First-degree AV block
A consistent prolongation of the PR interval in which every atrial impulse still reaches the ventricles. On its own it is a conduction delay rather than a true block of any beat.
Key ECG features
- ✓PR interval longer than normal
- ✓PR constant from beat to beat
- ✓Every P wave is followed by a QRS
- •Rhythm otherwise regular
✓ required · • typical
ECG characteristics
A P wave precedes every QRS with no dropped beats, and the PR interval is longer than normal but constant from beat to beat. The rhythm is otherwise regular.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Often high vagal tone or drug effect. It can also accompany more advanced conduction system disease, atrial enlargement, or electrolyte disturbance.
Why it matters
Usually benign in isolation and rarely symptomatic. Its value is as a marker - it can be the first visible sign of conduction disease that later progresses.
Differentiate from
- Second-degree AV blockFirst-degree block never drops a beat - every P conducts, just slowly. If a QRS goes missing, it is second-degree.