ECG Interpretation Review #48 (Chest Pain - STEMI - 1st Degree - 2nd Degree AV Block - Infero-lateral - 2-to-1 - Wraparound)

The 12-lead ECG shown in Figure 1 was obtained from 50-year-old woman with “chest tightness”.

• What two major conclusions should be reached regarding ECG interpretation of this tracing? 

Figure 1 – 12-lead ECG from a 50-year-old woman with chest tightness. (Reproduced from ECG-2014-ePub). – NOTE – Enlarge by clicking on Figures – Right-Click to open in a separate window.

INTERPRETATION: The two conclusions to be reached are: i) that there is a large acute evolving STEMI (ST-Elevation-Myocardial Infarction) in progress; and ii) there is 2nd degree AV block. It may be easiest to first interpret the changes of acute MI:

• The QRS complex is narrow.
• The axis is normal.
• There is no chamber enlargement.
• Q Waves/R Wave Progression: Q waves are present in each of the inferior leads (II,III,aVF). After a small (but definitely present) initial r wave in lead V1 – there is loss-of-R-wave – with a probable QS in V2 and a definite QS (with notch in downslope of S wave) by lead V3. Abnormal Q waves persist in leads V4-thru-V6.
• ST-T Wave Changes: There is marked ST elevation in each of the inferior leads. In addition – there is ST segment coving and elevation that appears to begin in lead V2, and persist through to lead V6. T wave inversion (sometimes marked) is seen in each of the leads with ST elevation.
• Reciprocal Changes – are seen in leads I and aVL. These reciprocal changes entail not only ST depression in lead aVL – but also T wave peaking (which is the “mirror-image” of the deep inferior symmetric T wave inversion).

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WHAT IS THE RHYTHM in FIGURE 1?

An extra P wave appears to be hiding in the tail portion of each QRS complex on this tracing (Figure 2). This extra P wave is most evident in lead II (RED arrows in Figure 2) – but it is also seen in other leads (BLUE arrows).

• The underlying atrial rhythm is rapid (~140/minute) and regular.
• The PR interval preceding each QRS complex is fixed (albeit prolonged to 0.26 second). Thus, every-other-P-wave is conducting – and the rhythm is 2nd degree AV block with 2:1 AV conduction.

Figure 2 – Addition of red and blue arrows to Figure 1 – revealing 2:1 AV conduction in the underlying rhythm (See text).

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IMPRESSION:  The ECG in Figure 1 suggests a large acutely evolving STEMI involving inferior, anterior and lateral precordial lead areas. Large Q waves have already developed in anterior leads (loss or r between V1-to-V2 – and QS in V2,V3). ST segment coving and elevation persists in these anterior lead areas – though ST elevation is not as marked as it is in inferior leads.

• Whether the extensive pattern of injury seen indicates acute occlusion of a large dominant circumflex artery vs an LAD (Left Anterior Descending) artery with “wraparound” (an LAD that also supplies the inferior wall) vs some combination of lesions (perhaps with collateralization affecting distribution) – is uncertain. What is certain – is that an extremely large infarction with AV block is acutely evolving.

We define the type of AV conduction disturbance seen in Figures 1,2 as 2nd degree AV block with 2:1 AV conduction:

• In theory – the conduction disturbance could be either Mobitz I (AV Wenckebach) or Mobitz II AV block. The reason we cannot absolutely distinguish between these two possibilities is that one never sees two beats conducted in a row (so that we can’t tell IF the PR interval would lengthen if it did have an opportunity to do so).
• That said – we strongly suspect the conduction disturbance represents 2nd degree AV block, Mobitz Type I (Wenckebach) – because: i) Mobitz I is far more common than Mobitz II – especially in the setting of acute inferior MI when there is also 1st degree AV block for conducting beats; and  ii) the QRS complex is narrow (as it almost always is with Mobitz I – vs QRS widening that is usually seen with Mobitz II).
• PEARL: It would be easy to overlook 2:1 AV block in this tracing. Awareness that various forms of AV Wenckebach commonly occur with acute inferior MI should increase your index of suspicion that this conduction disturbance may be “in hiding” whenever ST segment elevation is marked – and you either see: i) a pattern of grouped beating; or ii) a longer-than-expected PR interval preceding those beats that are conducting (as seen here). Use of calipers may prove invaluable for confirming your suspicion.

BOTTOM LINE: Acute reperfusion is urgently needed for this patient.

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NOTE: - Relevant PDFs on ECG features in this case

• Link to Section 10.0 – on Acute MI (from our ECG-2014-ePub).
• Link to Section 02.67  – on AV Block (from our ECG-2014-ePub).

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