Disturbances of cardiac rhythm are called arrhythmias or dysrhythmias. The term arrhythmia means a lack of rhythm, while a dysrhythmia is a disorder of rhythm. However, in practice they are often used synonymously. In the normal heart, rhythm is dependant on the discharge of the sinus node and is therefore known as sinus rhythm. In normal horses, disturbances from sinus rhythm are common due to high parasympathetic (vagal) tone. However, other arrhythmias may be associated with pathological processes such as myocardial disease, electrolyte imbalance, or toxaemia. Some arrhythmias have no identifiable cause.
Even if arrhythmias are associated with pathological change, they may not be sufficiently severe to result in clinical signs. However, they can reduce cardiac output directly, or they can be symptomatic of cardiac disease which itself is affecting cardiac function. A normal cardiac rhythm ensures that the filling and contraction of the heart is co-ordinated. An irregularity may reduce stroke volume by reducing the time available for ventricular filling during a short diainterval. In addition, cardiac arrhythmias may affect overall heart rate. In normal animals, the ability to alter heart rate is essential to maintain blood pressure at rest and to allow animals to respond to increased demands, for example during exercise. An abnormally high rate may prevent complete venfilling, so that stroke volume falls, in turn reducing cardiac output. An abnormally slow heart rate may reduce cardiac output sufficiently for clinical signs to be apparent. Usually this will be mediated by a drop in blood pressure, so the clinical signs seen are those of weakness or syncope, but it is very uncommon in horses.
7.1 Clinical examination of horses with arrhythmias
7.2 Identification of arrhythmias
7.3 Classification of arrhythmias
7.4 The pathophysiology of conduction abnormalities and arrhythmias
7.5 Principle of the treatment of arrhythmias
7.6 Normal cardiac rhythms
7.7 Arrhythmias which are frequently associated with high vagal tone
7.8 Pathological arrhythmias