Wednesday, January 12, 2011

ECG Blog #14 — The Cause of the Pause?

QUESTION: Interpret the Lead MCL-1 rhythm strip that is shown below in Figure-1.
  • What is the cause of the pauses in this tracing?  Is there AV block?
  • Why is the PR interval preceding beat #7 shorter than the PR for other sinus beats?

Figure-1: What is the cause of the pauses? (between beats #2-3 and between #6-7).

INTERPRETATION: The rhythm in Figure-1 is irregular in a pattern of group beating (with short pauses between beats #2-3 and #6-7). The QRS complex is narrow (ie, not more than half a large box in duration). The underlying rhythm appears to be sinus, with similar-looking P waves showing a fixed PR interval preceding beats #1, 2, 3, 4, 5, 6, 8, and 9 in this right-sided Lead MCL-1 rhythm strip.
  • Despite the presence of group beating — there is no evidence of Wenckebach or other form of AV block on this tracing.  Instead, the "cause" of the pause lies within the T waves of beats #2 and 6.

The Most Common Cause of a Pause: 
Although most premature supraventricular beats (PACs or PJCs) are conducted normally to the ventricles (ie, with a narrow QRS complex) — this is not always the case.  Instead, PACs (or PJCs) may sometimes occur so early in the cycle as to be "blocked" (non-conducted) — because the conduction system is still in an absolute refractory state.

  • This is the situation for premature impulse A in schematic Figure-2 (which shows A occurring during the ARP = Absolute Refractory Period).

Figure-2: Absolute and Relative Refractory Periods (ARP & RRP) — explaining why beat A is blocked — and beat B conducts with aberration.

  • At other times — premature (early) beats may occur during the RRP (Relative Refractory Period) — in which case aberrant conduction (with a wide and different-looking QRS) occurs.  This is the situation for premature impulse B in Figure 2.  Because impulse B occurs during the RRP — part (but not all) of the ventricular conduction system has recovered. Most often PACs occurring at Point B will conduct with some form of bundle branch block and/or hemiblock (reflecting that part of the conduction system which has not yet recovered).
  • Premature impulse C in Figure 2 occurs after the refractory period is over.  As a result — a PAC occurring at Point C will conduct normally (with a narrow QRS that looks identical to other sinus beats on the tracing).

KEY Clinical Point: 
The most common cause of a pause is a blocked PAC (corresponding to a PAC occurring at Point A in Figure 2). Blocked PACs occur much more often than any form of AV block.
  • Blocked PACs are often subtle and difficult to detect. That said — they will be found IF looked for (they'll often be hiding/notching a part of the preceding T wave).

Returning to the Questions in this Case: 
We illustrate our Answers in Figure 3:
  • The cause of the pause in this case is a blocked PAC (arrow in the T wave of beat #6 highlights the "telltale notching" of a PAC buried in this T wave).  A similar very early-occurring PAC (corresponding to a PAC at point B in Figure 2 can be seen notching the T wave of beat #2).
  • The occurrence of a PAC resets the sinus cycle, usually with a brief pause after the early beat. The reason the PR interval preceding beat #7 is shorter - is that beat #7 is a junctional escape beat that occurs just before before the P wave that precedes it is able to conduct to the ventricles. Normal sinus rhythm then resumes with beat #8.
  • Finally - is the subtle finding that the escape interval preceding beat #3 (ie, the distance between beats #2-3) is slight longer than the distance between beats #6-7. This accounts for why beat #3 is sinus-conducted (with a normal PR interval) — whereas slightly earlier occurring beat #7 is a junctional escape beat (that occurs just before the P wave preceding it is able to conduct to the ventricles).

Fgure 3: Answer to Figure 1 (See text).

    BOTTOM Line: 
    The commonest cause of a pause is a blocked PAC. Remembering this truism will hopefully remind you to always look carefully in the T wave at the onset of all pauses to see if the "telltale" notching of a blocked PAC is in hiding. 

    NOTE: See also ECG Blog Review #15 - and - Section 20.0 from ACLS-2013-ePub on AV Block - 


    1. Thank you for this great explanation, dr. Grauer.

      I just wanted to clarify. Does "telltale" notching of T-wave in beat #2 represent PAC during absolute refractory period or during relative one? You mentioned that it corresponds to point B on figure 2. But how is it different from APC of beat #6 (which is conducted during absolute refractory period? If PAC during beat #2 were to conduct during RELATIVE refractory period, wouldn't it have widened QRS with aberrant morphology as you've mentioned before? Thank you

      1. @ Artek — Thank you for your question. The notching on the T wave of beats #2 and #6 represents a “blocked” PAC — because no QRS complex occurs after these PACs. Therefore — we can presume that these PACs occur during the “absolute” refractory period.

        Of course — we do not know precisely where the absolute and relative refractory period lie — because that is an electrophysiologic concept … But in general — PACs that occur VERY early in the T wave are more likely to occur during the “absolute” refractory period (and therefore these early PACs are more likely to be non-conducted).