Vasodilators are now an important part of the treatment of heart disease, including CHF, in humans and small animals. They act by reducing afterload and preload. A variety of drugs are available which affect arterial or venous tone to differing degrees. Vasodilators can reduce the work of the heart and allow an increase in cardiac output and muscle perfusion, which may reduce signs of weakness. Angiotensin converting enzyme (ACE) inhibitors also reduce sodium and water retention. There is some evidence that ACE inhibitors improve both longevity and quality of life in humans and small animal patients with valvular and/or myocardial disease.
Unfortunately, there are very limited pharmacological data on vasodilators in horses. Drugs which could be used include arteriolar vasodilators such as hydralazine; mixed arteriolar and venodilators including acepromazine and ACE inhibitors (e.g. captopril and enalapril), and venodilators such as nitroprusside and nitroglycerine. Prolonged treatment would be very expensive in adult horses. Enalapril (an ACE inhibitor) has recently been licensed for use in dogs in the UK and, although it would be very expensive if it was used at the same dose, might find some use in horses, particularly if pharmacological studies were performed.
Without the benefit of experience of the action of these drugs in a substantial number of animals, treatment is fraught with potential danger. All these drugs are potentially hypotensive and need to be titrated in order that they have beneficial effects without precipitating profound hypotension. However, there could be circumstances in which, with informed owner consent, their use could be justified. Doses are empirical (see Appendix).