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 (similar to 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.

Automated summary

Second-degree atrioventricular (AV) block is a common cardiac arrhythmia in horses, and it can lead to syncope and collapse. The diagnosis and identification of the cause of this condition are difficult. In this report, a horse with a high burden of second-degree AV blocks and multiple collapses was examined. Various tests did not reveal any underlying cardiovascular disease, but an abnormality was found in the AV nodal region. The implantable loop recorder detected consecutive AV blocks and pauses. However, the exact cause of the collapses remains uncertain.