The ECG demonstrates grouped beats. P-waves precede the QRS complexes with constant but prolonged PR intervals. This indicates a first-degree AV block.
Please note that P-wave is absent during the pause between the grouped beats. This pattern suggests an SA node block, rather than a second-degree AV block, in which a P-wave is present during the pause but is not followed by a QRS complex.

For more advanced users:
The P-P interval progressively shortens prior to the dropped P wave. The duration of the PP interval during the pause is less than twice the length of the shortest PP interval which is observed prior to the pause. This suggests a second-degree, Wenckebach sinoatrial block.

Special thanks to Dr. Salvatore for his expert ECG interpretation and the informative diagram he provided.