7.4.1 Abnormal impulse conduction
Conduction blocks at the sinoatrial and atrioventricular nodes may result in a bradycardia. These arrhythmias are common in horses and are usually physioin origin. They reflect the large resting cardiac reserve in this species and are abolished by increased sympathetic and decreased parasympathetic tone. Abnormal conduction blocks can result from cell damage and lead to failure of the normal pathway to initiate ventricular contraction. In order to avoid asystole, a secondary pacemaker takes over. These are often seen as escape beats.
Re-entry arrhythmias are relatively common and can result in single or multiple premature beats or persistent arrhythmias. The most common of these in the horse is atrial fibrillation (AF). In a re-entry arrhythmia, an area of damaged tissue develops very slow conduction properties or a unidirectional block. This means that the impulse takes a long time to reach some areas of cardiac tissue in order to depolarise them. By the time that these areas are depolarised, adjacent areas, which would normally be refractory to further stimulus, have returned to their resting potentials and can be depolarised. This allows a loop to form so that the impulse can pass in a circuit and result in a premature depolarisation. The principle is demonstrated in Figure 7.1.