A 33 year old Afro-Caribbean male attends your cardiology clinic with shortness of breath. He begins to tell you that sometimes when he feels short of breath he can also feel his heart beat. He told you that yesterday when he walked up the stairs he also felt very dizzy and had to sit down. His observations are follows; 120/80 BP, HR 165, RR 12, temp 36.8, O2 99%. You note on his ECG that the left atrium is not depolarizing regularly, everything else is normal. What is your most likely diagnosis?