An 84 year old man is admitted to the internal medicine ward with COPD exacerbation. An ECG is done for new worsening of shortness of breath that is shown below: 

• Supraventricular Tachycardia with Left Bundle Branch Block (LBBB), ventricular rate 134/min  

Comment: Although p-waves are difficult to see, the QRS morphology is consistent with typical LBBB morphology, suggestive of a supraventricular origin. 

On the next day, the following ECG is recorded: 

• Sinus rhythm, 86/min 
• Left Bundle Branch Block 

Comment: The presence of LBBB during sinus rhythm confirms the diagnosis of SVT with aberrancy on the preceding ECG.