Validation and clinical application of implantable loop recorders for diagnosis of atrial fibrillation in horses.

BACKGROUND: Paroxysmal atrial fibrillation (pAF) occurs sporadically and can impair athletic performance. Gold standard for diagnosis is surface electrocardiography (ECG), however, this requires AF to be sustained. Implantable loop recorders (ILRs) are routinely used for AF detection in human medicine. While ILR placement has been studied in horses, its AF detection performance is unknown. OBJECTIVES: (I) Validation of ILRs for AF detection in horses. (II) Determining pAF incidence using ILRs and estimate the positive predictive value (PPV). STUDY DESIGN: (I) Experimental study; (II) Longitudinal observational study. METHODS: (I) Implantation of ILRs in 15 horses with AF and 13 horses in sinus rhythm. Holter ECGs were recorded at: 1, 4, 8, 12 and 16 weeks of AF. The ILR ECGs were compared with surface ECGs to assess diagnostic sensitivity and specificity. (II) Eighty horses (43 Warmbloods, 37 Standardbreds) with ILRs were monitored for 367 days [IQR 208-621]. RESULTS: (I) ILRs detected AF on all recording days, in horses with AF, with a sensitivity of 66.1% (95% CI: 65.8-66.5) and a specificity of 99.99% (95% CI: 99.97-99.99). The sensitivity remained consistent across all time points. (II) The incidence of pAF was 6.3% (5/80). In horses with pAF, the PPV ranged from 8% to 87%. Increased body condition score (BCS > 6/9) was associated with an increased number of false positive episodes (p = 0.005). MAIN LIMITATIONS: (I) Horses were stabled during the ECG recordings, and AF was induced, rather than naturally occurring pAF. (II) Integrated algorithm in this ILR is optimised for AF detection in humans using remote monitors. Additionally, sensing is affected by motion artefacts. CONCLUSION: The ILR reliably detected AF in resting horses, particularly in horses with normal BCS (6/9). The ILR proved useful to detect pAF and is recommended alongside Holter monitoring for diagnostic workup of horses with suspected pAF.

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.

Cartilaginous Intrusion of the Atrioventricular Node in a Quarter Horse with a High Burden of Second-Degree AV Block and Collapse: A Case Report.

Second-degree atrioventricular (AV) block is the most common cardiac arrhythmia in horses, affecting 40-90% depending on breed. Usually, the AV blocks occur while the horses are resting and disappear upon exercise and are, therefore, considered to be uneventful for horses. However, if the AV blocks occur frequently, this may result in syncope and collapse. Identifying the cause of second-degree AV block is difficult and often subscribed to high vagal tone. In this report, we present an eight-year-old Quarter horse with a high burden of second-degree AV blocks and multiple collapses. The clinical examination, including neurological examination, blood analysis, 24-h ECG recording and cardiac echocardiography, did not reveal any signs of general or cardiovascular disease besides a high burden of second-degree AV blocks (~300 blocks per hour) and a hyperechoic area in the AV nodal region. An implantable loop recorder (ILR) was inserted to monitor the cardiac rhythm. The ILR detected several consecutive second-degree AV blocks and pauses above 5 s. However, unfortunately, no recordings were available during the collapses. Eventually, the horse was euthanized and the heart inspected. The aortic root was severely cartilaginous and appeared to penetrate the AV node, especially in the His bundle region, possibly explaining the hampered AV conduction. Nevertheless, it is still uncertain if the AV nodal disruption caused the collapses and more knowledge on AV nodal diseases in horses is warranted.

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.