Talk to Cardiologist on Black Out

assalam o alikum I have a friend he is 28yrs old, complaining that he became blank during namaz (namaz in which he has to keep quiet behind the muezzin or in which voice came through the speaker). During this state he also experienced black out, trembling of feet n tachycardia. What could be the reason behind this ?? n what more qs do I have to ask him to reach any diagnosis?? He is a known hypertensive n vsd patient ( his vsd is diagnosed 7 months back )