The Patient     This ECG is taken from a teenage boy whose mother was concerned that he has an “irregular pulse”.  He is otherwise very healthy, and has no complaints.

The ECG     The rhythm is irregular.  With such a short strip (12-Lead ECGs are often about ten seconds long), it is difficult to determine the exact reason for the irregularity.  One possibility is a sinus rhythm with PACs, which would not be unexpected in a young person.  The more likely interpretation is sinus arrhythmia, also very common in young people.  In a longer strip, we might be able to associate the rhythmic slowing and speeding of the rate with the patient’s respirations.  The P waves all look alike in each of the twelve leads. Different P wave morphology in the early beats would indicate PACs, but the similar P waves do not exclude PACs.

The PR interval is normal, the QRS duration normal, the QTc is normal.  The rate (average 68 bpm) is normal.  The frontal plane QRS axis is normal, with Lead II having the tallest QRS in the frontal plane (limb leads), and aVL small and biphasic.  R wave progression in the precordial leads is good, with the transition from negative to positive in V₃.