Going through your history and investigations viz.ECG and stress test,it is quite evident that you don t have any serious heart problem.PVC should be picked up on ECG.In absence of any such evidence don t worry about it.As such there is respiratory variation in heart rate,it increases in inspiration and decrease in expiration, so many a times if you are anxious and take deep breath ,you feel this variation and create undue worry.just relax and maintain healthy life style avoid smoking,tobacco ,coffee if you are overusing!