Classification

The maximum efficacy in removing salt and water that any drug can achieve is related to its site of action, and it is clinically appropriate to rank diuretics according to their natriuretic capacity, as set out below. The percentages quoted in this rank order refer to the highest fractional excretion of filtered sodium under carefully controlled conditions and should not be taken to represent the average fractional sodium loss during clinical use.

High efficacy

Frusemide (furosemide) and the other (loop) diuretics can cause up to 25% of filtered sodium to be excreted. Their action impairs the powerful urine-concentrating mechanism of the loop of Henle and confers higher efficacy compared to drugs that act in the relatively hypotonic cortex (see below). Progressive increase in dose is matched by increasing diuresis, i.e. they have a high 'ceiling' of effect. Indeed, they are so efficacious that overtreatment can readily dehydrate the patient. Loop diuretics remain effective at glomerular filtration rates below 10 ml/min (normal 120 ml/min).

Moderate efficacy

The thiazide family, including bendrofluazide (bendroflumethiazide) and the related chlorthalidone, clopamide, indapamide, mefruside, metolazone and xipamide, cause 5-10% of filtered sodium load to be excreted. Increasing the dose beyond a small range produces no added diuresis, i.e. they have a low 'ceiling' of effect. Such drugs tend to be ineffective once the glomerular filtration rate has fallen below 20 ml/min (except metolazone).

Low efficacy

Potassium sparing triamterene, amiloride and spironolactone, cause 5% of the filtered sodium to be excreted. They are usefully combined with more efficacious diuretics to prevent the potassium loss, which other diuretics cause.

Osmotic diuretics, e.g. mannitol, also fall into this category.

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