ECG Interpretation Review #67 (PAC – PVC – 12 Leads Better than One)

     The rhythm strip shown in Figure 1 was obtained from a patient with palpitations. The patient was hemodynamically stable.

• Does Figure 1 show atrial trigeminy (every 3rd beat a PAC )?

Figure-1: Lead II rhythm strip obtained from a patient with palpitations. Is this atrial trigeminy? (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version- pp 229-230). NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window (See text).

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Interpretation of Figure 1:

     The underlying rhythm in Figure 1 is sinus — as determined by the presence of upright P waves with fixed PR interval preceding beats #1,2; 4,5; 7,8; 10,11; and 13 in this lead II monitoring lead. The QRS complex of sinus beats is narrow.

• Every-third-beat occurs earlier-than-expected and looks slightly different. That is — the QRS complex of beats #3, 6, 9 and 12 each have a smaller r wave and less deep S wave than do sinus beats. The QRS complex for each of these early beats looks to be narrow and preceded by a premature P wave (red arrow in Figure 1). IF this were the case — the rhythm would be atrial trigeminy (every third beat a PAC [Premature Atrial Contraction] ).

QUESTION: Do you agree with the above assessment?

• HINT #1: Do we have enough information from Figure 1 to determine IF the QRS complex of each early beat is truly narrow?
• HINT #2: Look at Figure 2 — in which we have added a simultaneously recorded lead I rhythm strip. Does the QRS of each early beat still look narrow?

Figure-2: A simultaneously recorded lead I rhythm strip has been added to Figure-1. Note from the vertical time line that the notch which looked to be a premature P wave in Figure-1 is actually the initial part of the QRS complex in simultaneously recorded lead I. (Figure reproduced from ACLS-2013-Arrhythmias: Expanded Version- pg 230).

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12 Leads are Better than One

     We fully acknowledge that this is a trick tracing. We nevertheless have chosen it to emphasize a number of important points:

• POINT #1: 12 Leads are Better than One. It is easy to get fooled when you are not provided with complete information. Part of the QRS complex may sometimes lie on the baseline. When this happens — the QRS complex may look narrow in one lead — whereas in reality it is actually quite wide. At other times (as in Figure 1) — what looks like a preceding “P wave” may actually be the initial part of the QRS complex. Use of a simultaneously recorded multi-lead rhythm strip (or 12-lead ECG) may be invaluable in such cases for shedding light on the true nature the rhythm being assessed. With the extra information provided by lead I in Figure 2 — it is now apparent that every third beat (ie, beats #3,6,9,12) occurs early, is wide, and is not preceded by any premature P wave. Every third beat in Figure 2 is therefore a PVC. The rhythm is ventricular trigeminy.
• POINT #2: Assume that a Premature Beat is “Guilty” (ie, a PVC) until Proven Otherwise.  Statistically — most early occurring different-looking beats that are not clearly preceded by a premature P wave will be ventricular in etiology. The “onus of proof” therefore rests with the interpreter to establish that any abnormal-looking beats are aberrantly conducted (ie, supraventricular) — rather than the other way around. Assume a ventricular etiology until proven otherwise. And remember that you can not “prove” aberrant conduction with use of the incomplete information provided from a single monitoring lead.