7.7.1 First degree atrioventricular block
First degree AV block (10AVB) is caused by delayed conduction of the sinus impulse through the AV node resulting in prolongation of the P-R interval. Auscultation may reveal a prolonged A-S1 interval. The condition is difficult to define in the horse because of the wide variation of normality, but an interval of >0.5 sec should be considered abnormal. In the horse it is usually due to high vagal tone, but it can also be caused by drugs such as xylazine, detomidine, and digoxin. It is rarely caused by AV nodal disease. 10AVB is primarily an ECG diagnosis of little practical relevance in horses.