ECG Review: What a Difference a Lead Makes

By Ken Grauer, MD

Figure. Telemetry rhythm strip obtained from a 67-year-old woman with heart failure

Clinical Scenario: The telemetry rhythm strip shown in the Figure was obtained from a 67-year-old woman who presented with heart failure. A permanent pacemaker had been implanted a number of years earlier. Interpret the tracing initially by looking only at lead MCL1. How does the addition of a second simultaneously recorded lead (lead II) help in your interpretation? How many findings can you identify on this two-lead telemetry tracing? (Hint: Some of these findings are very subtle!)

Interpretation/Answer: Accurate interpretation of this tracing would be virtually impossible if one only had access to a single MCL1 monitoring lead. This is because the QRS complex looks similar for virtually all beats in MCL1, and pacer spikes are practically nondetectable in this lead. This highlights the importance of viewing arrhythmias from more than the limited perspective of a single monitoring lead. Addition of lead II to our database allows identification of regular ventricular pacing spikes at a rate of 80/minute for much of the tracing (beats #4 through 10 and 12-13 are paced). Each pacer spike is followed by a QRS complex and T wave, indicating appropriate capture. Several spontaneous beats are seen on the tracing (beats #1, 2, 3 and 11). From the complete absence of P waves, the presence of fine undulations in the ECG baseline, and apparent irregularity of spontaneous beats #1, 2, and 3, the underlying rhythm appears to be atrial fibrillation. Appropriate sensing of the pacemaker is suggested by the absence of pacer spikes during the spontaneous rhythm, with appropriate return of pacer spikes following the two brief pauses that occur after beat #3 and beat #11. Note that the R-R interval preceding the pacer spikes occurring after these two pauses (ie, the R-R interval between beats #3-4 and 11-12) is virtually the same as the R-R interval during the 7-beat sequence of consecutively paced beats (that occurs between beats # 4-10). This confirms that the pacemaker is appropriately sensing as well as capturing the ventricles.

The final finding of interest relates to the presence of fusion beats. The importance of recognizing this finding on a pacemaker tracing is primarily so that one does not misinterpret the changes in QRS morphology that may result as indicative of ventricular ectopy or pacer malfunction. Fusion beats are commonly seen in patients with pacemakers (especially when the underlying spontaneous rhythm is atrial fibrillation), since the presence of the pacemaker itself predisposes to a situation in which some spontaneous beats are likely to occur (by chance alone) in close temporal proximity to paced impulses. The result of near simultaneous occurrence of a spontaneously occurring supraventricular impulse (from the patient's atrial fibrillation) with an impulse originating for the ventricles (from a paced beat) is a "fusion" complex that manifests characteristics of both the spontaneous beat and the paced QRS complex. Thus, the paced QRS complexes of both #4 and 5 are clearly not as wide in lead II as the other paced beats—a result of fusing QRS morphology of spontaneous beats with completely paced complexes. Note that the T wave in lead II of these fusion beats (ie, the T wave of beats #4 and 5) is not as prominent as the T wave of fully paced beats #6, 7, 9, 10, 11, and 12. In addition to a fusion effect on QRS morphology, near simultaneous occurrence of a supraventricular beat and a paced ventricular complex may also produce a fusion effect on T wave appearance. Awareness of this last point supports our suspicion that the slight alteration in QRS and T wave morphology of paced beat #8 also reflects some degree of fusion between this paced beat and a spontaneously occurring impulse from this patient’s underlying atrial fibrillation.

Dr. Grauer, Professor, Assistant Director, Family Practice Residency Program, University of Florida, is Associate Editor of Internal Medicine Alert.