This is the most common physiologic arrhythmia in the horse. Approximately 15% of horses on a resting ECG and up to 44% of horses on a 24-hour ECG will show a second-degree AV block. The heart rate is usually normal to low normal (20-40). The rhythm is regularly irregular. The first and second heart sound (S1 and S2) are clearly audible, with a clear fourth heart sound (S4) associated with atrial contraction immediately prior to each S1.
In the diastolic pause during the AV block, S4 is audible, but there is no following S1 and S2. The ECG shows a regular R-R interval and normal QRS complexes, each preceded by a P wave. Occasional P waves are not followed by QRS complexes, usually every 3-4 beats. The interval between the P wave and the QRS complex can be consistent (Mobitz Type II) or show gradual prolongation prior to the dropped beat (Mobitz Type I, Wenckebach, most common).
The block through the AV node can be easily reversed with sympathetic stimulation (exercise, excitement) or blockage of the parasympathetic system (glycopyrolate, atropine).
Tags: Cardiology