ECG Challenge: A Counterclockwise Rotation

The diagnosis is NSR, Mobitz type I second-degree atrioventricular (AV) block (Wenckebach), nonconducted (blocked) PACs, and early transition (counterclockwise rotation).

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Figure 2. Image courtesy of Dr Podrid

Discussion

The rhythm is irregular, but there is a repeating pattern of longer and shorter RR intervals and group beating with two QRS complexes followed by a pause. Hence, the rhythm is regularly irregular. The average rate is 54 beats/min. All of the QRS complexes are identical, and they have a normal duration (0.08 sec) and normal axis between 0° and +90° (positive QRS complex in leads I and aVF).

The morphology is normal, except for a very tall R wave in lead V2 (←). This represents early transition or counterclockwise rotation of the electrical axis in the frontal plane. This is determined by imagining the heart as if viewed from under the diaphragm.

When there is counterclockwise rotation, left ventricular forces are shifted anteriorly and they appear earlier in the precordial leads (ie, in lead V2) with a tall R wave. Although this might be considered evidence of right ventricular hypertrophy, the R wave in lead V1 is not particularly increased in amplitude. However, the very tall R wave in leads V2-V3 is suggestive of left ventricular hypertrophy.

The QT/QTc intervals are normal (440/420 msec). There are P waves before each QRS complex (+). The P waves are positive in leads I, II, aVF, and V4-V6. The PP interval is constant, at a rate of 78 beats/min. Therefore, this is NSR. However, the PR intervals are not constant (└┘,↔). The PR interval with the first QRS complex of the pair is 0.26 sec (└┘), and the next PR interval of the second QRS complex of the pair is longer (0.48 sec) (↔). This is a pattern of Mobitz type I second-degree AV block (Wenckebach).

After this second QRS complex, there is a pause. However, the pause is not the result of an on-time but nonconducted P wave, as is typical with Mobitz type I. An abnormal waveform is seen at the end of this QRS complex (^), which is negative in leads II, aVF, and V4-V6. Therefore, this is a nonconducted P wave, or blocked PAC. Because it occurs after every second QRS complex, it is a trigeminal rhythm. Alternatively, the negative P wave may represent a retrograde P wave, in which case the preceding QRS complex would be junctional (ie, a junctional escape complex) resulting from complete AV block (accounting for the very long PR interval).

Given the RR interval of this complex, a second junctional escape would be expected. However, it is not present, and instead, there is an on-time and conducted P wave. Therefore, the more likely diagnosis is blocked PAC in a trigeminal pattern. These blocked PACs interrupt the Wenckebach cycle.

Any views expressed above are the author's own and do not necessarily reflect the views of WebMD or Medscape.

Cite this: ECG Challenge: A Counterclockwise Rotation - Medscape - Jan 09, 2019.

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