Mechanical properties, collagen and glycosaminoglycan content of equine superficial digital flexor tendons are not affected by training.

Physical activity can activate extracellular matrix (ECM) protein synthesis and influence the size and mechanical properties of tendon. In this study, we aimed to investigate whether different training histories of horses would influence the synthesis of collagen and other matrix proteins and alter the mechanical properties of tendon. Samples from superficial digital flexor tendon (SDFT) from horses that were either (a) currently race trained (n = 5), (b) previously race trained (n = 5) or (c) untrained (n = 4) were analysed for matrix protein abundance (mass spectrometry), collagen and glycosaminoglycan (GAG) content, ECM gene expression and mechanical properties. It was found that ECM synthesis by tendon fibroblasts in vitro varied depending upon the previous training history. In contrast, fascicle morphology, collagen and GAG content, mechanical properties and ECM gene expression of the tendon did not reveal any significant differences between groups. In conclusion, although we could not identify any direct impact of the physical training history on the mechanical properties or major ECM components of the tendon, it is evident that horse tendon cells are responsive to loading in vivo, and the training background may lead to a modification in the composition of newly synthesised matrix.

Atrial fibrillation as a risk factor for exercise-induced pulmonary haemorrhage following a standardised exercise test.

BACKGROUND: Atrial fibrillation (AF) has been proposed as a risk factor for exercise-induced pulmonary haemorrhage (EIPH) due to increased pressure in the left atrium. OBJECTIVE: To evaluate if AF was associated with EIPH following a standardised exercise test (SET) to fatigue. STUDY DESIGN: Two-arm controlled experiment. METHODS: Ten untrained Standardbred mares mean (standard deviation [SD]) age 6 (2) years performed a SET on the treadmill in sinus rhythm (SR) (SET1) and 25-44 days after induction of self-sustained AF (SET2). AF was induced by tachypacing using a pacing device. Endoscopy, including tracheal wash and bronchoalveolar lavage (BAL), was performed 48-72 h before and 24 h after the two SETs. In addition, endoscopic grading of tracheal blood was performed 2 h after each SET. RESULTS: After SET1, none of the horses showed blood in the trachea, and two horses showed erythrophagocytosis. Following SET2, two horses had grade 1 blood in the trachea and free erythrocytes and erythrophagocytosis in the BAL, while another two horses had erythrophagocytosis in the BAL. In SET2, the overall performance on the treadmill was decreased with a lower maximum velocity (SET1 10.3 ± 0.8 m/s vs. SET2 8.9 ± 0.9 m/s, p = 0.004), a higher heart rate (284 ± 21 vs. 221 ± 18 bpm, p = 0.003) and more abnormal QRS complexes (p < 0.001) compared with SET1. CONCLUSIONS: Two horses showed signs of EIPH, resulting in visible blood in the trachea, when exercising in AF compared with SR. However, a possible link between EIPH, pulmonary pressure and AF needs to be further elucidated.

Muscarinic acetylcholine receptors M2 are upregulated in the atrioventricular nodal tract in horses with a high burden of second-degree atrioventricular block.

Background: Second-degree atrioventricular (AV) block at rest is very common in horses. The underlying molecular mechanisms are unexplored, but commonly attributed to high vagal tone. Aim: To assess whether AV block in horses is due to altered expression of the effectors of vagal signalling in the AV node, with specific emphasis on the muscarinic acetylcholine receptor (M2) and the G protein-gated inwardly rectifying K+ (GIRK4) channel that mediates the cardiac IK,ACh current. Method: Eighteen horses with a low burden of second-degree AV block (median 8 block per 20 h, IQR: 32 per 20 h) were assigned to the control group, while 17 horses with a high burden of second-degree AV block (median: 408 block per 20 h, IQR: 1,436 per 20 h) were assigned to the AV block group. Radiotelemetry ECG recordings were performed to assess PR interval and incidence of second-degree AV block episodes at baseline and on pharmacological blockade of the autonomic nervous system (ANS). Wenckebach cycle length was measured by intracardiac pacing (n = 16). Furthermore, the expression levels of the M2 receptor and the GIRK4 subunit of the IKACh channel were quantified in biopsies from the right atrium, the AV node and right ventricle using immunohistochemistry and machine learning-based automated segmentation analysis (n = 9 + 9). Results: The AV block group had a significantly longer PR interval (mean ± SD, 0.40 ± 0.05 s; p < 0.001) and a longer Wenckebach cycle length (mean ± SD, 995 ± 86 ms; p = 0.007) at baseline. After blocking the ANS, all second-degree AV block episodes were abolished, and the difference in PR interval disappered (p = 0.80). The AV block group had significantly higher expression of the M2 receptor (p = 0.02), but not the GIRK4 (p = 0.25) in the AV node compared to the control group. Both M2 and GIRK4 were highly expressed in the AV node and less expressed in the atria and the ventricles. Conclusion: Here, we demonstrate the involvement of the m2R-IK,ACh pathway in underlying second-degree AV block in horses. The high expression level of the M2 receptor may be responsible for the high burden of second-degree AV blocks seen in some horses.

Long-Term Training Increases Atrial Fibrillation Sustainability in Standardbred Racehorses.

Atrial fibrillation (AF) is more prevalent in athletes, and currently, the mechanisms are not fully understood. Atrial fibrillation inducibility and stability was investigated in trained and untrained Standardbred racehorses. The horses underwent echocardiography for evaluation of atrial size. High-density mapping during AF was performed, and the presence of structural remodeling, as well as the expression of inflammatory and pro-inflammatory markers in the atria, was studied. Atrial fibrillation sustained significantly longer after tachypacing in the trained horses, whereas no difference in AF inducibility was found. The untrained horses displayed a significant difference in the AF complexity when comparing right and left atria, whereas such difference was not observed in the trained animals. No evidence of increased structural remodeling or inflammation could be identified. Left atrial dimensions were not significantly increased. The increased AF sustainability in trained horses was not related to fibrosis or inflammation as seen in other animal exercise models.

Electrocardiographic Changes in a Horse with Induced Myocardial Infarction.

During acute myocardial infarction (AMI), the ischemia and necrosis of the infarcted tissue result in local electrophysiological changes, which bring about deviations of the ST segment and T wave. In this case report, the aim was to investigate whether these changes could be detected with a 12-lead electrocardiogram (ECG) during acute occlusion of the coronary artery in a 15-year-old Standardbred mare (scheduled for euthanasia due to non-cardiac health problems). The left anterior descending (LAD) coronary artery was occluded using an angioplasty balloon catheter guided through the carotid artery. Two coronary occlusions of 30 min were induced, separated by a 10-min reperfusion phase. AMI led to ST deviations and T-wave amplitude changes (maximum ST deviation was 1.98 mV; T-wave amplitude increased from 6.58 to 9.25 mV). The ST segment almost returned to the baseline during the reperfusion phase. The ECG changes seen after the infarction were comparable to those reported in other species with AMI, suggesting that the 12-lead-ECG can potentially be used to detect signs of myocardial infarction in horses.

Electrocardiographic characteristics of trained and untrained standardbred racehorses.

BACKGROUND: Long-term exercise induces cardiac remodeling that potentially influences the electrical properties of the heart. HYPOTHESIS/OBJECTIVES: We assessed whether training alters cardiac conduction in Standardbred racehorses. ANIMALS: Two hundred one trained and 52 untrained Standardbred horses. METHODS: Cross-sectional study. Resting ECG recordings were analyzed to assess heart rate (HR) along with standard ECG parameters and for identification of atrial and ventricular arrhythmias. An electrophysiological study was performed in 13 horses assessing the effect of training on sinoatrial (SA) and atrioventricular (AV) nodal function by sinus node recovery time (SNRT) and His signal recordings. Age and sex adjustments were implemented in multiple and logistic regression models for comparison. RESULTS: Resting HR in beats per minute (bpm) was lower in trained vs untrained horses (mean, 30.8 ± 2.6 bpm vs 32.9 ± 4.2 bpm; P = .001). Trained horses more often displayed second-degree atrioventricular block (2AVB; odds ratio, 2.59; P = .04). No difference in SNRT was found between groups (n = 13). Mean P-A, A-H, and H-V intervals were 71 ± 20, 209 ± 41, and 134 ± 41 ms, respectively (n = 7). We did not detect a training effect on AV-nodal conduction intervals. His signals were present in 1 horse during 2AVB with varying H-V interval preceding a blocked beat. CONCLUSIONS AND CLINICAL IMPORTANCE: We identified decreased HR and increased frequency of 2AVB in trained horses. In 5 of 7 horses, His signal recordings had variable H-V intervals within each individual horse, providing novel insight into AV conduction in horses.

Atrial fibrillatory rate as predictor of recurrence of atrial fibrillation in horses treated medically or with electrical cardioversion.

BACKGROUND: The recurrence rate of atrial fibrillation (AF) in horses after cardioversion to sinus rhythm (SR) is relatively high. Atrial fibrillatory rate (AFR) derived from surface ECG is considered a biomarker for electrical remodelling and could potentially be used for the prediction of successful AF cardioversion and AF recurrence. OBJECTIVES: Evaluate if AFR was associated with successful treatment and could predict AF recurrence in horses. STUDY DESIGN: Retrospective multicentre study. METHODS: Electrocardiograms (ECG) from horses with persistent AF admitted for cardioversion with either medical treatment (quinidine) or transvenous electrical cardioversion (TVEC) were included. Bipolar surface ECG recordings were analysed by spatiotemporal cancellation of QRST complexes and calculation of AFR from the remaining atrial signal. Kaplan-Meier survival curve and Cox regression analyses were performed to assess the relationship between AFR and the risk of AF recurrence. RESULTS: Of the 195 horses included, 74 received quinidine treatment and 121 were treated with TVEC. Ten horses did not cardiovert to SR after quinidine treatment and AFR was higher in these, compared with the horses that successfully cardioverted to SR (median [interquartile range]), (383 [367-422] vs 351 [332-389] fibrillations per minute (fpm), P < .01). Within the first 180 days following AF cardioversion, 12% of the quinidine and 34% of TVEC horses had AF recurrence. For the horses successfully cardioverted with TVEC, AFR above 380 fpm was significantly associated with AF recurrence (hazard ratio = 2.4, 95% confidence interval 1.2-4.8, P = .01). MAIN LIMITATIONS: The treatment groups were different and not randomly allocated, therefore the two treatments cannot be compared. Medical records and the follow-up strategy varied between the centres. CONCLUSIONS: High AFR is associated with failure of quinidine cardioversion and AF recurrence after successful TVEC. As a noninvasive marker that can be retrieved from surface ECG, AFR can be clinically useful in predicting the probability of responding to quinidine treatment as well as maintaining SR after electrical cardioversion.

Increased fibroblast accumulation in the equine heart following persistent atrial fibrillation.

BACKGROUND: Fibroblasts maintain the extracellular matrix homeostasis and may couple to cardiomyocytes through gap junctions and thereby increase the susceptibility to slow conduction and cardiac arrhythmias, such as atrial fibrillation (AF). In this study, we used an equine model of persistent AF to characterize structural changes and the role of fibroblasts in the development of an arrhythmogenic substrate for AF. MATERIAL AND METHODS: Eleven horses were subjected to atrial tachypacing until self-sustained AF developed and were kept in AF for six weeks. Horses in sinus rhythm (SR) served as control. In terminal open-chest experiments conduction velocity (CV) was measured. Tissue was harvested and stained from selected sites. Automated image analysis was performed to assess fibrosis, fibroblasts, capillaries and various cardiomyocyte characteristics. RESULTS: Horses in SR showed a rate-dependent slowing of CV, while in horses with persistent AF this rate-dependency was completely abolished (CV•basic cycle length relation p = 0.0295). Overall and interstitial amounts of fibrosis were unchanged, but an increased fibroblast count was found in left atrial appendage, Bachmann's bundle, intraatrial septum and pulmonary veins (p < 0.05 for all) in horses with persistent AF. The percentage of α-SMA expressing fibroblasts remained the same between the groups. CONCLUSION: Persistent AF resulted in fibroblast accumulation in several regions, particularly in the left atrial appendage. The increased number of fibroblasts could be a mediator of altered electrophysiology during AF. Targeting the fibroblast proliferation and differentiation could potentially serve as a novel therapeutic target slowing down the structural remodeling associated with AF.

RNA sequencing and immunofluorescence of the myotendinous junction of mature horses and humans.

The myotendinous junction (MTJ) is a specialized interface for transmitting high forces between the muscle and tendon and yet the MTJ is a common site of strain injury with a high recurrence rate. The aim of this study was to identify previously unknown MTJ components in mature animals and humans. Samples were obtained from the superficial digital flexor (SDF) muscle-tendon interface of 20 horses, and the tissue was separated through a sequential cryosectioning approach into muscle, MTJ (muscle tissue enriched in myofiber tips attached to the tendon), and tendon fractions. RT-PCR was performed for genes known to be expressed in the three tissue fractions and t-distributed stochastic neighbor embedding (t-SNE) plots were used to select the muscle, MTJ, and tendon samples from five horses for RNA sequencing. The expression of previously known and unknown genes identified through RNA sequencing was studied by immunofluorescence on human hamstring MTJ tissue. The main finding was that RNA sequencing identified the expression of a panel of 61 genes enriched at the MTJ. Of these, 48 genes were novel for the MTJ and 13 genes had been reported to be associated with the MTJ in earlier studies. The expression of known [COL22A1 (collagen XXII), NCAM (neural cell adhesion molecule), POSTN (periostin), NES (nestin), OSTN (musclin/osteocrin)] and previously undescribed [MNS1 (meiosis-specific nuclear structural protein 1), and LCT (lactase)] MTJ genes was confirmed at the protein level by immunofluorescence on tissue sections of human MTJ. In conclusion, in muscle-tendon interface tissue enriched with myofiber tips, we identified the expression of previously unknown MTJ genes representing diverse biological processes, which may be important in the maintenance of the specialized MTJ.

Detection of atrial fibrillation with implantable loop recorders in horses.

BACKGROUND: Cardiac arrhythmias in horses are diagnosed by auscultation or electrocardiogram (ECG), which results in a low sensitivity for detecting arrhythmias that occur sporadically. Implantable loop recorders (ILRs) are small ECG devices placed subcutaneously, to automatically detect arrhythmias in human patients. OBJECTIVES: To test ILRs ability to detect atrial fibrillation (AF) in horses. Furthermore, we hypothesised that anatomical location of the implant site might influence signal quality. Signal quality was evaluated both during exercise and over time. STUDY DESIGN: Experimental study. METHODS: In five Standardbred mares, eleven ILRs were implanted subcutaneously in up to three different positions (Front: pectoral region, Left-6: sixth left intercostal space and Ventral: xiphoid region) and AF induced. The R- and T-wave amplitudes were measured in all positions over time during AF. AF burden automatically registered by the ILRs over a 2-month period was compared with selected Holter ECG recordings. RESULTS: All three positions had stable R- and T-wave amplitudes during the study period and were of sufficient quality to allow AF detection at rest. The position Left-6 showed significantly higher R- and T-wave amplitudes compared with the other positions. During submaximal exercise only the Left-6 position was able to record ECG signals of diagnostic quality. No position yielded diagnostic signals at maximum exercise due to artefacts. MAIN LIMITATIONS: Few horses and ILRs included and no spontaneous AF episodes were studied. CONCLUSIONS: This preliminary study indicates that ILRs can be used for AF detection in horses, but the anatomical location is important for optimal ECG quality. Despite insufficient quality during exercise, ILRs were suitable for AF detection at rest. Therefore, the ILR may be a valuable diagnostic tool for detecting paroxysmal AF in horses.

First catheter-based high-density endocardial 3D electroanatomical mapping of the right atrium in standing horses.

BACKGROUND: Three-dimensional electroanatomical mapping is of potential interest in equine cardiology to identify arrhythmia mechanisms, characterise electroanatomical substrates and guide ablation strategies. OBJECTIVES: To describe three-dimensional electroanatomical mapping in standing horses. STUDY DESIGN: Research methodology, proof of concept study. METHODS: Four Standardbred horses (2 geldings, 2 mares, median age 4.5 [4-9] years, mean bodyweight 485 [440-550] kg) were sedated and placed in stocks. Via the jugular vein, a high-density multipolar grid catheter (Advisor™ HD Grid Mapping Catheter with EnSite VelocityTM, Abbott Medical) was used for endocardial mapping of the right atrium. The P-wave on the surface ECG was used as a timing reference for simultaneous local activation time- and bipolar voltage-mapping. For a positional reference a 10-pole catheter (Abbott Medical) was placed in the caudal vena cava. RESULTS: Endocardial right atrial mapping guided by the three-dimensional mapping system and local electrograms was successfully performed in all four horses. A median of 32719 [25499-65078] points, covering the entire right atrium, were collected. Three-dimensional electroanatomical mapping provided detailed information about activation patterns and electrogram-characteristics of the sinoatrial node, intervenous tubercle and cavotricuspid isthmus. Additionally, transvenous biopsy forceps connected to the mapping system were visualised on screen to guide biopsy collection. MAIN LIMITATIONS: The feasibility of electroanatomical mapping for the left atrium and in larger breeds requires further study. CONCLUSIONS: High-density three-dimensional electroanatomical mapping of the right atrium is feasible in the standing horse.

A novel approach for obtaining 12-lead electrocardiograms in horses.

BACKGROUND: In equine medicine, 12-lead electrocardiograms (ECGs) rarely are used, which may in part be a result of shortcomings in the existing guidelines for obtaining 12-lead ECGs in horses. The guidelines recommend placing the limb leads on the extremities, which is inappropriate because the ventricular mean electrical axis is then perpendicular to the limb leads, leading to large variations in ECG configuration even among healthy horses. From an electrophysiological point of view, the leads instead should be parallel to the electrical axis to minimize variability. OBJECTIVE: Develop an improved method for obtaining 12-lead ECGs in horses based on electrophysiology and cardiac electrical vectors relevant to horses. ANIMALS: Thirty-five healthy Standardbred horses. METHODS: Two ECGs obtained at rest; 1 ECG with the electrodes placed according to the method developed in the present study, the Copenhagen method, and 1 ECG following existing guidelines. RESULTS: In the Copenhagen method, we repositioned the limb electrodes to the thorax to better capture the electrical activity of the heart. Variation in the mean electrical axis decreased dramatically with the Copenhagen method (SD decreased from 24.6° to 1.6°, P < .001). Consequently, this new method provided stable ECGs with repeatable configurations. CONCLUSIONS AND CLINICAL IMPORTANCE: With this novel method, the ECG is recorded with respect to the electric axis to fully realize the potential of 12-lead ECG in horses. The Copenhagen method delivered more consistent and reliable ECG recordings compared to existing guidelines. The Copenhagen method potentially allows for expanded use of 12-lead ECGs in equine medicine.

Implantable loop recorders can detect paroxysmal atrial fibrillation in Standardbred racehorses with intermittent poor performance.

BACKGROUND: Limited information is available on paroxysmal atrial fibrillation (PAF) in the horse. Indeed, undiagnosed PAF could result in poor performance. Due to the intermittent occurrence, PAF is difficult to diagnose. However, implanting a small ECG device (implantable loop recorder, ILR) subcutaneously, allows the continuous and automatic detection of PAF. OBJECTIVES: The aim was to investigate the potential of ILRs as a tool for diagnosing PAF in horses with poor performance. STUDY DESIGN: Prospective field study. METHODS: Twelve racing Standardbred trotters with intermittent reduced performance (mean age: six years) were enrolled prospectively. The ILR was implanted subcutaneously at the fifth or sixth left intercostal space and data from the ILR was collected during the study period in which the horses were followed for a median duration of 7.5 month (range 6-28). RESULTS: The ILR was able to detect PAF in four out of twelve racehorses. The ILR also detected sustained atrial fibrillation (AF) in one horse during the study. The ILRs rely on RR detection and R waves were correctly identified in 96%. One hundred episodes were categorised as AF by the ILRs and subsequently visual ECG inspection categorised 12 as sinus rhythm (SR), 28 as sinus arrhythmia (SA), 14 as other arrhythmias and 46 as AF episodes. The Root Mean Square of the Successive Differences (RMSSD) values were significantly increased for AF compared to SR and SA. MAIN LIMITATIONS: Few horses included and duration of study period varied among the horses. Further it was not possible to assess the sensitivity of the device in the current study and the ILRs proved to have a high rate of false positive misclassifications. CONCLUSIONS: This study indicates that ILRs can be used for detection of PAF episodes and could be a useful ECG tool for horses presenting with poor performance. This methodology provides a platform to facilitate the long-term assessment of AF development and quantification of AF burden in horses. Further studies including both healthy and poor performing horses are needed in order to learn more about PAF prevalence in racehorses.

Comparison of Tenocyte Populations from the Core and Periphery of Equine Tendons.

Tendon is a highly organized, dense connective tissue that has been demonstrated to have very little turnover. In spite of the low turnover, tendon can grow in response to loading, which may take place primarily at the periphery. Tendon injuries and recurrence of injuries are common in both humans and animals in sports. It is unclear why some areas of the tendon are more susceptible to such injuries and whether this is due to intrinsic regional differences in extracellular matrix (ECM) production or tissue turnover. This study aimed to compare populations of tenocytes derived from the tendon core and periphery. Tenocytes were isolated from equine superficial digital flexor tendons (SDFTs), and the proliferation capacity was determined. ECM production was characterized by immuno- and histological staining and by liquid chromatography-mass spectrometry-based proteomics. Core and periphery SDFT cultures exhibited comparable proliferation rates and had very similar proteome profiles, but showed biological variation in collagen type I deposition. In conclusion, the intrinsic properties of tenocytes from different regions of the tendon are very similar, and other factors in the tissue may contribute to how specific areas respond to loading or injury.

Effect of selective IK,ACh inhibition by XAF-1407 in an equine model of tachypacing-induced persistent atrial fibrillation.

BACKGROUND AND PURPOSE: Inhibition of the G-protein gated ACh-activated inward rectifier potassium current, IK,ACh may be an effective atrial selective treatment strategy for atrial fibrillation (AF). Therefore, the anti-arrhythmic and electrophysiological properties of a novel putatively potent and highly specific IK,ACh inhibitor, XAF-1407 (3-methyl-1-[5-phenyl-4-[4-(2-pyrrolidin-1-ylethoxymethyl)-1-piperidyl]thieno[2,3-d]pyrimidin-6-yl]azetidin-3-ol), were characterised for the first time in vitro and investigated in horses with persistent AF. EXPERIMENTAL APPROACH: The pharmacological ion channel profile of XAF-1407 was investigated using cell lines expressing relevant ion channels. In addition, eleven horses were implanted with implantable cardioverter defibrillators enabling atrial tachypacing into self-sustained AF. The electrophysiological effects of XAF-1407 were investigated after serial cardioversions over a period of 1 month. Cardioversion success, drug-induced changes of atrial tissue refractoriness, and ventricular electrophysiology were assessed at baseline (day 0) and days 3, 5, 11, 17, and 29 after AF induction. KEY RESULTS: XAF-1407 potently and selectively inhibited Kir 3.1/3.4 and Kir 3.4/3.4, underlying the IK,ACh current. XAF-1407 treatment in horses prolonged atrial effective refractory period as well as decreased atrial fibrillatory rate significantly (~20%) and successfully cardioverted AF, although with a decreasing efficacy over time. XAF-1407 shortened atrioventricular-nodal refractoriness, without effect on QRS duration. QTc prolongation (4%) within 15 min of drug infusion was observed, however, without any evidence of ventricular arrhythmia. CONCLUSION AND IMPLICATIONS: XAF-1407 efficiently cardioverted sustained tachypacing-induced AF of short duration in horses without notable side effects. This supports IK,ACh inhibition as a potentially safe treatment of paroxysmal AF in horses, suggesting potential clinical value for other species including humans.

Longitudinal study of electrical, functional and structural remodelling in an equine model of atrial fibrillation.

BACKGROUND: Large animal models are important in atrial fibrillation (AF) research, as they can be used to study the pathophysiology of AF and new therapeutic approaches. Unlike other animal models, horses spontaneously develop AF and could therefore serve as a bona fide model in AF research. We therefore aimed to study the electrical, functional and structural remodelling caused by chronic AF in a horse model. METHOD: Nine female horses were included in the study, with six horses tachypaced into self-sustained AF and three that served as a time-matched sham-operated control group. Acceleration in atrial fibrillatory rate (AFR), changes in electrocardiographic and echocardiographic variables and response to medical treatment (flecainide 2 mg/kg) were recorded over a period of 2 months. At the end of the study, changes in ion channel expression and fibrosis were measured and compared between the two groups. RESULTS: AFR increased from 299 ± 33 fibrillations per minute (fpm) to 376 ± 12 fpm (p < 0.05) and atrial function (active left atrial fractional area change) decreased significantly during the study (p < 0.05). No changes were observed in heart rate or ventricular function. The AF group had more atrial fibrosis compared to the control group (p < 0.05). No differences in ion channel expression were observed. CONCLUSION: Horses with induced AF show signs of atrial remodelling that are similar to humans and other animal models.

Effects of dofetilide and ranolazine on atrial fibrillatory rate in a horse model of acutely induced atrial fibrillation.

INTRODUCTION: The atrial fibrillatory rate is a potential biomarker in the study of antiarrhythmic drug effects on atrial fibrillation (AF). The purpose of this study was to evaluate whether dose-dependent changes in the atrial fibrillatory rate can be monitored on surface electrocardiography (ECG) following treatment with dofetilide, ranolazine, and a combination of the two in an acute model of AF in horses. METHODS AND RESULTS: Eight horses were subjected to pacing-induced AF on 4 separate days. Saline (control), dofetilide, ranolazine, or a combination of dofetilide and ranolazine was administered in four incremental doses. Atrial fibrillatory activity was extracted from surface ECGs using spatiotemporal QRST cancellation. The mean atrial fibrillatory rate before drug infusion was 297 ± 27 fpm. Dofetilide reduced the atrial fibrillatory rate following the infusion of low doses (0.89 µg/kg, P < 0.05) and within 5 minutes preceding cardioversion (P < 0.05). Cardioversion with ranolazine was preceded by a reduction in the atrial fibrillatory rate in the last minute (P < 0.05). The combination of drugs reduced the atrial fibrillatory rate in a similar manner to dofetilide used alone. A trend toward a lower atrial fibrillatory rate before drug infusion was found among horses cardioverting on low doses of the drugs. CONCLUSION: The atrial fibrillatory rate derived from surface ECGs showed a difference in the mode of action on AF between dofetilide and ranolazine. Dofetilide reduced the atrial fibrillatory rate, whereas ranolazine displayed a cardioverting mechanism that was distinct from a slowing of the fibrillatory process.

Time-dependent antiarrhythmic effects of flecainide on induced atrial fibrillation in horses.

BACKGROUND: Pharmacological treatment of atrial fibrillation (AF) in horses can be challenging because of low efficacy and adverse effects. Flecainide has been tested with variable efficacy. OBJECTIVE: To test whether the efficacy of flecainide is dependent on AF duration. ANIMALS: Nine Standardbred mares. METHODS: Factorial study design. All horses were instrumented with a pacemaker and assigned to a control or an AF group. On day 0, all horses were in sinus rhythm and received 2 mg/kg flecainide IV. Atrial fibrillation subsequently was induced in the AF group by pacemaker stimulation. On days 3, 9, 27, and 55, flecainide was administered to all horses, regardless of heart rhythm. RESULTS: All horses in AF cardioverted to sinus rhythm on days 3 and 9. On day 27, 5/6 horses cardioverted, whereas only 2/6 cardioverted on day 55. The time from the start of flecainide infusion to cardioversion (range, 3-185 min, log transformed) showed linear correlation with the cumulative duration of AF (r2 = .80, P < .0001). Flecainide induced abnormal QRS complexes in 4/6 AF horses and 1/3 controls. A positive correlation was found between heart rate before flecainide infusion and number of abnormal QRS complexes (0.14, P < .05). One horse suffered from cardiac arrest and died after flecainide infusion. CONCLUSIONS AND CLINICAL IMPORTANCE: Flecainide is effective for cardioversion of short-term induced AF, but the effect decreases with AF duration. Controlling heart rate may minimize adverse effects caused by flecainide, but the drug should be used with great caution.

Effect of induced chronic atrial fibrillation on exercise performance in Standardbred trotters.

BACKGROUND: Atrial fibrillation (AF) is the most common arrhythmia affecting performance in horses. However, no previous studies have quantified the performance reduction in horses suffering from AF. OBJECTIVES: To quantify the effect of AF on maximum velocity (Vmax ), maximum heart rate (HRmax ), heart rate recovery (T100 ), hematologic parameters and development of abnormal QRS complexes. ANIMALS: Nine Standardbred trotters. METHODS: Two-arm controlled trial. Six horses had AF induced by means of a pacemaker and 3 served as sham-operated controls. All horses were subjected to an exercise test to fatigue before (SET1) and after (SET2) 2 months of AF or sham. The Vmax and HRmax were assessed using a linear mixed normal model. Abnormal QRS complexes were counted manually on surface ECGs. RESULTS: Atrial fibrillation resulted in a 1.56 m/sec decrease in Vmax (P < .0001). In the AF group, HRmax ± SD increased from 226 ± 11 bpm at SET1 to 311 ± 27 bpm at SET 2. The AF group had higher HRmax at SET2 compared with controls (P < .0001), whereas no difference between the control and AF groups was observed at SET1 (P = .96). Several episodes of wide complex tachycardia were observed during exercise in 3 of the AF horses during SET2. CONCLUSIONS AND CLINICAL IMPORTANCE: Atrial fibrillation resulted in a significant reduction in performance, an increase in HR and development of abnormal QRS complexes during exercise, which may be a risk factor for collapse or sudden cardiac death.

Antiarrhythmic Effects of Combining Dofetilide and Ranolazine in a Model of Acutely Induced Atrial Fibrillation in Horses.

BACKGROUND: Antiarrhythmic compounds against atrial fibrillation (AF) often have reduced efficacy and may display cardiac and/or noncardiac toxicity. Efficacy can be improved by combining 2 compounds with distinct mechanisms, and it may be possible to use lower doses of each compound, thereby reducing the likelihood of adverse side effects. The purpose of this study was to investigate whether the effective doses of dofetilide and ranolazine can be reduced if the drugs are combined. METHODS: Dofetilide, ranolazine, and a combination of these were administered in 4 incremental dosing regimens to horses with acutely pacing-induced AF. Time to cardioversion, atrial effective refractory period, and AF vulnerability and duration were assessed. RESULTS: Of 8 horses, 6 cardioverted to sinus rhythm after infusion with a combination of 0.889 µg/kg dofetilide and 0.104 mg/kg ranolazine. Two horses cardioverted with 0.104 mg/kg ranolazine alone, and 3 cardioverted with 0.889 µg/kg dofetilide alone. The combination therapy decreased AF vulnerability (P < 0.05) and AF duration (P < 0.05). No change in atrial effective refractory period was detected with any of the drugs. CONCLUSIONS: The combination of dofetilide and ranolazine showed increased antiarrhythmic effects on acutely induced AF in horses, affecting time to cardioversion, AF vulnerability, and AF duration.