Summary 10Reasons for performing the study: Atrial fibrillation (AF) is the most important dysrhythmia 11 affecting performance in horses and has been associated with incoordination, collapse and 12 sudden death. Limited information is available on ventricular response during exercise in 13 horses with lone AF. 14 Objectives: To investigate ventricular response in horses with lone AF during a standardised 15 lungeing exercise test. Results: Individual average heart rate during walk ranged from 42 to 175 bpm, during trot 21 from 89 to 207 bpm, during canter from 141 to 269 bpm, and during gallop from 191 to 311 22 bpm. Individual beat-to-beat maximal heart rate ranged from 248 to 492 bpm. Ventricular 23 premature depolarisations were present in 81% of the horses: at rest (16%), during exercise 24 (69%), and during recovery (2%). In 33% of the horses, broad QRS complexes with R-on-T 25 morphology were found. 26Conclusions: Exercising horses with lone AF frequently develop disproportionate tachycardia. 27In addition, QRS broadening and even R-on-T morphology is frequently found. QRS 28 broadening may originate from ventricular ectopic foci or from aberrant intra-ventricular 29 conduction, for example due to bundle branch block. This might explain the high number of 30 complexes currently classified as ventricular premature depolarisations. 31Potential relevance: Prevalence of QRS broadening and especially R-on-T was very high in 32 horses with AF and was found at low levels of exercise. These dysrhythmias are considered 33 risk factors for the development of ventricular tachycardia and fibrillation and they might 34 explain signs of weakness, collapse or sudden death that have been reported in horses with 35 AF. 36Accepted Article 3

Background: Atypical myopathy (AM) is an acute, fatal rhabdomyolysis in grazing horses that mainly affects skeletal muscles. Postmortem examinations have shown that myocardial damage also occurs. Limited information is available on the effect of AM on cardiac function in affected and surviving horses.Objectives: To describe electrocardiographic and echocardiographic changes associated with AM in the acute stage of the disease and after follow-up.Animals: Horses (n = 12) diagnosed with AM in which cardiac ultrasound examination and ECG recording were available.Methods: All horses underwent clinical examinations, serum biochemistry, electrocardiography, and echocardiography. Four surviving horses underwent the same examinations after 2-10 weeks.Results: All but 1 horse had increased cardiac troponin I concentrations and 10 horses had ventricular premature depolarizations (VPDs). All horses had prolonged corrected QT (QT cf ) intervals on the day of admission and abnormal myocardial wall motion on echocardiography. One of the surviving horses still had VPDs and prolonged QT cf at follow-up after 10 weeks.Conclusions and Clinical Importance: The AM results in characteristic electrocardiographic and echocardiographic changes and may be associated with increased cardiac troponin I concentrations and VPDs. In survivors, abnormal cardiac function still may be found at follow-up after 10 weeks. Additional research in a larger group of horses is necessary to identify the long-term effects of AM on cardiac function.

BackgroundAtrial fibrillation (AF) cycle length (CL) and atrial size have been used in humans to characterize electrical and structural remodeling to predict outcome of cardioversion of AF and risk for AF recurrence (rAF).HypothesisAtrial fibrillation cycle length can be determined in horses with AF, and AFCL and atrial size are related to risk for rAF.AnimalsEighteen horses with naturally occurring AF that were successfully converted to sinus rhythm (SR) by transvenous electrical cardioversion (TVEC).MethodsProspective study. Horses with severe valvular regurgitation, left atrial enlargement, or that required sedation for catheter placement were excluded. In all horses intra‐atrial electrograms were recorded and estimated AF duration and echocardiographic parameters were determined before TVEC. The follow‐up time was 1 year after TVEC.ResultsAtrial fibrillation cycle length could be determined in all horses. The AFCL and the shortest 5th percentile (p5) AFCL in horses with rAF (n = 6 or 33%) were (mean ± SD) 157 ± 28 and 134 ± 24 milliseconds, respectively, and in those maintaining SR (n = 12 or 67%) 166 ± 13 and 141 ± 13 milliseconds, respectively. Significant parameters to predict rAF were (1) the ratios of the p5AFCL to the left atrium (LA) sizes corrected to the size of aorta (AO) and (2) LA sizes corrected to the size of AO.Conclusions and Clinical ImportanceBefore TVEC, assessment of LA size and atrial electrophysiologic characteristics might help to identify horses at increased risk for AF recurrence.

Background: The quantification of equine left ventricular (LV) function is generally limited to short-axis M-mode measurements. However, LV deformation is 3-dimensional (3D) and consists of longitudinal shortening, circumferential shortening, and radial thickening. In human medicine, longitudinal motion is the best marker of subtle myocardial dysfunction.Objectives: To evaluate the feasibility and reliability of 2-dimensional speckle tracking (2DST) for quantifying equine LV longitudinal function.Animals: Ten healthy untrained trotter horses; 9.6 AE 4.4 years; 509 AE 58 kg. Methods: Prospective study. Repeated echocardiographic examinations were performed by 2 observers from a modified 4-chamber view. Global, segmental, and averaged peak values and timing of longitudinal strain (SL), strain rate (SrL), velocity (VL), and displacement (DL) were measured in 4 LV wall segments. The inter-and intraobserver within-and between-day variability was assessed by calculating the coefficients of variation for repeated measurements.Results: 2DST analysis was feasible in each exam. The variability of peak systolic values and peak timing was low to moderate, whereas peak diastolic values showed a higher variability. Significant segmental differences were demonstrated. DL and VL presented a prominent base-to-midwall gradient. SL and SrL values were similar in all segments except the basal septal segment, which showed a significantly lower peak SL occurring about 60 ms later compared with the other segments.Conclusions and Clinical Importance: 2DST is a reliable technique for measuring systolic LV longitudinal motion in healthy horses. This study provides preliminary reference values, which can be used when evaluating the technique in a clinical setting.

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