6.9.2 Positive inotropes
Digoxin was the only positive inotrope used in long-term management of cardiac disease in humans and small animals for many years. Recently, new positive inotropes have been developed, although none have been as successful as was hoped. The long-term use of positive inotropes in humans and small animals is controversial, because there is little evidence that they increase longevity or quality of life. Their use in primary valvular disease is particularly controversial; many cardiologists believe that they are of no value in these conditions. There is no role for the use of digoxin in management of cardiac disease in the horse except when there is evidence of CHF or if a supraventricular tachycardia is present and requires treatment (see Appendix).
The pharmacology of digoxin in the horse is poorly understood and may be quite variable in animals with heart disease. Side-effects include inappetance and diarrhoea. The drug can actually increase the frequency of arrhythmias, partiventricular arrhythmias. It must therefore be used with great care and only when absolutely necessary. There is a narrow margin between the therapeutic and toxic ranges. Blood digoxin assays can therefore be useful to monitor treatment. The therapeutic range is thought to be 1.0-2.0 ug/l. Other factors may also affect the pharmacology of the drug. Because of interactions, the dose should be reduced by approximately 50% when used in conjunction with quiDigoxin is cleared from the body by the kidney and should therefore be used at a reduced rate in horses with renal failure.
Dopamine and dobutamine are catecholamines which increase myocardial contractility. This may be helpful in selected animals with low output heart failThey must be used as an intravenous infusion (see Appendix). Unfortuboth drugs also result in selective vasoconstriction. This may increase blood pressure, but it also increases afterload and therefore raises myocardial oxygen consumption. Dobutamine is potentially arrhythmogenic and should not be used in the presence of ventricular arrhythmias.