Diuretics in critically ill patients: a narrative review of their mechanisms and applications

Diuretics remain the cornerstone therapy of critically ill patients with volume overload as a result of cardiac failure, acute kidney injury or aggressive fluid resuscitation. This review summarises the principles of applied renal physiology, describing the mechanisms of action, the clinical applications, and the adverse effects of commonly used diuretics during critical illness. Loop diuretics, and in particular furosemide, remain the most popular, despite evidence of any effect on mortality or, indeed, on the need for renal replacement therapy. The efficacy of loop diuretics after administration depends on three factors. Firstly, the tubular concentration of the diuretic: continuous infusion of furosemide seems to provide a higher and more stable tubular concentration of furosemide with respect to bolus injection. Secondly, the interaction with albumin both in the plasma and in the renal tubule: despite a strong physiological rationale supporting this approach, albumin supplementation in hypoalbuminaemic patients does not seem to result in a higher diuretic efficacy. Thirdly, diuretic resistance, which can be addressed by optimising loop diuretic dose and by using combination therapy with other agents, including thiazides or thiazide-like diuretics or carbonic anhydrase inhibitors. These drugs constitute a useful adjunct to overcome loop diuretic resistance. Other agents such as distal potassium-sparing diuretics and osmotic diuretics can also be considered. The latter have been used successfully in hypokalaemia, rhabdomyolysis-associated acute kidney injury or to prevent ischaemia-reperfusion injury in kidney transplantation. Finally, this review provides the basic concepts of the interplay between acid-base equilibrium and diuretic therapy.