💊 Loop Diuretics
Drug Class & Overview
Loop diuretics are a class of potent diuretics that act on the thick ascending limb of the loop of Henle. They are primarily used to treat fluid overload states such as heart failure and oedema.
Mechanism of Action
These drugs inhibit the Na+/K+/2Cl- co-transporter (NKCC2) in the luminal membrane of the thick ascending limb. This prevents the reabsorption of sodium, potassium, and chloride, leading to increased excretion of water and electrolytes. The high capacity of this segment for reabsorption makes loop diuretics very effective.
Key Indications
Loop diuretics are indicated for the treatment of oedema associated with heart failure, liver cirrhosis, and renal disease. They are also used in the management of hypertension, particularly in patients with impaired renal function, and in acute pulmonary oedema.
Contraindications
Absolute contraindications include anuria and severe hypovolaemia or dehydration. They should be used with caution in patients with severe electrolyte imbalances, hepatic coma, or pre-coma due to the risk of exacerbating these conditions. Hypersensitivity to sulphonamides is a relative contraindication for some loop diuretics.
Adverse Effects
Common adverse effects include electrolyte disturbances such as hypokalaemia, hyponatraemia, and hypomagnesaemia. Dehydration, hypotension, and dizziness are also frequent. Ototoxicity, particularly with rapid intravenous administration or in renal impairment, is a serious but less common side effect.
Monitoring
Regular monitoring of electrolytes (potassium, sodium, magnesium, calcium) and renal function (urea, creatinine) is essential. Blood pressure and fluid balance should also be closely monitored, especially during initiation and dose adjustments, to prevent hypotension and dehydration.
Prescribing Safety (OSCE)
When prescribing, always check for sulphonamide allergy if prescribing furosemide or bumetanide. Counsel patients on the importance of regular blood tests and to report symptoms of dehydration or dizziness. Be aware of interactions with NSAIDs (reduced efficacy) and digoxin (increased toxicity due to hypokalaemia).
MLA High-Yield Notes
High-yield facts include their use in acute pulmonary oedema, their potent natriuretic effect, and the characteristic electrolyte disturbances they cause (hypokalaemia, hypomagnesaemia). Remember that they can cause ototoxicity. Furosemide is the most commonly prescribed loop diuretic in the UK.
Common SBA Themes
SBAs often focus on distinguishing loop diuretic side effects from other drug classes, particularly electrolyte imbalances like hypokalaemia. Questions may present a patient with oedema and ask about the most appropriate diuretic or the expected electrolyte abnormality. Another common theme is the management of acute pulmonary oedema.
References
- BNF (British National Formulary)
- NICE (National Institute for Health and Care Excellence)
- MHRA (Medicines and Healthcare products Regulatory Agency)