Horses cardiac drugs

The use of cardiac therapeutic agents in the horse is less developed than in other species and these agents are often used empirically by extrapolation of data from other species. With a few exceptions, there is no difference between the arsenal of cardiac drugs used currently in equine medicine and that used 20 years ago. This chapter describes the current practice in equine cardiac therapeutics and also discusses some newer agents used in other species that are currently under preliminary evaluation in the horse. 

Adverse CNS effects occur when the procaine portion of the procaine benzylpenicillin (procaine penicillin) formulation is given intravascularly. The signs of toxicity include hyperexcitability, muscle tremors, ataxia, apnea and cardiac arrest. There is no specific treatment for procaine toxicity one can only attempt to prevent the horse from injuring itself and others imtil the effects of the procaine wear off. The CNS reaction can be prevented by pretreatment with diazepam. The solubility of the procaine fraction of procaine benzylpenicillin (procaine penicillin) formulations increases with increasing ambient temperature, so these products should be stored in a cool place to reduce the risk of reactions. Procaine is a common cause of positive drug tests in racehorses and other performance horses. Procaine benzylpenicillin (procaine penicillin) should be avoided in these animals. 

Rapid i.v. administration of tetracyclines can result in hypotension and collapse. This has been attributed to intravascular chelation of calcium and/or a decrease in blood pressure owing to the drug vehicle. The i.v. administration of doxycycline to horses causes tachycardia, systemic arterial hypertension, collapse and death. This reaction may be caused by the highly lipid-soluble doxycycline chelating intracellular calcium, resulting in cardiac neuromuscular blockade. 

In horses, diminazene aceturate should be administered by deep i.m. injection divided between several injection sites. The injection sites should be massaged in order to promote drug absorption. At the dose rates suggested here, signs of toxicity are imcommon but local reactions, caused by muscle necrosis, may be severe. Diminazene aceturate should be avoided in horses unless other drugs are either ineffective or unavailable. Toxic doses result in respiratory distress, depression, cardiac signs, hypersalivation and diarrhea. Toxic doses may be treated with calcium salts. 

Quinidine, the d-isomer of quinine, has been used in horses since 1924 (Roos 1924) and has a wide variety of properties. It is currently the drug of choice for the treatment of atrial fibrillation (AF) in horses but can be used for the treatment of a variety of re-entrant and ectopic arrhythmias. Quinidine affects heart rate, cardiac rhythm and vascular tone by a range of mechanisms and also produces a variety of non-cardiac effects. In addition to the class la activity, which prolongs the effective refractory period, the drug is vagolytic as a result of its antimuscarinic properties. 

Propranolol is a lipid-soluble drug that undergoes rapid hepatic metabolism, resulting in a short half-life (2h). Rapid first-pass metabolism results in a low bioavailability and therapeutic plasma concentrations are rarely achieved in the horse after oral administration (Muir Mcguirk 1987). The drug can only administered i.v. and plasma concentrations are increased if hepatic blood flow is reduced, e.g. by significant cardiac disease. 

Muir W W 1995 The haemodynamic effects of milrinone HCI In halothane anaesthetized horses. Equine Veterinary Journal Supplement 19 108-113 Muir W W, Mcguirk S M 1985 Pheirmacology and pharmacokinetics of drugs used to treat cardiac disease in horses. Veterinary Clinics of North America Equine Practice 1 335-352 Muir W W D, Mcguirk S 1987 Cardiovascular drugs. Their pharmacology and use In horses. Veterinary Clinics of North America Equine Practice 3 37-57 Muir W W D, Reed S M, Mcguirk S M 1990 Treatment of atrial fibrillation in horses by intravenous administration of quinidine. Journal of the American Veterinary Medical Association 197 1607-1610... 

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