How would you interpret the rhythm in the figure below? How certain are you of your diagnosis? Why does beat 8 look so different from all other beats in this tracing? What clinical situation commonly is associated with arrhythmias such as the one shown here?

Unfortunately, no clinical information is provided for the rhythm strip shown in the figure. Nevertheless, to facilitate interpretation, we number the beats and highlight with red arrows the atrial activity that clearly is present.

The easiest way to begin interpretation of a complex arrhythmia, such as the one shown here, is by looking to see if there is an underlying rhythm. The key lies with recognition of the two consecutively conducted sinus beats in the middle of the tracing (beats 6 and 7). Additional sinus beats manifest a similarly shaped, narrow QRS complex and similar P wave shape with constant PR interval preceding beats 1, 3, 9, 11, and 13. Thus, the underlying rhythm is sinus.

Beats 2, 4, 5, 10, and 12 are wide. These beats are either not preceded by any P wave or preceded by an on-time P wave that notches the very beginning of the QRS complex with a PR interval that is too short to conduct. Therefore, these beats must be ventricular in etiology. We call these beats premature ventricular contractions (PVCs), even though they occur relatively late in the cycle (usually just before the next on-time sinus P wave would be able to conduct).

Clinically, this late-cycle feature of these PVCs is similar to the phenomenon of accelerated idioventricular rhythm (AIVR), in which a ventricular rhythm at a slightly accelerated rate (usually between 60-110 beats/minute) is seen in patients with recent acute infarction who have just reperfused the infarct-related artery. Note that this is the picture we see for ventricular beats 4 and 5, which, if they were followed by additional ventricular beats at similar R-R interval spacing, would constitute AIVR at a rate of ~75 beats/minute. Even though we are not given any clinical information about this patient, the finding of a bigeminal pattern of late-cycle PVCs with two consecutive ventricular beats at a rate consistent with AIVR is characteristic enough to strongly suggest consideration that the rhythm in the figure might represent a reperfusion rhythm.

We save assessment of beat 8 for last. Note that this beat is preceded by an on-time P wave with a PR interval that is too short for normal conduction. Note also that both QRS and T wave morphology of beat 8 is intermediate between morphology of the pure ventricular beats on this tracing and the sinus-conducted beats. Beat 8 is a fusion beat, and it proves beyond doubt that widened beats in this tracing are ventricular in etiology.

Additional discussion of this case is available at: http://tinyurl.com/KG-Blog-129. Please also see the ECG Review in the October 15 issue of Internal Medicine Alert for further review on recognition and the clinical significance of fusion beats: http://bit.ly/2dpfhZ6.

ECG Figure