There are a handful of situations for which I find myself looking at an ECG and asking, “Why am I unsure about what this tracing shows?”. Despite feeble attempts to reassure myself in the knowledge that as a teacher of electrocardiography and clinician who has interpreted literally thousands of difficult tracings — these perplexing encounters continue to occasionally pose problems for me. One of these situations is AFib (Atrial Fibrillation). While most of the time, the ECG diagnosis of AFib is obvious — sometimes this is not the case. The most notorious example of this phenomenon, is that ECG without prior comparison tracing — in which the rhythm looks almost regular; the ventricular response is controlled (ie, between 60-to-80/minute); P waves are absent; and the baseline is clean and devoid of those “telltale” undulations known as “fib waves”. More than once in such cases have I found myself asking whether I was looking at AFib or a slightly accelerated junctional response or simple sinus rhythm with extremely small P waves.

The other ECG encounter that occasionally has me wondering — is that patient with a pacemaker whose device leaves no more than the tiniest clue of mechanical stimulation. This tracing presented by Dawn provides an excellent example of this latter phenomenon. When I first looked at this ECG — I thought it represented some type of atypical LBBB. The clinical reality, is that we do not always know at the time we are asked to interpret a tracing whether or not the patient has a pacemaker. The patient’s medical record may not be available — and sometimes even the patient doesn’t know if they have a pacemaker … While pacemaker spikes are usually easy to see (at least in some leads …) — they are not at all obvious in any of the leads on this ECG. But, pacemaker spikes are present, and they are present in multiple leads:

• We know that the tiny dark spot at the onset of the QRS in lead V2 is real (and represents a ventricular pacing spike) — because this tiny mark is also present at the exact same relative point of the QRS complex in leads V3, V4 and V5. Close scrutiny of this tiny mark in these 4 leads (especially in lead V5), reveals consistent overflow of this dark area beyond the limits of the QRS complex.
• Importantly, as we look vertically down from the chest leads at the simultaneously-recorded long lead II rhythm strip — We do not see any indication at all in lead II that ventricular pacing is present. Sometimes, some leads simply do not show pacemaker spikes.
• Knowing from the chest leads that ventricular pacing spikes are present — allows me to surmise that the even-more-subtle miniscule deflections preceding the QRS in certain limb leads (ie, leads I, III, aVL) also reflects ventricular pacing. Admittedly, had I not seen the chest leads — I would probably have overlooked these limb lead ventricular spikes.
• Finally — atrial pacing is also present, albeit atrial spikes are well seen only in the long lead II rhythm strip. That said, had I not been clued in (by the presence of ventricular spikes) to the need to look especially close just before the onset of the P wave deflection — I most probably would not have detected this subtle evidence of atrial pacing. 

BOTTOM LINE: The art of ECG interpretation is both fascinating and humbling. Failure to appreciate that a pacemaker is present in a tracing such as the one shown here — would be a fundamental oversight that could lead to a dramatically different clinical interpretation and management approach. Awareness of how this can happen, and the focus one must routinely devote to detect subtle pacemaker spikes will hopefully help to avoid this potential pitfall. It will also explain why even experienced clinicians sometimes ask, “Does this patient whose ECG I am trying to interpret have a pacemaker?”.

In response to Ken's comments about the occasional genuine difficulty in being sure whether a rhythm is paced, the heart rate can provide a clue. Here the HR is EXACTLY 60/min throughout the recording. This doesn't prove anything but it certainly supports a suspicion of pacing. Of course, the converse is not true: A HR other than 60/min does not in any way exclude pacing.

Certainly I agree with Drs. Grauer and Richley, but I think the key word in Dr. Richley's comment (which he conveniently capitalized) is "EXACTLY" and not the rate of 60/min. I'm not so concerned about the actual rate as the fact that the rhythm is perfectly - EXACTLY - regular. One of the first things I have my students do in my advanced ECG class is to use their calipers and try to find a sinus rhythm that remains exact for more than 6 or 8 beats. So, even if the rate were 68/min, 70/min or 75/min - the fact that it is EXACT throughout the tracing is an excellent hint that a pacemaker is present. I gave up thinking I could determine the presence of a pacemaker by the miniscule pacing spike a long time ago. I really think the pacemaker manufacturers have done healthcare providers a real disservice in not making the pacemaker artifacts more visible.

Of course, an escape rhythm could be "exact" for the 10 seconds or so duration of an ECG and reentrant rhythms can be exact (though, obviously, much faster). So, as Dr. Grauer pointed out, knowing the circumstances surrounding the acquisition of the ECG is extremely helpful.