Diuretics alleviate the congestive signs seen in heart failure by reducing preload. They may also improve oxygenation of peripheral tissues by removing excess interstitial fluid. A reduction in preload reduces the hydrostatic pressure which is driving fluid out into the extracellular spaces, reducing oedema. It also reduthe dilutional effects of excessive water retention which is found in animals with CHF. This benefits the animal because the main clinical signs of CHF are due to oedema formation. Diuretics can be life-saving in animals with acute pulmonary oedema and dyspnoea as a result of flooding of alveoli (left-sided CHF). They are less effective in reducing oedema which has formed through the effects of increased hydrostatic pressure in the systemic capillaries (right-sided CHF).
A significant side-effect of reducing preload by the use of diuretics is that, if the fall in preload exceeds compensatory mechanisms, cardiac output can be decreased. Preload reserve is lost in CHF and as a result of diuresis, so a delicate balance must be struck. Box rest ensures that demands for increased cardiac output are kept to a minimum.
The choice of diuretics which are suitable for use is the horse is limited. Frusemide is the only diuretic which has been widely used and there seems little to be gained by trying other drugs. The dosage can be varied according to individual requirements (see Appendix). Intravenous use is required to relieve acute pulmonary oedema; intramuscular and oral preparations can be used for maintenance.