FRANK J. MILNE STATE-OF-THE-ART LECTURE


Fig. 57. Lead I ECG demonstrating a slow VT/AIVR at a rate of 80/minute with a single VPC of another form (large arrow), originating in the previous T-wave (R on T complex). Notice the slower atrial rate (small arrows) with some of the P-waves visible and some buried in the QRS and T complexes. 25 mm/sec paper speed.


useful in the evaluation of equine myocardial function in horses following an episode of sus- tained VT. Paroxysms of VT in horses performing a maxi-


mal exercise stress test on the treadmill to fatigue results in their rapid deceleration due to the rapid drop in cardiac output during exercise (Fig. 61). Although an association between complex ar- rhythmias and poor performance has been made,293 these may coexist in poorly performing horses without being negatively associated with athletic performance.292 Ongoing research by a number of investigators will shed more light on the significance of ventricular arrhythmias during exercise, both on their role in poorly performing horses and hopefully in horses experiencing SCD.


Horses with uniform VT and no evidence of struc-


tural heart disease often recover and return to normal work but recurrences can occur.39 These horses should be treated for their arrhythmias, rested for a minimum of 4 weeks, and a cardiac reevaluation per- formed before returning the horse to work. A contin- uous 24-hour ECG should also be performed before returning the horse to work, followed by an exercising ECG if the continuous 24-hour ECG is normal.39 Asecond exercisingECGshould be performed once the horse has returned to full work.39 Annual continuous 24-hour ECG monitoring and exercise testing is rec- ommended in these horses, along with echocardio- graphic reexamination. Horses with complex ventricular arrhythmias


should also be treated for their arrhythmias and rested for 4 weeks before cardiac reevaluation.


can be driven or ridden by an informed adult rider.39 Rigorous athletic work is not recommended for horses that have underlying structural heart dis- ease (myocardial fibrosis or moderate to severe AR) and VT, and these horses should only be used by an informed adult rider.39 Annual echocardiographic evaluation, continuous 24-hour ECG monitoring, and exercise testing is recommended in these horses.


Often, multiple cardiac reevaluations are necessary before there is a minimal amount of ventricular ectopy at rest. These horses should not be ridden or driven until their rhythm has returned to normal on a continuous 24-hour ECG and then is normal during an exercise test. A second exercise test should also be performed in these horses once the horse is back in full work. Periodic continuous 24- hour ECG monitoring and exercising ECGs of these horses is recommended to look for increased ventric- ular ectopy (at least annually and with more com- plex ventricular arrhythmias, at least biannually). Echocardiographic reexamination should also be performed at least annually in horses with a history of complex ventricular arrhythmias. These horses should only be ridden by an informed adult rider, as their safety is uncertain.39 Horses with AIVR that is overdriven with exercise


8. Sudden Cardiac Death


Sudden death in horses during or shortly after ex- ercise has a myriad of causes.311–313 Massive inter- nal hemorrhage associated with vessel rupture, including aortic rupture, are common cardiovascu- lar causes of sudden death.311–316 Horses that ex-


Fig. 58. Lead I ECG demonstrating uniform VT at a rate of 150/minute with relatively narrow QRS and T complexes. Notice the slower atrial rate (small arrows) with some of the P-waves visible and some buried in the QRS and T complexes. 25 mm/sec paper speed.


AAEP PROCEEDINGS  Vol. 64  2018 131


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