Second-degree atrioventricular block observed in a Thoroughbred foal on 2.5 months of age.

The research monitored electrocardiographic changes in 10 Thoroughbred foals from fetal stages to one year after birth, during which one filly developed second-degree atrioventricular block with sinus arrhythmia at 2.5 months old. The study found that the rate of dropped beats and irregularity of sinus arrhythmia were influenced by the atrial rate, leading to multiple variations of the Wenckebach phenomenon.

Research Overview

The study focused on the ongoing monitoring of electrocardiographic measurements in 10 Thoroughbred horses, starting from their fetal stages and up until they were one year old. This was done to observe any heart-related irregularities, specifically the development of atrioventricular (A-V) block.

Observation of A-V Block in a Filly

During the study, one notable anomaly that was observed was a filly that showed signs of second-degree A-V block and sinus arrhythmia about 2.5 months after birth. A-V block refers to a condition where the electrical signals that regulate the heartbeat are partially or completely blocked from reaching the ventricles.

• Second-degree A-V block is a more severe form of this condition, where not all electrical signals reach the ventricle, causing an irregular heartbeat.
• Sinus arrhythmia is a minor irregularity in heart rhythm that varies with a person’s breathing and is generally considered normal.

Atrial Rate Influence

One of the key findings from the study was that the frequency of dropped beats and the irregularity of sinus arrhythmia during the A-V block in the filly were impacted by the atrial rate, which is the rate at which the heart’s upper chambers, the atria, beat.

• When the atrial rate increases, it can cause more electrical signals to be blocked leading to an increase in dropped beats.
• The irregularity of the sinus arrhythmia is also influenced by changes in the atrial rate.

Wenckebach Phenomenon Variations

The effect of the atrial rate on the A-V block and sinus arrhythmia resulted in several variations of the Wenckebach phenomenon in the foal.

• The Wenckebach phenomenon is a pattern of progressively prolonged conduction time until a beat drops in a heart’s electrical activity, typically seen in the pattern of second-degree A-V block.
• This highlights how the atrial rate can influence the presentation of the A-V block in this foal, adding another layer of complexity to understanding this heart condition in foals.

Comparison with Adult horses

According to the authors of the study, the electrocardiographic characteristics of the second-degree A-V block witnessed in the filly mirrored those more frequently seen in Thoroughbred adult horses, showing a possible pattern or prevalence of this type of A-V block in this breed of horse.