7.7.4 Sinus block or arrest
Sinus block or arrest are manifestations of high vagal tone which are much less frequent than AV block (see below), but which are not uncommon. Horses with these arrhythmias have a slow heart rate with pauses less than or equal to or greater than two normal R-R intervals. During these pauses no A sound will be heard and no P wave is seen on the ECG (Figure 7.6). Sinus block or arrest are very seldom pathological. In extreme cases, junctional or ventricular escape beats may be seen (see below). However, if syncopal episodes which can be attributed to the presence of these arrhythmias do occur, treatment with atropine is indicated in the short-term. In the long-term, implantation of an artificial pacemaker could be required if the syncope can be directly related to the dysIt is very rare that any treatment is required.