This is a normal 12-Lead ECG with two PACs that are aberrantly conducted in a right bundle branch block pattern. (Sixth and ninth beats). In the PACs, the QRS is slightly wider than the normal beats. The aberrantly-conducted beats have an rsR' pattern in V1, and a wide little S wave in aVL. No PACs are seen in Lead I to demonstrate the wide S wave. This represents a right bundle branch block pattern, which is a common form of aberrancy, and is rate-related. That is, the PAC occurs early in the cycle, catching the right bundle branch is a refractory state and unable to depolarize. Slower beats are easily acommodated by the right bundle branch.
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.
This series of rhythm strips demonstrates a supraventricular tachycardia with a wide QRS. The SVT stops abruptly after a 6 mg dose of adenosine is given IVP. This confirms the diagnosis of SVT vs sinus tachycardia. When the tachycardia stops, a sinus rhythm resumes, and the QRS is now narrow. This confirms that the bundle branch block was rate-dependent. This is caused by differing refractory periods in the fasicles - the right bundle branch was able to recover its function quickly during the tachycardia, but the left bundle was not able to keep up at that rate.
