Monday, March 14, 2011

ECG Interpretation Review #17 (Older Woman with a Pacemaker and Chest Discomfort)

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):
Figure 2 - Labeling of the tracing in Figure 1.
Click to enlarge. Right-Click to open in a separate window.
We interpret this tracing by making the following points:
  • 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).
BOTTOM LINE:  This interesting ECG shows that despite the presence of a pacemaker - it is still possible to assess pacemaker sensing and pacing function, determine the underlying rhythm - and detect ischemic T wave inversion of clinical concern in this older woman with chest discomfort.


  1. The T-wave changes in V1 and V2's paced beats are also concerning. They do not appear to have the appropriate discordance. Although, without seeing the patient's "resting" 12-lead, it is difficult to say exactly what I'm seeing in those leads.

  2. YES - The ST-T wave changes in V1,V2,V3 paced beats are eye-catching (these are the paced beats corresponding to [above] beat #6 in the Lead II Rhythm Strip). That said - these are PACED beats. The QRS is wide because of the ventricular origin of these paced beats - and interpretation of ST-T wave morphology is unfortunately not reliable. Although possible (on rare occasions) to gain insight about ischemia/infarction from paced beats - in the overwhelming majority of cases, you cannot draw reliable conclusions. The point of this tracing is to highlight how looking for non-paced beats may provide information about ischemia/infarction on a paced tracing (BLUE Ovals). The fusion beat (BLUE oval in V3) is particularly interesting - in that the shallow T wave inversion represents fusion of ST elevation and T wave peaking from the paced beat - and symmetric T inversion of the spontaneous beat ... THANKS for your Comment Christopher! - Ken Grauer, MD

  3. Agreed, after identifying paced complexes my first task is usually to find non-paced complexes (if any). I always enjoy fusion complexes, reminds me of studying superimposition of waves in physics and analog signal processing.

  4. It was very helpful , thank you very much

  5. I will share by paraphrasing this comment just received from Dr. Ekaterina Pervova of Moscow, Russia: "I have a comment. I think we should talk to doctors about the impossibility of interpreting ST-T wave changes on the ECG in non-paced beats as indicative of myocardial ischemia. ECG changes such as these should be interpreted as the phenomenon of ECG "Cardiac memory" (in Russia, this is called "ECG phenomenon Chattergjee"). Rather than risk false diagnosis of acute MI in a paced ECG - the physician should take into account the tracing with the history of chest pain - and reserve final diagnosis for confirmatory tests (cardiac enzymes, echocardiography, coronary angiography, etc.). THANK YOU Dr. Pervova for your excellent comment! I should have been a little less definitive in my statements ... Your point is well taken that pacing may affect the reliability of ECG interpretation of non-paced beats in assessing for ischemia - BUT while clearly not definitive - in a patient like this one with chest discomfort - finding the ST segment shape as is seen in a number of leads on this tracing (in conjunction with the small but real q wave and T inversion in the lead II rhythm strip) I believe should certainly at least raise the question of acute ischemia/recent infarction. I agree completely with you that definitive tests (troponins and probably cath) will ultimately be needed for confirmation - Ken Grauer, MD (12-18-2011).

  6. Great ECG as always!
    Aren't v1 and v2 paced beats meeting the Sgarbossa Criteria?
    Niko G

  7. Great ECG as always Ken!
    Aren't v1 and v2 paced beats meeting the sgarbossa criteria for STEMI?
    Niko G

    1. Excellent question Niko. Opinions on the answer vary. There are those who say that Sgarbossa criteria may be applied to paced tracings. In my opinion — that is an extrapolation that is not well validated. I honestly don't see how it could be "validated" — since there are so many different types/sites for pacing, that doing a controlled, prospective study would be exceedingly difficult (to say the least). On the other hand — assessment of QRST morphology can at times be helpful with pacer tracings — and when you see clear ST elevation that shouldn't be in certain leads, the ECG of paced beats can even be diagnostic. In my opinion — this is not one of those times ... The ST-T waves in leads V2,V3 for the paced beats in Figure 1 is suspicious, but to me, just not diagnostic of acute stemi given paced etiology. On the other hand, ischemic-looking ST-T waves in spontaneous beats support that there is ischemia, though the findings are again not diagnostic of acute ongoing stemi in my opinion. Thanks for your interest!