first-degree AV block
First-degree AV block is the mildest form of heart block — really just a slow lane rather than a roadblock. Every signal from the atria still reaches the ventricles, but it takes a little longer than usual to make the trip. Because no beats are actually lost, the heart rate is normal and the person usually feels nothing at all.
On the ECG it is defined purely by a prolonged PR interval — the time from the start of the P wave to the start of the QRS — exceeding about 200 milliseconds, while every P wave is still followed by a QRS. The delay typically lies within the AV node itself, where conduction is naturally slow and easily slowed further.
Most first-degree block is benign and needs no treatment; it is common in healthy older people, athletes with high vagal tone, and as an effect of rate-slowing drugs. It matters mainly because it can occasionally be the first hint of more advanced conduction disease, so it is noted and watched rather than ignored.
A fit marathon runner's pre-race ECG shows a PR interval of 230 ms with every P wave conducted; this first-degree block reflects high vagal tone and requires no action.
In athletes, first-degree block usually reflects high vagal tone, not disease.
By definition every atrial beat is still conducted in first-degree block — only the timing is delayed. It is therefore not a true “block” in the sense of dropped beats, despite its name.