QUESTION: The patient is an older woman with a pacemaker who presents to the office with atypical chest discomfort. A 12-lead ECG and Lead II rhythm strip was obtained (below).
- How would you interpret her office ECG and rhythm strip?
- Does the pacemaker prevent you from interpreting her ECG?
- Is the pacemaker working?
- Clinically - Is anything acute likely to be going on?
|Figure 1 - ECG from an older woman with a pacemaker and chest discomfort.|
- Note - Enlarge by clicking on Figures - Right-Click to open in a separate window.
INTERPRETATION: This challenging tracing is from a patient with a VVI pacemaker. As such - it senses the Ventricles - paces the Ventricles - and is Inhibited by a spontaneous beat. Operation and function of VVI pacemakers is simpler than that of newer DDD pacemakers that sense and pace both atria and ventricles, and may either be triggered or inhibited by a spontaneous beat. When interpreting a pacemaker tracing - we seek to determine IF the pacemaker is both sensing and pacing appropriately. This will often be extremely difficult to do from the surface ECG with newer DDD models showing atrial and ventricular spikes, especially if the pacing rate varies due to rate-responsiveness functions. VVI pacing (as seen in Figure 1) is the one time you can often feel confident from inspection of the 12-lead ECG alone (without formal interrogation of the device) that the pacemaker is or is not appropriately functioning (Figure 2):
We interpret this tracing by making the following points:
|Figure 2 - Labeling of the tracing in Figure 1.|
Click to enlarge. Right-Click to open in a separate window.
- Beats #1, 2, and 3 in Figure 2 are paced (each QRS complex for these first 3 beats is preceded by a tiny vertical pacer spike best seen in the lead II rhythm strip). The rate of the pacemaker is ~60/minute (the R-R interval from one pacer spike to the next for these first 3 beats is about 5 large boxes).
- Beat #4 is a spontaneous beat. Its QRS complex is narrow and not preceded by a pacer spike.
- The pacemaker appears to be sensing appropriately because the R-R interval between beats #3-to-4 is shorter than the pacer R-R interval (and therefore the pacer appropriately does not fire).
- That the pacer is sensing appropriately is confirmed by beat #5 which is again paced, and which is preceded by an R-R interval of 5 large boxes (which is the appropriate amount of time for the pacer to wait before firing).
- Beats #8, 9, and 10 appear to be spontaneous (they are not preceded by pacer spikes).
- Beat #7 is probably a fusion beat (since it appears to be preceded by a pacing spike, and manifests a QRS complex intermediate in morphology between paced beats and spontaneous beats).
- We surmise that the underlying rhythm in this tracing is atrial fibrillation - since we do not see definite P waves in the lead II rhythm strip and the spontaneous beats are irregular (though we acknowledge that baseline artifact makes it difficult to be sure that there truly are no P waves ... ).
- Finally - a very interesting aspect of this tracing is that the spontaneous beats suggest ischemia! One cannot say anything about ischemia from paced complexes (ie, from beats #1,2,3,5, or 6). We cannot be sure about ST-T wave morphology from the fusion beat (beat #7). However, the T wave of spontaneous beats #4, 8, 9, and 10 in the Lead II Rhythm Strip manifests a worrisome appearance of fairly deep symmetric T wave inversion. Use of time lines (vertical RED lines in Figure 2) show there is corresponding ischemic-looking T wave inversion in leads aVL, V3, and V4 (T wave inversion within the BLUE ovals).