How would you interpret the lead II rhythm strip shown in the figure below? How certain are you of your diagnosis? Are the P waves preceding beats six and seven conducting?

The easiest way to approach the interpretation of more challenging arrhythmias such as this one is to begin with the part of the tracing that is most evident and save the most difficult part for last. To do this, mentally block out the first seven beats on this tracing.

Beats 8 through 13 clearly show sinus tachycardia at a rate of 110/minute. Red arrows highlight upright sinus P waves in this lead II rhythm strip. Assessing the first five beats in this tracing also is straightforward. These beats show a regular, wide QRS rhythm at a rate between 100-105/minute, without preceding P waves. This suggests a ventricular etiology. Since the usual rate of an idioventricular escape rhythm is much slower (in the range of 20-40/minute), we describe the initial five beats as an accelerated idioventricular rhythm (AIVR).

This leaves us with the middle two beats (i.e., beats 6 and 7). Note that a P wave (red arrow) precedes beat 6. However, the PR interval preceding this beat is much shorter than the PR interval preceding each of the other sinus beats. This means that there simply was not enough time for the sinus impulse preceding beat 6 to complete its conduction pathway before “something else” must have happened. That “something else” must have come from the other direction, arising in the ventricles. Beat 6 is a fusion beat. Its QRS complex and T wave are intermediate in morphology between the much wider and taller morphology of ventricular beats (1 through 5), and the narrow, predominantly negative sinus-conducted beats (8 through 13).

Beat 7 also is a fusion beat. Since its preceding PR interval is longer and its QRS and T wave morphology more closely resembles the QRS and T wave morphology of subsequent sinus beats, fusion between near-simultaneous appearance of sinus and ventricular impulses occurred later in the cycle for beat seven.

Beat 5 actually is also a fusion beat. It, too, is preceded by a P wave (blue arrow highlighting the small bump deforming the initial part of the QRS). Note that beat 5 is not as tall (and its T wave is not as deep) as the first four beats in this tracing, which are pure ventricular beats.

Clinically, the reason recognition of fusion beats is important is that it proves the widened beats in a tracing must be of ventricular etiology. For a further discussion of this case, please visit: http://tinyurl.com/KG-Blog-128.

ECG Review