This 78-year-old woman with a longstanding history of poorly controlled hypertension, type 2 diabetes, and left ventricular dysfunction, presents to the endocrinology clinic for follow-up, and a routine ECG is obtained:

•  Sinus tachycardia, 105/min with occasional premature atrial complexes
•  Intermittent left bundle branch block  (2:1)
•  T wave abnormality, consider anterior ischemia  

The PR interval is constant (except for the PACs, that have a slightly longer PR).  This is an important finding to distinguish aberrantly conducted sinus complexes from late premature ventricular complexes, which usually have a shorter PR.

A second ECG was obtained 90 minutes later. The patient remained symptom-free:

• Sinus tachycardia, 110/min
• Left bundle branch block