ECG Challenge: What's Irregular About This Rhythm?

Philip J. Podrid, MD

Disclosures

January 05, 2018

Discussion

The diagnosis is normal sinus rhythm, WPW pattern, PACs in bigeminal pattern, and pseudo inferior wall infarction.

Figure 2. Courtesy of Dr Podrid.

The rhythm is irregular and there is a pattern of group beating (ie, two QRS complexes followed by a pause). The average rate is 66 beats/min. The first of the QRS complexes has a P (+) wave with a stable PR interval (0.12 sec). The P wave is positive in leads I, II, aVF, and V4-V6. Hence, this is a normal sinus rhythm. The P wave before the second QRS complex (*) has a different morphology. This is a PAC. Because every other QRS complex is a PAC, this is atrial bigeminy. The PR associated with the PAC is shorter (0.10 sec). All of the QRS complexes have the same morphology, but the first of the two (ie, the sinus complex) is slightly narrower (0.12 sec) than the second (0.14 sec)—the PAC. Both QRS complexes have a slurred upstroke (↑,^), making the base of the QRS complex wider than the peak. The slurred upstroke is due to a delta wave; along with the short PR interval, this is a WPW pattern. The PAC has a slightly shorter PR interval and is more preexcited (the delta wave is more prominent and the PR interval is shorter) as a result of slowing of atrioventricular (AV) nodal conduction. This results in more of the ventricular myocardium being activated via the accessory pathway. The PAC arrives at the AV node earlier, and with decremental conduction it is conducted through the node more slowly than the sinus complex.

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