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


horse may be used by an informed adult rider.39 Riding horses in which the 2°AVB does not com- pletely disappear during exercise is not recom- mended. Frequent monitoring of the horse’s heart rate and rhythm is recommended both at rest and during exercise.39 Monitoring the horse’s rhythm at rest can be performed by monitoring the arterial pulse or ausculting the heart with a stethoscope, which most clients can do successfully. However, a smartphone-enabled ECG recording device is useful for obtaining an actual ECG tracing of the rhythm that can then be emailed to the veterinarian for interpretation. During exercise, a heart rate mon- itor is used to determine if the horse’s heart rate is appropriate for the degree of exercise performed. Horses that are symptomatic at rest are not safe


for ridden or driven work and should be given a guarded prognosis.39 Progression of high-grade 2°AVB to complete (3°AVB) heart block can occur as the conduction system disease advances. With 3°AVB, none of the impulses originating at the sinus node and conducted across the atria are conducted through the AV node to the ventricle. The atrial and ventricular rates are totally independent, with a very slow ventricular rate. The insertion of a tem- porary transvenous pacemaker is suggested in col- lapsing horses, but intermittent pacing can be a problem due to the large size of the equine ventri- cle.188 Implantation of a permanent pacemaker is the treatment of choice for horses with complete AVB.189,190 Permanent pacing is also an option for horses who are symptomatic with advanced 2°AVB. Horses with high-grade 2°AVB that does not im-


prove with exercise or excitement and horses with complete heart block are at risk for collapse or SCD. Myocardial inflammation or degeneration of the AV nodal tissues has been detected histopathologically in affected horses. Treatment with a course of cor- ticosteroids in decreasing dosages may be effective if myocardial inflammation is present in acute cases. If sedation of a horse with high-grade AVB is neces- sary,2 receptor adrenergic agonist drugs should be avoided, if possible. If use of these drugs is desired, atropine should be used prior to their use to see if the heart rate increases appropriately.191,192


Atrial Fibrillation


Atrial fibrillation (AF) is a common arrhythmia in horses, with a prevalence of 0.6–5.3%.193–198 In horses finishing their race, the frequency of parox- ysmal AF is 0.029% compared with a prevalence of 1.39% in slow finishing and nonfinishing Thorough- bred racehorses.199 There was a higher frequency of paroxysmal AF in Standardbred racehorses (0.14%) but a similar frequency in those with poor performance (2.0%).199,200 AF is most frequently associated with exercise intolerance or poor performance but can also be an incidental finding detected on routine examination. A sudden slowing of speed during the middle or toward the end of the race is common. The onset of


114 2018  Vol. 64  AAEP PROCEEDINGS


AF is the most likely reason for this sudden de- crease in racing performance.201–203 Other presenting complaints include tachypnea, dyspnea, coughing exer- cise-induced pulmonary hemorrhage (EIPH), myopathy, colic, ataxia (rare), collapse (rare), vocalization (also rare), and clinical signs of CHF.194,204–206 EIPH, com- monly detected on endoscopic examination in most elite athletes, often occurs as marked epistaxis dur- ing or immediately after intense exercise in horses with AF.194,206–208 In most horses with AF-associ- ated EIPH and epistaxis, the severe EIPH and epi- staxis resolve once the AF is converted and does not return when the horse returns to work.206 In horses with AF-associated EIPH and epistaxis, the epistaxis recurs with a recurrence of AF.206,207 During exercise, pulmonary artery wedge pressure increases and is significantly higher in horses with AF than in normal horses.208 Stress failure of the pulmonary capillaries occurs during exercise, and this is probably more severe in AF horses due to the increased transmural pressure across the pulmo- nary capillaries.206,207 Conversion of AF results in a decrease in pulmonary arterial and right atrial pressures.209 The detection of wide QRS tachycardia during exer-


cise occurs in some horses with AF in naturally occur- ring and chronic induced AF.210,211 Fatal ventricular arrhythmias may arise from this wide QRS tachycar- dia and be responsible for SCD, butSCDis very rare in horses with AF during or after exercise. There is a genetic predisposition to AF in Standard-


breds, particularly in male horses and in pacers.212–214 Although there are several stallions that are signifi- cant contributors to the genetics of Standardbreds with AF, AF is most likely in the descendants of one Standardbred sire.214 Oldenburg and Westphalian horses were slightly more likely to present with AF in one study, but this effect was not strong, whereas Quarter Horses were at low risk of presenting with AF.212 In the older literature, draft horses were more likely to present with AF compared with light breed horses,197 but that was not the case in a more recent hospital-based study.212 Many horses with AF have little or no detectable underlying cardiac disease. Horses with no detect- able heart disease have often been referred to as having “lone” AF. This terminology was coined in human medicine long before the pathophysiology of AF was understood. “Lone” AF referred to individuals in which no underlying cardiac disease was found.215 It is likely that “lone” AF may represent AF where the heart disease is below the limit of detection, rather than AF in the absence of underlying cardiac disease, and that microstructural disease or channelopathies are present in these people. It is likely that the sim- ilar situation is present in our equine patients. We know that AF induces rapid electrical and contractile remodeling in horses.216 AFis also a common finding in horses with valvular heart disease, particularly MR (most common) and TR, as well as in those with CHF secondary to the severe valvular heart dis-


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