The correct interpretation of this ECG includes normal sinus rhythm with a right bundle branch block (RBBB) and a left anterior fascicular block (LAFB). The patient also has T-wave inversions in the inferior leads.
Normal sinus rhythm is evidenced by a ventricular rate ≥ 60 beats/min and < 100 beats/min, as well as a P wave for every QRS complex, a QRS complex for every P wave, and a consistent PR interval.
Criteria for a RBBB include a QRS > 120 ms with a biphasic RSR’ seen in leads V1–V3 and slurred S waves in leads I, aVL, V5, and V6. An LAFB is defined by left-axis deviation (–48° on this ECG), small Q waves with tall R waves in leads I and aVL, and small R waves and deep S waves in leads II, III, and aVF.
An RBBB plus an LAFB constitutes bifascicular block. In such cases, the ventricles are depolarized from one remaining fascicle: the left posterior. Affected patients are susceptible to complete heart block, particularly in the presence of congestive heart failure.
For those inexperienced with ECG interpretation, fascicular blocks may be confusing. Recall that evidence of Q waves in leads II, III, and aVF is associated with inferior myocardial infarction; by substituting the Q waves with S waves in the same leads, you won’t miss an LAFB.