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. 

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

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