A good example of aberrantly conducted premature beats (PACs or possibly PJCs) that are conducted with a left anterior fascicular block. The underlying rhythm is sinus at about 80/min. The timing of the premature beats is best seen in the Lead II rhythm strip at the bottom, as this ECG machine does not print the 12 leads in an uninterrupted manner. You will see interruptions each time the leads change.
The first beat on the ECG is one of the premature beats. You can observe the left axis deviation without pathological Q waves. Lead I shows the premature beats with an Rs pattern, and Leads II and III have rS. The early beats have caught the anterior fascicle of the left bundle branch refractory from the preceding beat. It recovers for the normally-timed sinus beats.