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


ease.38,39,194,197,205,206 Although AR was not asso- ciated with AF in one large hospital-based study,65 when horses with AR develop AF, signif- icant left atrial enlargement should be suspected. The left atrial enlargement is usually a sequelae to moderate or severe longstanding AR. AF has been reported in a horse with AR due to a flail aortic valve leaflet and MR.68 Although postmortem examination of the first


horse reported with AF in 1911 revealed no signifi- cant myocardial lesions,197 myocardial fibrosis and vacuolar degeneration were reported in the next series of horses with AF.217 Visible atrial myocar- dial disease (patchy or diffuse fibrosis, dilation, thin- ning, and/or ballooning) was detected in 80% of horses with AF at gross postmortem examination.197 Varying degrees of fibrosis was the most common lesion detected histopathologically while 24% of horses had myocardial degeneration and round cell infiltration with minimal fibrosis. One-third of the horses with no gross lesions had a small amount of fibrosis detected microscopically. However, in the remaining horses, the histopathological findings were minimal or nonexistent.197,206,218 Valvular lesions (mostly mitral) were common in this group of horses, present in 80%, and, in most, the valvular regurgitation was significant. Similar findings of atrial myocardial fibrosis have


been found in many studies since, with patchy fibro- sis and microscopic focal myocardial fibrosis or fibro- fatty infiltration in the atria.218,219 Confounding the picture is the fact that similar histopathological findings have also been reported in the atria of horses without AF. Horses are predisposed to the development of AF


may vary in intensity due to variable length of ven- tricular filling time and the lack of an atrial contrac- tion.230 If the resting heart rate in AF exceeds 60 bpm, significant, usually severe, underlying struc- tural heart disease is likely.194 The intensity of the peripheral arterial pulses is also irregularly irregu- lar. In some horses, the rhythm is more regular and is initially difficult to distinguish from 2°AVB because there is a more patterned AV conduc- tion.231–233 Pulse deficits may be present, partic- ularly in horses with two conducted beats occurring in rapid succession. If the resting heart rate is elevated, underlying heart disease, sympathetic stimulation due to another concur- rent problem, or an accessory (bypass) tract may be present.39,234 The heart should be carefully ausculted for murmurs of MR, TR, and, less fre- quently, AR. These valvular insufficiencies may be present in horses with AF and influence their prognosis. MR and TR murmurs are common findings in horses with AF, with AR murmurs occurring less frequently.194,196,197,205,206,235 In some horses with AF and moderate to severe un- derlying valvular heart disease, the murmurs of valvular regurgitation can be quite soft and easily missed.


Electrocardiogram


due to their high resting vagal tone,220 large atrial mass,221,222 and autonomic imbalance associated with exercise.206,209 Horses with atrial enlarge- ment, therefore, are at an increased risk for AF. Although the risk is likely small, it is an important consideration in horses performing in high-intensity athletic events, particularly in the higher risk sports. Transient potassium depletion secondary to the administration of furosemide or excessive sweating is also a known predisposing fac- tor.206,223,224 Hyperthyroidism, although rare, has also been associated with AF in horses receiving Thyro-L or iodine-containing supplements, such as those containing kelp or ground shellfish. Shorten- ing of the effective refractory period, atrial inhomo- geneity, and APCs are other predisposing factors for the development of AF.197,201,206,225,226 Atrial tachycardia and atrial flutter are precursor arrhyth- mias for AF.225 Pulmonary hypertension has been reported as a rare cause of AF in young horses.227 AF can be induced by rapid atrial pacing221,228 and atrial burst pacing with a programmable pulse generator.216,225,229 Horses with AF usually have normal resting heart


rates (28–44 bpm), although the rhythm is irregu- larly irregular and no S4 is produced.206,230 S1


The ECG is diagnostic for AF and reveals irregularly irregular R-R intervals, no P-waves, and normal- appearing QRS complexes (Fig. 38).230 Rapid base- line fibrillation “f” waves are usually present which may be small (fine) or large (coarse). In approxi- mately 10% of horses with AF, QRS complexes of different morphology that may be aberrantly con- ducted or originating from the ventricle are present at rest.194,196,205,236


Echocardiogram


The echocardiogram of a horse in AF usually reveals a reduction in left ventricular shortening fraction that improves with cardioversion.136 The M-mode of the mitral valve lacks the A peak, caused by atrial contraction (the A peak is also lacking in the tricus- pid valve as well).136,237 Left atrial and left ven- tricular internal diameters and areas may be increased, as there are many horses presenting with AF that have mild to moderate MR. Similarly, al- though less common, there may be right atrial and right ventricular enlargement in AF horses with mild to moderate TR.


Exercise


Although the cardiac output in horses with AF and no significant underlying cardiac disease is normal at rest,209,238 a drop in cardiac output occurs during exercise.210 The exercising heart rate, which is of- ten 40–60 bpm higher than if the horse was in sinus rhythm, is a major component of the drop in cardiac output during exercise (Fig. 39).239,240 Maximal heart rate during exercise to fatigue increased to


AAEP PROCEEDINGS  Vol. 64  2018 115


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