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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.

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.

Baggrund: Sandsynligheden for overlevelse og død kan være svær at forudsige i føl med sepsis, her kan forskellige prognostiske markører og parametre være brugbare, men studier heri er begrænset. Formål: At undersøge om forskellige markører kan bruges til tidlig påvisning af sepsis og som markører for overlevelsesrate i føl. Sammenligning af forskellige komplikationer i relation til sepsis og overlevelse. Bestemmelse af effekten af antibiotika behandling forud for hospitalisering ift. overlevelse i septiske føl. Metodeafsnit: Retrospektivt studie med data fra 177 føl ≤ 30 dage indlagt på Universitets Hospitalet for Store Husdyr i Danmark fra 2018 til 2022. Følgende parametre blev undersøgt: komplikationer, antibiotika behandling forud for indlæggelse og prognostiske markører som serum amyloid A (SAA), laktat, neutrofile granulocytter, IgG, jern og hæmoglobin. Antibiotika behandling blev analyseret via en 2x2-tabel. Komplikationer og prognostiske markører blev undersøgt via en generaliseret lineær model med binomial variable og testet via ANOVA, samt pairs (hvis ANOVA-test blev fundet signifikant). Resultater: Et hundrede syvoghalvfjerds (54 cases og 123 kontroller) føl ≤ 30 var inkluderet i studiet. SAA var signifikant for sepsis (p < 0.01) men ikke for overlevelse. Laktat var signifikant for sepsis (p < 0.01) og for overlevelse i septiske og alle føl (p < 0.01; < 0.01, henholdsvis). Neutrofile var signifikant for sepsis og overlevelse i alle føl (p < 0.02; < 0.02, henholdsvis). IgG var signifikant for overlevelse i septiske og alle føl (p < 0.01; < 0.01, henholdsvis). Høj serum jern var associeret med bakteriæmi (p < 0.01) - hæmoglobin og jern/hæmoglobin kombineret var ikke signifikant for bakteriæmi. Omphalitis gav øget chance for overlevelse ift. septisk artritis, FPT og asfyksi (p ≤ 0.02). Septisk artritis var ikke signifikant ift. andre komplikationer (undtagen omphalitis). Antibiotika behandling forud for in, Background: Survival and non-survival can be difficult to predict in equine neonates with sepsis and different parameters and prognostic markers can be helpful - but studies in these are limited. Objectives: To determine if different clinicopathological variables can be used in early detection of sepsis and in prediction of survival rates. Comparison of different complications related to sepsis and survival rates. Determine if antibiotic treatment prior to hospitalization affects the survival in septic foals. Methods: Retrospective study with data from 177 foals ≤ 30 days of age admitted to the Large Animal Teaching Hospital in Denmark from 2018 to 2022. We evaluated complications, antibiotic treatment prior to admission and clinicopathological variables including serum amyloid A (SAA), lactate, neutrophil count, IgG, iron, and hemoglobin. Antibiotic treatment prior to referral were analyzed by a two-by-two table. Complications and clinicopathological variables were analyzed by a generalized linear model with binomial variables and tested by ANOVA and if significant effect, pairs was also used. Results: One hundred seventy-seven (54 cases and 123 controls) foals ≤ 30 days were included in the study. SAA was significant for diagnosing sepsis (p < 0.01) but not for survival. Lactate was significant for diagnosing sepsis (p < 0.01) and for survival in septic and all foals (p < 0.01; < 0.01, respectively). Neutrophils was significant for sepsis and survival in all foals (p < 0.02; < 0.02, respectively). IgG was significant for survival in septic foals and all foals (p < 0.01; < 0.01, respectively). High iron was associated with bacteremia (p < 0.01) - hemoglobin and iron/hemoglobin combined were not prognostic for bacteremia. The complication omphalitis was associated with increased chance of survival compared to septic arthritis, FPT, and asphyxia (p ≤ 0.02). Septic arthritis was not significant compared to all ot

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

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.

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.

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

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.

The myotendinous junction (MTJ) is a specialised interface for transmitting high forces between 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 cryo-sectioning 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-SNE plots were used to select the muscle, MTJ and tendon samples from 5 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 expression of a panel of 61 genes enriched at the MTJ. 48 of these 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, NCAM, POSTN, NES, OSTN) and previously undescribed (MNS1 and LCT) 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 expression of previously unknown MTJ genes representing diverse biological processes, which may be important in the maintenance of the specialized MTJ.

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

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.

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.

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. Further, 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, 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 were 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 to 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.

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.

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.

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.

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 (r 2 = .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.

Background Atrial fibrillatory cycle length has been considered one of the indices of atrial electrical remodelling during atrial fibrillation (AF), which can be assessed from surface ECG by computer-assisted calculation of atrial fibrillatory rate (AFR). Horses have been suggested as a bona fide model for AF studies since horses too, develop lone AF, however data on AF characteristics in horses are extremely sparse and non-invasive characterization of AF complexity using surface ECG processing has not been reported. Aim The aim was to study characteristics of induced AF and its modification by flecainide. Methods The study group consisted on 3 horses with spontaneous persistent AF and 13 with pace-induced AF. Seven horses were treated with saline (control) and eight with flecainide (2 mg/kg). ECGs were analysed using spatiotemporal cancellation of QRST complexes and calculation of AFR from the residual atrial signal. Results At AF onset, AFR was 295 ± 52 fibrillations per minute (fpm) in the horses with induced AF treated with flecainide, 269 ± 36 fpm in the control group (ns), and 364 ± 26 fpm in the horses with spontaneous persistent AF (P < 0.05 compared to the control group). Flecainide caused a decrease in AFR in all animals and restored sinus rhythm in the animals with induced AF. In the control animals, AFR increased from 269 ± 36 fpm to a plateau of 313 ± 14 fpm before decreasing to 288 ± 28 fpm during the last 10% of the AF episodes preceding spontaneous conversion (P < 0.05). Conclusion AFR in horses with induced AF resembles AFR in humans with paroxysmal AF. Flecainide caused a rapid decrease in AFR in all horses, further supporting the method to be a non-invasive technique to study the effect of antiarrhythmic compounds.

Background: Although premature beats are a matter of concern in horses, the interpretation of equine ECG recordings is complicated by a lack of standardized analysis criteria and a limited knowledge of the normal beat-to-beat variation of equine cardiac rhythm. The purpose of this study was to determine the appropriate threshold levels of maximum acceptable deviation of RR intervals in equine ECG analysis, and to evaluate a novel two-step timing algorithm by quantifying the frequency of arrhythmias in a cohort of healthy adult endurance horses.Results: Beat-to-beat variation differed considerably with heart rate (HR), and an adaptable model consisting of three different HR ranges with separate threshold levels of maximum acceptable RR deviation was consequently defined. For resting HRs <60 beats/min (bpm) the threshold level of RR deviation was set at 20%, for HRs in the intermediate range between 60 and 100 bpm the threshold was 10%, and for exercising HRs >100 bpm, the threshold level was 4%. Supraventricular premature beats represented the most prevalent arrhythmia category with varying frequencies in seven horses at rest (median 7, range 2–86) and six horses during exercise (median 2, range 1–24).Conclusions: Beat-to-beat variation of equine cardiac rhythm varies according to HR, and threshold levels in equine ECG analysis should be adjusted accordingly. Standardization of the analysis criteria will enable comparisons of studies and follow-up examinations of patients. A small number of supraventricular premature beats appears to be a normal finding in endurance horses. Further studies are required to validate the findings and determine the clinical significance of premature beats in horses.

The inwardly rectifying potassium current (IK1) conducted through Kir2.X channels contribute to repolarization of the cardiac action potential and to stabilization of the resting membrane potential in cardiomyocytes. Our aim was to investigate the effect of the recently discovered IK1 inhibitor PA-6 on action potential repolarization and refractoriness in isolated rat hearts. Transiently transfected HEK-293 cells expressing IK1 were voltage-clamped with ramp protocols. Langendorff-perfused heart experiments were performed on male Sprague–Dawley rats, effective refractory period, Wenckebach cycle length, and ventricular effective refractory period were determined following 200 nmol/L PA-6 perfusion. 200 nmol/L PA-6 resulted in a significant timelatency in drug effect on the IK1 current expressed in HEK-293 cells, giving rise to a maximal effect at 20 min. In the Langendorff-perfused heart experiments, PA-6 prolonged the ventricular action potential duration at 90% repolarization (from 41.8 6.5 msec to 72.6 21.1 msec, 74% compared to baseline, P < 0.01, n = 6). In parallel, PA-6 significantly prolonged the ventricular effective refractory period compared to baseline (from 34.8 4.6 msec to 58.1 14.7 msec, 67%, P < 0.01, n = 6). PA-6 increased the shortterm beat-to-beat variability and ventricular fibrillation was observed in two of six hearts. Neither atrial ERP nor duration of atrial fibrillation was altered following PA-6 application. The results show that pharmacological inhibition of cardiac IK1 affects ventricular action potential repolarization and refractoriness and increases the risk of ventricular arrhythmia in isolated rat hearts.

OBJECTIVE To evaluate heart rate, heart rate variability, and arrhythmia frequency as well as changes in cardiac biomarker values and their association with heart rate in horses before and after an endurance ride. DESIGN Cross-sectional study. ANIMALS 28 Arabian horses competing in a 120- or 160-km endurance ride. PROCEDURES ECG recordings were obtained from each horse before (preride) and after (recovery) an endurance ride to evaluate changes in heart rate and the SD of normal R-R intervals (SDNN) during the initial 12 hours of recovery. Frequencies of supraventricular and ventricular premature complexes before and after the ride were evaluated. Blood samples were obtained before the ride and twice during recovery. Hematologic analyses included measurement of serum cardiac troponin I concentration and creatine kinase isoenzyme MB activity. RESULTS Heart rate was significantly increased and SDNN was decreased during the recovery versus preride period. Frequency of ventricular premature complexes increased during recovery, albeit not significantly, whereas frequency of supraventricular premature complexes was not significantly different between preride and recovery periods. Serum cardiac troponin I concentration and creatine kinase isoenzyme MB activity were significantly increased in the recovery versus preride period. No associations were identified between cardiac biomarkers and velocity, distance, or mean heart rate. CONCLUSIONS AND CLINICAL RELEVANCE Heart rate increased and SDNN decreased in horses after completion of an endurance ride. These and other cardiac changes suggested that prolonged exercise such as endurance riding might have cardiac effects in horses. Additional studies are needed to clarify the clinical relevance of the findings., OBJECTIVE: To evaluate heart rate, heart rate variability, and arrhythmia frequency as well as changes in cardiac biomarker values and their association with heart rate in horses before and after an endurance ride. DESIGN: Cross-sectional study. ANIMALS: 28 Arabian horses competing in a 120- or 160-km endurance ride. PROCEDURES: ECG recordings were obtained from each horse before (preride) and after (recovery) an endurance ride to evaluate changes in heart rate and the SD of normal R-R intervals (SDNN) during the initial 12 hours of recovery. Frequencies of supraventricular and ventricular premature complexes before and after the ride were evaluated. Blood samples were obtained before the ride and twice during recovery. Hematologic analyses included measurement of serum cardiac troponin I concentration and creatine kinase isoenzyme MB activity. RESULTS: Heart rate was significantly increased and SDNN was decreased during the recovery versus preride period. Frequency of ventricular premature complexes increased during recovery, albeit not significantly, whereas frequency of supraventricular premature complexes was not significantly different between preride and recovery periods. Serum cardiac troponin I concentration and creatine kinase isoenzyme MB activity were significantly increased in the recovery versus preride period. No associations were identified between cardiac biomarkers and velocity, distance, or mean heart rate. CONCLUSIONS AND CLINICAL RELEVANCE: Heart rate increased and SDNN decreased in horses after completion of an endurance ride. These and other cardiac changes suggested that prolonged exercise such as endurance riding might have cardiac effects in horses. Additional studies are needed to clarify the clinical relevance of the findings.

Background: Atrial fibrillation (AF) is the most important supraventricular arrhythmia in horses and the most common arrhythmia in clinical practice in humans. To learn more about the AF in both horses and man it is desirable to develop an AF animal model, which makes it possible to study the pathogenesis over time and new treatment methods. Objectives; 1) To evaluate the horse as a model of chronic AF induced by a pacemaker. 2) To compare methods for obtaining atrial fibrillation cycle length (AFCL) by intra-atrial electrocardiography (EGM), tissue Doppler imaging (TDI) and atrial fibrillatory rate (AFR) by spatiotemporal cancellation of QRST-complexes from a surface electrocardiogram (ECG). 3) To compare performance in the horse before and after induction of AF, to evaluate whether AF results in exercise intolerance. Method; Six clinically healthy horses were included in this study (Mean age: 9.7 years, ranging from 5-14 years. Mean bodyweight: 494 kg, ranging from 399-572 kg). 1) In all horses a pacemaker was implanted, and used for induction of AF. 2) Measurements were done on 5 procedure days; day 1, day 3, day 9, day 27 and day 55. The AFCLEGM was obtained by using the pacemaker leads, the AFCLTDI was measured from TDI velocity curves from the following three imaging planes; Interatrial septum from a left parasternal long axis view (IASL), interatrial septum from a right parasternal long-axis four chamber view (IAS4CH) and left atrial free wall from a right parasternal long-axis four chamber view (LAFW4CH). The AFR was measured from a surface ECG using spatiotemporal cancellation of QRST-complexes. Tukey’s honest significant difference test was used on a linear mixed effect model for comparing the five days and three methods. 3) Two maximal exercise tests (MET) were performed for each horse; the first MET before pacemaker implementation and the second MET with AF after procedure day 55. For each MET an ECG was recorded to evaluate heart rate (HR) and blood sam, Baggrund: Atrieflimmer (AF) er den vigtigste supraventrikulære arrytmi hos heste og den mest almindelige arrytmi i human klinisk praksis. For at lære om AF hos både hos og menneske er det ønskeligt at udvikle en AF dyremodel, som gør det muligt at studere patogenensen over tid og nye behandlingsmetoder. Formål; 1) At evaluere hesten som model for kronisk AF induceret af en pacemaker. 2) At sammenligne metoder for målinger af atrieflimmer cykluslængde (AFCL) fra intrakardielt elektrokardiografi (EGM), vævsdoppler ekkokardiografi (TDI) og målingen af atriefibrillationsraten (AFR) vha. spatiotemporal fjernelse af QRST-komplekser fra et overflade elektrokardiografi (EKG). 3) At sammenligne kondition hos heste før og efter induktion af AF, for at evaluere hvorvidt AF resulterer i motions intolerance. Metode; Seks klinisk raske heste var en del af studiet. (Gennemsnitlig alder 9.7 år, imellem 5-14 år. Gennemsnitlig kropsvægt 494 kg, imellem 399-572 kg). 1) I alle heste blev en pacemaker indopereret og brug til induktion af AF. 2) Målingerne blev lavet over fem forsøgsdage; dag 1, dag 3, dag 9, dag 27 og dag 55. Målingerne af AFCLEGM blev lavet ved brug af pacemaker ledninger, AFCLTDI blev målt ved brug af TDI hastighedskurver fra følgende billedplaner: Venstre atries frivæg fra et parasternalt fire-kammer billede 2 (LAFW4CH), intraatrialt septum fra et parasternalt fire-kammer billede (IAS4CH), intraatrielt septum fra et venstre parasternalt billede (IASL). Atriefibrillationsraten blev målt fra et overflade EKG ved brug af spatiotemporal fjernelse af QRST-komplekser. Tukey’s Honest significant difference test blev brug på en lineær mixed effekt model til at lave en statistisk sammenligning indenfor de fem forskellige forsøgsdage og indenfor de tre metoder. 3) To maksimale motions tests blev udført for hver hest; den første motionstest før indoperation af pacemaker, og den anden motionstest med AF efter proceduredag 55. For hver motionstest blev et EKG optaget for at evalu