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

Development of an atrial transseptal puncture procedure in horses to access the left heart: An ultrasound-guided jugular vein and transhepatic approach

BACKGROUND

Radiofrequency ablation has been successfully applied to treat right atrial arrhythmias in horses. Ablation of left-sided arrhythmias requires a retrograde transarterial approach which is complicated. In human medicine, the left atrium is accessed through transseptal puncture (TSP) of the fossa ovalis (FO) using a caudal approach via the femoral vein.

OBJECTIVES

To develop a zero fluoroscopy TSP technique for horses using a jugular vein (cranial) and transhepatic (caudal) approach.

STUDY DESIGN

In vivo experimental study.

METHODS

Transseptal puncture was performed in 18 horses admitted for euthanasia and donated for scientific research under general anaesthesia: using a jugular vein approach (10 horses), a transhepatic approach (2 horses) or both (6 horses). Radiofrequency energy was applied on a guidewire to perforate the FO and allow sheath advancement under intracardiac and transthoracic echocardiographic guidance. Puncture lesions were inspected post-mortem.

RESULTS

Transseptal puncture was successful in 17/18 horses, of which 15/16 jugular vein approaches and 5/8 transhepatic approaches. Failure was due to technical malfunction, inability to advance the guidewire toward the heart and inability to advance the sheath through the FO. Intracardiac echocardiography was essential to safely guide the puncture process. Atrial arrhythmias caused by the TSP occurred in 13/18 horses. Puncture lesions were found in the right atrium in the FO region, and left atrium ventral to pulmonary vein ostium III.

MAIN LIMITATIONS

Because in several horses two approaches were tested consecutively, it cannot be excluded that the second TSP was performed at the previous puncture site. Due to the developmental nature of the study the approaches were not randomised and did not allow comparison.

CONCLUSION

Transseptal puncture is feasible in horses using ultrasound guidance and allows for electrophysiological exploration of the left heart. Further studies are needed to evaluate post-operative follow-up.

Lesion size index-guided radiofrequency catheter ablation using an impedance-based three-dimensional mapping system to treat sustained atrial tachycardia in a horse

Sustained atrial tachycardia at an atrial rate of 191/min on the surface ECG was detected in a 6-year-old Warmblood mare. The vectorcardiogram obtained from a 12-lead ECG suggested a caudo-dorsal right atrial origin of the arrhythmia. Impedance-based three-dimensional electro-anatomical mapping, using the EnSite™ Precision Cardiac Mapping System revealed a clockwise macro-reentry around a line of conduction block in the caudomedial right atrium. Ten radiofrequency applications were applied to isolate the caudal vena cava myocardial sleeves at a power of 35 W and mean contact force of 14 ± 3 g until a lesion size index of 6 was reached. Sinus rhythm was restored at the first energy application. Successful isolation was confirmed by demonstrating entrance and exit block. Holter monitoring 5 days post-ablation revealed no abnormalities. To date, 9 months after treatment, no recurrence has been observed. The use of lesion size index-guided ablation and isolation of the arrhythmogenic substrate in the caudal vena cava may minimise the risk of recurrence.

Morphological and histological investigation of the conduction system in the equine atrial muscle sleeve of pulmonary veins

BACKGROUND

Atrial fibrillation is the most common arrhythmia in horses causing poor performance. The role of pulmonary vein triggers in the pathogenesis has been identified in horses. Ablation methods have been investigated, but the available information on anatomical, histological and immunohistochemical assessment of the pulmonary vein ostia and the conduction system of the myocardial sleeve is still limited.

OBJECTIVES

The aim of the study was to describe the morphological properties of the myocardial sleeve in healthy horses.

STUDY DESIGN

Cross-sectional.

METHODS

Eighty-three equine hearts were dissected. The number and diameters of pulmonary vein ostia were determined, and anatomical localisation was described. Fifty-eight tissue samples were collected for routine histology and 12 of these were used for immunohistochemistry (connexin 43, 45, S100, and tyrosine hydroxylase antibodies).

RESULTS

The mean number of pulmonary vein ostia was 4.5 (4 veins: 46 horse, 5 veins: 31 horses, 6 veins: 6 horses). Diameters (mean ± SD) of the main ostia were as follows: vein I: 20.2 ± 7.0 mm, vein II: 32.7 ± 7.1 mm, vein III: 33.4 ± 5.9 mm, vein IV: 18.1 ± 4.5 mm. Diameters of supernumerary vein ostia varied between 3.0 and 28.0 mm (11.5 ± 5.5 mm). Early branching was found in 26 horses (31.3%) and 30 veins (vein I: 14, vein II: 9, vein III: 5, vein IV: 2). Histology confirmed the presence of a muscle sleeve composed of myocardial tissue in each pulmonary vein. S100 and TH positivity was detected in each vein, and it confirmed the presence of adrenergic and non-adrenergic nerve fibres within the myocardial sleeve. Cx43 and 45 positivity were also found in each vein indicating the presence of gap junctions.

MAIN LIMITATIONS

The effect of bodyweight on pulmonary vein dimensions is unknown.

CONCLUSIONS

Future ablation techniques should consider that conductive tissue is present in the entire myocardial sleeve in all pulmonary vein ostia.

Structural and electro-anatomical characterization of the equine pulmonary veins: implications for atrial fibrillation

INTRODUCTION/OBJECTIVES

Spontaneous pulmonary vein (PV) activity triggers atrial fibrillation (AF) in humans. Although AF frequently occurs in horses, the origin remains unknown. This study investigated the structural and electro-anatomical properties of equine PVs to determine the potential presence of an arrhythmogenic substrate.

ANIMALS, MATERIALS AND METHODS

Endocardial three-dimensional electro-anatomical mapping (EnSite Precision) using high-density (HD) catheters was performed in 13 sedated horses in sinus rhythm. Left atrium (LA) access was obtained retrogradely through the carotid artery. Post-mortem, tissue was harvested from the LA, right atrium (RA), and PVs for histological characterization and quantification of ion channel expression using immunohistochemical analysis.

RESULTS

Geometry, activation maps, and voltage maps of the PVs were created and a median of four ostia were identified. Areas of reduced conduction were found at the veno-atrial junction. The mean myocardial sleeve length varied from 28 ± 13 to 49 ± 22 mm. The PV voltage was 1.2 ± 1.4 mV and lower than the LA (3.4 ± 0.9 mV, P < 0.001). The fibrosis percentage was higher in PV myocardium (26.1 ± 6.6 %) than LA (14.5 ± 5.0 %, P = 0.003). L-type calcium channel (CaV1.2) expression was higher in PVs than LA (P = 0.001). T-type calcium channels (CaV3.3), connexin-43, ryanodine receptor-2, and small conductance calcium-activated potassium channel-3 was expressed in PVs.

CONCLUSIONS

The veno-atrial junction had lower voltages, increased structural heterogeneity and areas of slower conduction. Myocardial sleeves had variable lengths, and a different ion channel expression compared to the atria. Heterogeneous properties of the PVs interacting with the adjacent LA likely provide the milieu for re-entry and AF initiation.

Development of a three-dimensional computer model of the equine heart using a polyurethane casting technique and in vivo contrast-enhanced computed tomography

INTRODUCTION/OBJECTIVES

Insight into the three-dimensional (3D) anatomy of the equine heart is essential in veterinary education and to develop minimally invasive intracardiac procedures. The aim was to create a 3D computer model simulating the in vivo anatomy of the adult equine heart.

ANIMALS

Ten horses and five ponies.

MATERIALS AND METHODS

Ten horses, euthanized for non-cardiovascular reasons, were used for in situ cardiac casting with polyurethane foam and subsequent computed tomography (CT) of the excised heart. In five anaesthetized ponies, a contrast-enhanced electrocardiogram-gated CT protocol was optimized to image the entire heart. Dedicated image processing software was used to create 3D models of all CT scans derived from both methods. Resulting models were compared regarding relative proportions, detail and ease of segmentation.

RESULTS

The casting protocol produced high detail, but compliant structures such as the pulmonary trunk were disproportionally expanded by the foam. Optimization of the contrast-enhanced CT protocol, especially adding a delayed phase for visualization of the cardiac veins, resulted in sufficiently detailed CT images to create an anatomically correct 3D model of the pony heart. Rescaling was needed to obtain a horse-sized model.

CONCLUSIONS

Three-dimensional computer models based on contrast-enhanced CT images appeared superior to those based on casted hearts to represent the in vivo situation and are preferred to obtain an anatomically correct heart model useful for education, client communication and research purposes. Scaling was, however, necessary to obtain an approximation of an adult horse heart as cardiac CT imaging is restricted by thoracic size.

Training vs. racing: A comparison of arrhythmias and the repeatability of findings in Thoroughbred Chuckwagon racehorses

Arrhythmias are commonly reported in exercising horses, however due to regulatory constraints electrocardiograms (ECGs) are acquired during training but not competition, raising questions about the repeatability of findings. The aims were (1) compare training and competition arrhythmias and (2) describe the repeatability of arrhythmias during maximal-intensity exercise. A convenience sample of 52 healthy Thoroughbreds (aged 8.7 ± 2.5 years) competing in the World Professional Chuckwagon Association were obtained, totaling 152 training or competition ECGs (2-7 ECGs/horse). Speed, heart rate (HR) and arrhythmias (supraventricular premature complex, SVPC; ventricular premature complex, VPC) were examined. Pre- and post-recovery (approximately 6 min) blood samples measured lactate and high-sensitivity troponin-T. Training and competition arrythmias were compared (Friedman's test) and reliability of repeated ECGs assessed (intraclass correlation; P < 0.05). Training vs. competition: Forty horses had clean tracing from training and competition (n = 80 ECGs); the number and type of arrhythmias were not different. In training, VPCs were present in 7/40 horses (median [interquartile range, IQR]/ECG; range; 0 [0,0]; 0-4) and 9/40 horses (0 [0,0]; 0-5) in active-recovery. In competition, VPCs were present in 7/40 horses (0 [0,0]; 0-8) and 8/40 horses (0 [0,0]; 0-5) in active-recovery. Arrhythmias were primarily single premature complexes. Training and competition speed, HR, lactate and troponin-T did not differ however, sampling was too early for peak serum Troponin-T levels. Repeatability: total arrhythmias between serial ECGs did not differ. The reliability to detect SVPCs and VPCs was poor to moderate, and poor, respectively. Overall, the total number of arrhythmias was repeatable, but the reliability of arrhythmia type was poor to moderate.

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.

Paroxysmal Atrial Fibrillation in Horses: Pathophysiology, Diagnostics and Clinical Aspects

Atrial fibrillation (AF) is the most common arrhythmia in horses causing poor performance. As in humans, the condition can be intermittent in nature, known as paroxysmal atrial fibrillation (pAF). This review covers the literature relating to pAF in horses and includes references to the human literature to compare pathophysiology, clinical presentation, diagnostic tools and treatment. The arrhythmia is diagnosed by auscultation and electrocardiography (ECG), and clinical signs can vary from sudden loss of racing performance to reduced fitness or no signs at all. If left untreated, pAF may promote electrical, functional and structural remodeling of the myocardium, thus creating a substrate that is able to maintain the arrhythmia, which over time may progress into permanent AF. Long-term ECG monitoring is essential for diagnosing the condition and fully understanding the duration and frequency of pAF episodes. The potential to adapt human cardiac monitoring systems and computational ECG analysis is therefore of interest and may benefit future diagnostic tools in equine medicine.

Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation

Atrial fibrillation (AF) is the most common pathological arrhythmia in horses. Although it is not usually a life-threatening condition on its own, it can cause poor performance and make the horse unsafe to ride. It is a complex multifactorial disease influenced by both genetic and environmental factors including exercise training, comorbidities or ageing. The interactions between all these factors in horses are still not completely understood and the pathophysiology of AF remains poorly defined. Exciting progress has been recently made in equine cardiac electrophysiology in terms of diagnosis and documentation methods such as cardiac mapping, implantable electrocardiogram (ECG) recording devices or computer-based ECG analysis that will hopefully improve our understanding of this disease. The available pharmaceutical and electrophysiological treatments have good efficacy and lead to a good prognosis for AF, but recurrence is a frequent issue that veterinarians have to face. This review aims to summarise our current understanding of equine cardiac electrophysiology and pathophysiology of equine AF while providing an overview of the mechanism of action for currently available treatments for equine AF.

Anti-arrhythmic investigations in large animal models of atrial fibrillation

Atrial fibrillation (AF) constitutes an increasing health problem in the aging population. Animal models reflecting human phenotypes are needed to understand the mechanisms of AF, as well as to test new pharmacological interventions. In recent years, a number of large animal models, primarily pigs, goats, dog and horses have been used in AF research. These animals can to a certain extent recapitulate the human pathophysiological characteristics and serve as valuable tools in investigating new pharmacological interventions for treating AF. This review focuses on anti-arrhythmic investigations in large animals. Initially, spontaneous AF in small and large mammals is discussed. This is followed by a short presentation of frequently used methods for inducing short- and long-term AF. The major focus of the review is on anti-arrhythmic compounds either frequently used in the human clinic (ranolazine, flecainide, vernakalant and amiodarone) or being promising new AF medicine candidates (I_K,Ach , I_SK,Ca and I_K2P blockers).

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.