Patients taking a loop diuretic can be considered relevant for a medicines use review (MUR) if they fall into the national target group of patients who are at risk of, or have been diagnosed with, cardiovascular disease and are regularly being prescribed at least four medicines.
They can also fall into the target group for post-discharge reviews, following the transition of a patient taking two or more medicines from hospital to community care.
What to know before advising patients
Loop diuretics are a group of diuretics (furosemide and bumetanide) that cause powerful diuresis (more powerful than the diuresis caused by thiazide diuretics).
To understand the mechanism of action, it is important to understand some of the kidney’s functions. The kidney is made up of nephrons that filter and balance fluid and electrolytes between blood and urine. Generally, the electrolytes are reabsorbed into the body in the loop of Henlé, which promotes the reabsorption of water as it follows the higher concentration of electrolytes.
At a pharmacological level, these diuretics inhibit the reabsorption of electrolytes from the ascending limb of the loop of Henlé in the renal tubule. As the filtrate in the ascending limb contains sodium and other solutes, water is not drawn out of the tubule. This leads to an increased excretion of water from the body in the form of urine.
What are loop diuretics used for?
Loop diuretics are primarily used for oedema (swelling in the ankles, feet and legs caused by a build-up of fluid in these areas). They are also used in various cardiovascular conditions – including fluid in the lungs due to inefficient heart pumping in heart failure – and in high blood pressure.
Furosemide and bumetanide have similar diuretic activity. They begin to act within one hour, and the effect is usually completed after around six hours. The diuretic effects are dose related.
How should patients take their loop diuretic?
Doses are usually taken once daily in the morning, but loop diuretics can be prescribed twice daily if additional diuresis is needed. Patients can be advised to adjust the timing of their doses for convenience with respect to their social and work life.
Generally, patients don’t need to drink more fluids while taking a diuretic (unless they have been advised to exercise fluid restriction by their doctor). You can advise patients to drink as much as they normally do and in response to their feelings of thirst.
Side effects of loop diuretics
The most common side effects are:
- electrolyte imbalance
- metabolic alkalosis
- muscle spasms
Pregnancy and breastfeeding
Loop diuretics are not suitable for use during pregnancy, due to the risk of hypovolaemia (low blood volume due to fluid depletion) in the mother.
Loop diuretics are passed into breast milk, so should not be used in breastfeeding patients. In addition, the BNF states that furosemide and bumetanide may inhibit lactation.
Loop diuretics can exacerbate gout (due to increased uric acid levels), whereas latent diabetes may be exposed by treatment with a loop diuretic and insulin requirements may change in patients with established diabetes.
Lower doses should be used in elderly patients, especially when initiating, as they are particularly susceptible to the side effects. The dose also needs to be adjusted based on the renal function of the patient.
Due to the mechanism of action, loop diuretics can lead to low levels of potassium in the blood, which is problematic for patients with cardiovascular disease. For patients taking digoxin, the low potassium levels increase the risk of digoxin toxicity, resulting in an increased risk of abnormal cardiac rhythm. For this reason, loop diuretics are often taken with a potassium-sparing diuretic (eg furosemide is available as a combination product with the potassium-sparing diuretic amiloride).
Renal function must be monitored regularly during treatment, especially during initiation (eg renal function should be checked after the first week). If decreases in renal function are detected, dose reduction and withdrawal may be necessary.
Because of the increase in diuresis, loop diuretics should be used with caution in patients who have urinary retention, such as men with benign prostatic hypertrophy.
Diuretics can also be problematic in patients who are susceptible to hypotension.
Loop diuretics have a number of important interactions with other drugs. Patients should be advised to seek the advice of a doctor or pharmacist before taking additional medicines. For a full list of interactions and clinical relevance, consult the BNF.
What else can you recommend to patients?
Due to the risks of dehydration with diuretics, patients are often advised that if they develop diarrhoea and vomiting while taking these drugs, they should maintain their fluid intake and stop the diuretic for 1-2 days until they recover. Stopping treatment for a short time is thought to avoid dehydration, hypotension and acute kidney injury (and should not cause a sudden deterioration in heart function). If diarrhoea and vomiting persist beyond two days, they should see a GP and have their blood checked.
If a patient experiences dizziness during treatment (due to the reduced volume of fluid in their body), they should avoid driving. If symptoms of hypotension, dizziness, light headedness or confusion are frequent or persistent, it may be a sign of excessive diuresis and the patient must be referred to their GP.
You can advise patients that if they forget to take a dose of their diuretic, they should take it as soon as they remember it and then continue the following morning as normal. They should not take a double dose to make up for a forgotten dose.
They should also be careful when drinking alcohol while taking diuretics, because it may cause their blood pressure to fall. Alcohol also has a diuretic effect (by blocking the release of an antidiuretic hormone) and this may lead to lower blood pressure.