Cardiac Pathologies
Atrial Fibrillation (AF)
Atrial fibrillation (AF) is considered the most common cardiac pathology in the horse and is the most clinically significant __________________ in horses. AF may be idiopathic, or it may be caused by mitral valve deficiencies, bradycardia, high tone of the vagus nerve, or by electrolyte imbalances. Frequent yet random stimulation of the ____________________ (AV) node causes numerous, asynchronous areas of depolarization & repolarization within the atria. The random nervous input causes rapid, weak, highly irregular & asynchronous atrial contractions that do not function to pump blood into the ventricles. Instead ventricular filling occurs _________________, resulting in a twenty to thirty percent decrease in stroke volume. This means that there is less blood available for the ventricles to pump out to other areas of the body. With increased heart rates, such as is seen with exercise, the inefficiency of ventricular fill causes the pulse rate to be considerably lower than heart rate, which is called a _______________________ . When atrial fibrillation is seen with poor performance it is suspected that electrolyte imbalance disrupts the resting membrane potential of the cardiac muscle cells and the Purkinje fibres, inhibiting the normal right to left flow of depolarization in the atria or that a lack of calcium ion prevents the cardiac muscle cells from contracting in response to electrical stimulus. Atrial fibrillation is seen most commonly in _______________horses, and in other large breeds, but is sometimes seen in Standardbred or Thoroughbred racehorses in combination with poor performance. Atrial fibrillation is seen clinically as an irregularly irregular _________________________ , bradycardia or tachycardia, syncope, poor performance, exercise intolerance and an irregular ECG showing absent P-waves, rapid F-waves and irregularly irregular R to R intervals.
Diagnosis of AF can be made through auscultation, and ECG.
Atrial fibrillation is treated by treating any underlying conditions such as electrolyte imbalances or valvular defects. Once underlying pathologies are treated the arrhythmia can be treated through the administration of ___________________ , or through the direct electrical stimulation of the atria. It is important to note that horses affected by AF and have a low resting heart rate are unsuitable for riding or driving as they are prone to syncope.
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