UC Davis

Electrical disturbances in the heart, known as cardiac arrhythmias, are common in the horse buttheir clinical signficance remains a topic of debate. Due to their electric nature, arrhythmias have a tendency to go unnoticed, as there are often a lack of clincal signs associated with them. While arrhythmias have been associated with sudden cardiac death, in other instances the presence of a cardiac arrhythmia is not indicative of disease. This discrepency inspires the need to determine when cardiac arrhythmias are clinically relevant and what factors may predispose a horse to develop them. Perhaps more iminently, there is a need to scratch the surface on what a veterinarian may consider “normal”.

I begin this thesis by exploring, in Chapter 1, the prevalence and risk factors for cardiac arrhythmias in a sedentary mixed breed equine population over the course of 24-hour electrocardiogram recordings. This chapter served as an exploration of what is to be expected in a sedentary herd of horses and aims to provide baseline data on prevalence in an average herd of North American horses. Our data shows that arrhythmias occur at a high prevalence despite a lack of clinical signs, and that increased body condition may put a horse at risk for the development of arrhythmias.

Cardiac remodeling, due to stress from disease or exercise, alters the structure of the heart, which in turn effects electrical conduction in the heart. The relationship between arrhythmogenesis and cardiac remodeling remains unclear. In Chapter 2, the investigation of arrhythmogenesis continues by introducing and investigating the relationship between structure and electric functionality of the heart. Here, we demonstrate that myocardial staining for fibrosis and the matricellular protein tenascin C may serve as valuable indicators of myocardial remodeling following damage from exercise or disease.

The continued investigation of equine arrhythmogenesis may provide the basis for the development of screening protocols to reduce the incidence of potential life-threatening cardiac changes. Collectively this work demonstrates the complexity involved in both diagnosing and categorizing cardiac disease status in the horse, as well as providing inspiration for the work that remains to be done.