease blood pressure and ameliorate edema.
Loop diuretics are principally used in the following indications (Rossi, 2004):
They are also sometimes used in the management of severe hypercalcaemia in combination with adequate rehydration (Rossi, 2004).
Loop diuretic resistance
Renal insufficiency causes decreased bloodflow to the kidneys, which decreases the glomerular filtration rate (GFR) and reduces the ability of loop diuretics to reach their target organ, the loop of Henle. Similarly, non-steroidal anti-inflammatory drugs also decrease GFR with comparable results. In patients with reduced GFR, ceiling dosages of loop diuretics are increased proportional to the decrease in GFR. Simultaneous treatment with a thiazide diuretic such as hydrochlorothiazide (to inhibit sodium reabsorption at multiple sites in the nephron) is often successful.
Patients with congestive heart failure tend to retain sodium, also necessitating an increase in dosage. The same is true for patients with increased sodium intake.
The most common adverse drug reactions (ADRs) are dose-related and relate to the effect of loop diuretics on diuresis and electrolyte balance.
Common ADRs include: hyponatraemia, hypokalaemia, hypomagnesaemia, dehydration, hyperuricaemia, gout, dizziness, postural hypotension, syncope (Rossi, 2004).
Infrequent ADRs include: dyslipidaemia, increased serum creatinine concentration, hypocalcaemia, rash (Rossi, 2004).
Ototoxicity is a serious, but rare ADR associated with use of loop diuretics. This may be limited to tinnitus and vertigo, but may result in deafness in serious cases.
It should also be noted that loop diuretics may also precipitate renal failure in patients also concomitantly taking an NSAID and an ACE inhibitor - the so-called "triple whammy" effect (Thomas, 2000).
Examples of loop diuretics
- Loop Diuretic (http://www.fpnotebook.com/CV222.htm), from the Family Practice Notebook