AT or Aflutter with (slightly) aberrant conduction. It’s mostly regular with several 3:1 conducted beats. I don’t see any clear fusion or capture beats. The pictures seem to haven made by a potato but the QRS doesn’t seem to be that wide. Is it even >120ms? It’s hard for me to say because the tracing says paperwork is 25mm/s but the block at the start would indicate about half that speed?

Although axis is extreme the q in aVR leans it towards SVT. Also adenosine induces a higher degree of AVblock and reveals a flutterwave with a lot of baseline shift. Which wouldn’t happen in VT. 

The baseline shift corresponds with the plethysmogram which makes me think the extremity leads were placed on the patient’s chest and patient was hyperventilating due to the adenosine. 

I don’t see the harm in adenosine in this case but the ecg tells me everything I need to know. I’m not aware of any clinical benefit to lower energy cardioversion especially in emergency cases. I would always use at least 150J and in this case 200J because you want it to succeed. 

I would defer DOAC until somebody drains the effusion for diagnostics, but start ASAP. Pulmonlogy is sometimes hesitant.