Figure 2. Courtesy of Dr Podrid.
The correct answer is sinus pause due to sinus node exit block. The rhythm is regular at a rate of 80 beats/min, although two long RR intervals (└┘) with a rate of 40 beats/min are present in the precordial leads. Each QRS complex is preceded by a P wave (+) with a stable PR interval (0.16 sec). The P wave is sinus (positive in leads I, II, aVF and V4-V6), hence a normal sinus rhythm. The QRS complex is wide (0.12 sec).
There are two long PP (or RR intervals) present (└┘) and there are no P waves present during the pause. Hence, this is not an AV block but represents a sinus pause. A sinus pause may be due to either a sinus node exit block in which the sinus node activation occurs but the impulse fails to get into the atrium so there is no atrial activation and no P wave but the sinus rate remains unchanged (and the PP intervals around the pause are equal to 2 sinus intervals); or a sinus node arrest, in which the sinus node fails to activate on time and the sinus rate is slower (and the PP intervals around the pause is less than or greater than 2 sinus intervals). As the PP interval around the pause is equal to two sinus intervals (2PP), this represents a sinus node exit block.
Although the complex looks like a left bundle branch block (LBBB), certain features are not indicative of LBBB: a prominent septal R wave in lead V1 (indicating normal initial septal forces) and a broad terminal S wave in lead V6 (indicating terminal forces directed left to right).
Initial ventricular activation occurs in the left part of the septum and goes from right to left. The impulse originates from a septal or median branch that comes from the left bundle. This accounts for the small R wave in lead V1 and often small Q waves in leads I, aVL and V5-V6. With LBBB, septal forces are absent.
The broad terminal S wave in lead V6 is due to terminal forces directed from left to right which is not seen with LBBB, where all forces are directed from right to left. The terminal S wave is more consistent with a right bundle branch block (RBBB). Hence the QRS complex has an intraventricular conduction delay. The axis is extremely leftward, with a positive QRS complex in lead I and negative complex in leads II and aVF.
This may be due to either an old inferior wall myocardial infarction due to deep Q waves in leads II and aVF, or a left anterior fascicular block with an initial R wave and deep S wave. Hence, this is a left anterior fascicular block which can be diagnosed as the QRS complex is widened from an intraventricular conduction delay and not a LBBB (for the latter, there would be a block in both fascicles). There is slight downsloping ST segment depression in leads I and aVL (^), which is nonspecific.
Philip Podrid, MD, is an electrophysiologist, a professor of medicine and pharmacology at Boston University School of Medicine, and a lecturer in medicine at Harvard Medical School. Although retired from clinical practice, he continues to teach clinical cardiology and especially ECGs to medical students, house staff, and cardiology fellows at many major teaching hospitals in Massachusetts. In his limited free time he enjoys photography, music, and reading.
You can follow Dr Podrid on Twitter @PPodrid
Follow theheart.org | Medscape Cardiology on Twitter
Follow Medscape on Facebook, Twitter, Instagram, and YouTube
Medscape Cardiology © 2021 WebMD, LLC
Any views expressed above are the author's own and do not necessarily reflect the views of WebMD or Medscape.
Cite this: ECG Challenge: Irregular Rhythms After Head Trauma - Medscape - Aug 04, 2021.