Third-Degree Atrioventricular Block

Third-degree atrioventricular block is a condition in which the electrical signals that normally coordinate the heartbeat between the upper chambers (atria) and lower chambers (ventricles) are completely interrupted. The heart continues to beat, but the upper and lower chambers contract independently of one another, often resulting in a slow and inefficient rhythm. This form of heart block represents the most complete disruption of the heart's internal conduction system. Owners may notice episodes of weakness, collapse, or reduced exercise tolerance, though some animals show surprisingly few outward signs until the heart rate drops significantly. Because the ventricles generate their own rhythm when signals from above are blocked, the heart rate can vary depending on where that backup rhythm originates and how reliably it fires. The pattern can be persistent or intermittent, and the severity of signs often relates to how slow the heart becomes and whether the backup rhythm is stable. This page explores the signals that may accompany third-degree heart block, the underlying mechanisms that lead to complete conduction failure, the investigations used to confirm the diagnosis and assess the heart's function, and the range of approaches—including monitoring, medication, and pacing—that may be considered depending on the individual animal's circumstances.