Loop diuretics in heart failure: symptoms, not survival
Loop diuretics are strong diuretics that clear the fluid overload (excess fluid in the tissues and lungs) of heart failure, but they have no established effect on survival.
Remember
A loop diuretic eases the symptoms of heart failure by lowering preload (the stretch on the heart muscle before it contracts), but it has no established effect on survival.
Memory aid
loop diuretics lighten the load, they do not lengthen life
Use it to recall that loop diuretics lower preload and relieve symptoms, but they do not improve survival.
Loop diuretics such as furosemide block the NKCC2 transporter (a carrier protein that moves sodium, potassium and chloride out of the urine) in the thick ascending limb of the loop of Henle (the hairpin-shaped part of the kidney tubule that reabsorbs salt). Salt and water stay in the urine instead of returning to the blood, so the diuresis (the increase in urine output) is powerful.
In chronic heart failure the weakened heart leaves the kidneys holding on to sodium and water, so fluid collects as fluid overload (a build-up of excess fluid in the tissues and lungs). Passing more salt and water lowers the volume of circulating blood, so less blood returns to the heart and preload (the stretch on the heart muscle just before it contracts) falls. Congestion (fluid backing up into the lungs and legs) eases, and breathlessness and oedema improve.
Their benefit is symptom relief. Loop diuretics have no established effect on mortality, so they are added alongside drugs that improve prognosis. Beta-blockers and spironolactone are examples of drugs that reduce deaths in heart failure with reduced ejection fraction (a weak left ventricle that pumps out too little blood).
Loop diuretics are also the usual drug for acute pulmonary oedema (fluid in the lungs) from heart failure. Because they impair calcium reabsorption, they can also be used together with intravenous fluids to remove excess calcium in hypercalcaemia (high calcium). They also cause loss of potassium, calcium and magnesium, so electrolytes must be monitored.
Key facts
- Mechanism
- they block NKCC2 in the thick ascending limb of the loop of Henle, which gives a strong diuresis.
- Benefit
- a lower preload means less congestion, so breathlessness and oedema improve.
- Survival
- loop diuretics have no established effect on mortality in heart failure.
- Other uses
- acute pulmonary oedema, oedema from renal or liver disease, and hypercalcaemia.
- Electrolytes
- they cause loss of potassium, calcium and magnesium, so electrolytes must be monitored.
- Versus thiazides
- thiazide diuretics are less potent and reduce urinary calcium loss.
Exam traps
- Loop diuretics do not reduce mortality in chronic heart failure: they treat symptoms and have not been shown to reduce deaths, whereas drugs such as beta-blockers and spironolactone are the ones linked with lower mortality in heart failure with reduced ejection fraction.
- Loop diuretics do not relieve symptoms by raising contractility (the force of the heart's squeeze): that is the job of inotropes (drugs that raise the force of contraction), while loop diuretics act on the kidney to remove sodium and water.
- Loop diuretics mainly lower preload by removing fluid, not afterload (the resistance the left ventricle pumps against), and mixing the two up is the trap.
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