Fig. 38. Base-apex ECG of a horse with atrial fibrillation (A) and after conversion to normal sinus rhythm (B). 25 mm/sec paper speed. A, Notice the irregularly irregular rhythm, the lack of P waves and the baseline fibrillation (f waves; arrow). B, Notice the P-waves (arrow) that precede every QRS and T complex.
over 300 bpm in horses with chronic induced AF.200 Their maximal velocity also decreased by 1.56 m/sec (a 12% decrease) compared with when they were in normal sinus rhythm.200 VPCs may be detected at rest in horses with AF. Frequent VPCs (69%) or aberrantly conducted complexes and complexes with wide QRS morphol- ogy and R on T (33%) occurred in lunging Warm- blood horses with AF and in Standardbred racehorses with induced AF exercising to fa- tigue.210,211 These complexes have been detected in other horses during periods of excitement or exercise (Fig. 40). It is unknown whether these complexes are ventricular in origin or are aber-
rantly conducted. The detection of these com- plexes is a cause for concern, as it is unknown what risk they pose for triggering more malignant arrhythmias.210,211 Differentiation between VPCs and aberrantly conducted complexes is important, but it is not known how to reliably do this in the horse, unless a multiple lead ECG system can be obtained during intense exercise.
Types of AF
Paroxysmal AF Spontaneous conversion to normal sinus rhythm may occur in horses with paroxysmal AF, usually within
Fig. 39. Base-apex ECG of a horse with rapid AF. Notice the variable R-R intervals and the slight variation in the QRS and T-wave morphology (arrows). 25 mm/sec paper speed.
