Diuretics are indicated when the signs of congestion, oedema, ascites or volume overload cannot be controlled by reduced salt intake alone.
Typically dogs in Stages III and IV of heart failure will benefit from diuretics, but some individuals in Stage II may also require low dose diuretic administration.
Diuretics given in heart failure will reduce circulating blood volume (hypovolaemia) thereby reducing preload effects on the heart, and loop diuretics may cause peripheral vasodilation which also reduces cardiac workload. However, the reduced renal blood flow which also results may be undesirable as it decreases glomerular filtration rate and renal function, and may precipitate acute renal failure.
Other undesirable effects of diuretic use include hypokalaemia, hypohypocalcaemia and hypomagnesaemia. Relative overdosage may cause dehydration and if extracellular fluid is lost without bicarbonate loss, metabolic alkalosis. Some authors advocate routine monitoring of body weight, creatinine, acid-base balance and serum electrolytes (parpotassium) during diuretic therapy.
Hypokalaemia enhances cardiac glycoside toxicity, may cause cardiac dysrhythmias and impairs carbohydrate metabolism.
Hypomagnesaemia potentiates the cardiac effects of hypokalaemia.
Dietary potassium supplementation with salt substitute (KCI) may be helpful in avoiding hypokalaemia, and the use of potassium sparing diuretics such as spironolactone, amiloride or triamterene in conjunction with more potent diuretics such as frusemide, will also help reduce the chances of the development of hypokalaemia, but these drugs should not
be used in the presence of other age-related conditions which might predispose to hyperkalaemia such as renal failure or diabetes mellitus. They should also not be used in the presence of metabolic acidosis or alongside therapy with beta-blockers or angiotensin-converting enzyme (ACE) inhibitors (e.g. captopril).
To avoid iatrogenic hyperkalaemia, potassium supplementation should not be given at the same time as the potassium-sparing diuretics.
The efficacy of diuretics may be reduced in the presence of hypopro(as they are protein-bound), proteinuria or impaired renal function.
Osmotic diuretics such as mannitol are contraindicated in heart disease, as they may cause cardiac overload.