ECG Interpretation Review #49 (Chest Pain - STEMI - Culprit Artery - LAD - RBBB - LAHB - AFib - Nodal)

The 12-lead ECG and lead II rhythm strip shown in Figure 1 were obtained from 55-year-old man with “chest pressure”. He is hemodynamically stable.

• What is the rhythm?
• Can you identify the “culprit artery” ?

Figure 1 – 12-lead ECG from a 55-year-old man with chest pressure. 

INTERPRETATION: The first problematic aspect of the tracing in Figure 1 is to determine the rhythm. Given that the patient is hemodynamically stable – there is at least a moment to contemplate what is seen.

• The QRS complex looks slightly widened. Assessing QRS duration in leads where the complex appears to be widest (ie, leads V4,V5,V6) – we estimate QRS duration at 0.11 second (just over half a large box). That said – QRS morphology looks to be supraventricular (as we’ll discuss momentarily).
• The rhythm is not sinus. There is no upright P wave in lead II. In fact – there are no P waves anywhere … There are fine undulations of the baseline – although we can’t be sure if this represents baseline artifact, fine “fib waves”, or both.
• The rate is ~100/minute.
• The rhythm is not completely regular. This tracing provides an excellent example of how easy it is to be fooled into thinking a rhythm is regular when one does not measure (with calipers) consecutive complexes. Admittedly – variation in R-R interval from beat-to-beat is minimal – and there are a number of consecutive beats with the same R-R interval. But there is some variation.

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BOTTOM LINE: What is the Rhythm?

We are not certain what the rhythm in Figure 1 is based on this single tracing. Additional rhythm strips would be needed to know for sure. The differential diagnosis includes:

• Fascicular Tachycardia or VT (Ventricular Tachycardia).
• Accelerated AV Nodal (Junctional) Rhythm.
• AFib (Atrial Fibrillation).

We favor AFib with a controlled but regularized ventricular response as the most likely etiology of this rhythm – given supraventricular appearance, irregularity, and absence of P waves.

• We doubt the rhythm is VT or fascicular tachycardia – given its supraventricular appearance (See below).
• We can’t rule out the possibility of an accelerated junctional rhythm – given the “almost regularity” of the rhythm.
• We fully acknowledge that we are not certain of the rhythm.
• Given that this patient is at the moment hemodynamically stable – there are much more pressing problems than determination of the rhythm!
• Additional rhythm strips will almost certainly clarify rhythm diagnosis.

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What is Going On with the Rest of the Tracing? 
QRS morphology in Figure 1 suggests RBBB (Right Bundle Branch Block).

• The QRS complex is widened. Although a typical rSR’ pattern is not seen in lead V1 - the QR pattern that is seen serves as a “right bundle equivalent” in a patient who has lost the small initial positive r wave deflection due to septal infarction.
• S waves are seen in left-sided leads I and V6 (although admittedly the S in lead I is not nearly as wide as is usually the case with RBBB).

In addition – there is LAHB (Left Anterior HemiBlock), as determined by the rS complexes in the inferior leads producing a markedly leftward axis. The presence of RBBB/LAHB qualifies as “bifascicular block”.

     Of most concern – the ECG in Figure 1 suggests ongoing evolution of a large acute STEMI (ST Elevation Myocardial Infarction):

• Q waves are seen in septal leads V1 and V2.
• There is significant ST segment elevation in leads V1-thru-V4. There is also ST segment coving and elevation in lead aVL. ST elevation in lead V3 shows straightening of the ST upstroke and elevation of the J point by at least 4mm.
• There is also ST segment elevation in lead aVR.
• Reciprocal ST segment depression is clearly seen in each of the inferior leads (II,III,aVF).
• There appear to be hyperacute T waves in the inferior and lateral precordial leads.

IMPRESSION: The ECG shown in Figure 1 suggests acute proximal occlusion of the LAD (Left Anterior Descending) coronary artery causing an extensive anterior STEMI. Acute reperfusion is urgently needed for this patient.

• ST elevation in leads V1-thru-V4 suggests antero-septal involvement.
• ST elevation also in lead aVL suggests involvement of the 1st Diagonal Branch of the LAD.
• Septal necrosis with extensive damage is suggested by: i) RBBB (especially in view of the loss of the initial positive r wave deflection in lead V1 with this RBBB); ii) Q waves in leads V1,V2; iii) ST elevation in lead aVR; iv) presumably new bifascicular block (RBBB/LAHB); v) AFib. 
• This patient is at high risk of developing complete heart block and/or cardiogenic shock.

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ACKNOWLEDGMENT: My appreciation goes to Dr. Harsha Nagarajarao (of Cardiology Boards) for allowing me to use this tracing.

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NOTE: - Relevant PDF on ECG diagnosis of acute MI:

• Link to Section 10.0 from our ECG-2014-ePub.

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