64-year-old man with narrow and broad tachycardia

This case was sent to me by a colleague, and I think it is worth discussing it (basically electrocardiographic analysis, since the clinical data are poor). This is a 64-year-old man that was admitted in another service with “tachycardia” and pressure drop. He was referred for a tertiary care hospital, where he was admitted with the ECG1. Initially he did not respond to vagal maneuvers. As there was no adenosine, EV verapamil was administered + new massage of carotid sinus. He reverted to sinus rhythm for a few seconds, going immediately into the rhythm in ECG2. After some minutes, the patient presented symptoms of low output associated to ECG3. This time, electrical cardioversion was applied, and recorded in ECG4.
Note: Regrettably, there are no other clinical data.

Question

What is the diagnosis and management?