ECG Challenge: Reflux or a Heart Attack?

Philip J. Podrid, MD

Disclosures

February 05, 2019

The diagnosis is NSR, isorhythmic dissociation, acute ST-segment elevation MI (STEMI) of the lateral wall, and poor R-wave progression across the precordium (clockwise rotation).

Figure 2. Courtesy of Dr Podrid

Discussion

The rhythm is regular at a rate of 90 beats/min. The QRS complex duration is increased (0.12 sec). Although there appears to be left bundle branch block (LBBB) morphology, some features are not consistent. These include the right axis between +90° and +180° (negative QRS complex in lead I and positive in lead aVF) and a terminal S wave in leads V5 and particularly in lead V6 (^), indicating left-to-right forces, which are not seen in a LBBB because all of the forces are directed from left to right. A small R wave in lead V1 (*) indicates initial septal forces, which are not seen with a LBBB because the septal forces are a result of septal activation via a septal branch which comes from the left bundle. Last, the Q waves in leads I and aVL (^) account for the right axis and are consistent with a lateral MI. Lateral Q waves should not be seen with a LBBB because they also indicate left-to-right forces. Hence, the widened QRS complex is the result of an intraventricular conduction delay (IVCD) and not a LBBB.

In contrast to a LBBB, in which there is direct myocardial activation that precludes definitive diagnosis of any left ventricular abnormality, an IVCD results from activation through the normal His-Purkinje system, which is slower than normal. Therefore, abnormalities of the left ventricle can be interpreted. Indeed, along with the lateral Q waves, the ST-segment elevation in leads I and aVL (↓) suggest that this is an acute STEMI of the lateral wall. There is poor R-wave progression from leads V1-V6, which is consistent with clockwise rotation of the electrical axis of the heart in the horizontal plane. This is established by imagining the heart as if viewed from under the diaphragm. With clockwise rotation, left ventricular forces are shifted posteriorly and develop in the left lateral precordial leads. This has occasionally been diagnosed as an anterior wall MI, but small R waves are seen in the precordial leads and the T waves are normal.

The QT/QTc intervals are normal (360/440 and 340/420 when the prolonged QRS complex duration is considered). P waves (↑) can be seen before each QRS complex (although they are not seen before the first seven QRS complexes), and the PR interval is not constant. P waves become apparent before the eighth QRS complex and the PR interval progressively lengthens.

The variable PR intervals represent atrioventricular (AV) dissociation. AV dissociation can occur with complete (third-degree) heart block. In this situation, the atrial rate is faster than the rate of the QRS complexes, which are the result of an escape rhythm that may be either junctional or ventricular, based on the QRS complex morphology and not the rate of the escape rhythm. AV dissociation can also occur with an accelerated lower pacemaker (junctional or ventricular, based on the QRS complex morphology). In this situation, the atrial rate is slower than the rate of the QRS complexes.

In Figure 2, the atrial rate is constant (┌┐) at a rate of 90 beats/min and is identical to the ventricular rate (└┘). Because it cannot be established whether this is third-degree AV block or an accelerated rhythm (junctional in this case, given the QRS complex morphology), this is termed isorhythmic dissociation.

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