Exactly. Electrical alternans. Seen easier in some leads compared to others. 

The patient had h/o lung cancer and received chemotherapy 3 years ago. His echo showed massive effusion and tamponade. 

Presenting Echo:

during tab (bubble study):

after tab and drain:

er EKG after 400 cc of fluid removed is given below (and at the bottom I put the presenting EKG to compare them): 

Post Tab

Presenting EKG (alternans):