First-degree atrioventricular (AV) block is a type of heart block that occurs when the conduction of electrical signals from the atria to the ventricles is slowed. This can cause the heart to beat more slowly than normal. In first-degree AV block, the electrical signals are delayed, but they are still able to reach the ventricles and trigger a heartbeat. This type of heart block is usually not serious and does not cause any symptoms. However, it may be a sign of an underlying heart problem and may need to be monitored by a healthcare provider.