What Causes Irregular Heart Beats Along With Shortness Of Breath?

Husband has SVT s, PVC S & A-fib. Yesterday and today he s been having irregular heart beat - at times 125 +\-. Shortness of breath, sweating, low oxygen. He takes 2 Metoprolol daily, Eloquis & Lanoxin. He s been told he can take extra Metoprolol during episodes. None of this is new to us. He nearly passed out several times tonight. Hes just getting over a stomach virus and now he s constipated. His system is down right now. He said for me to give him nitroglycerin if his chest hurts bad or if he passes out. I m pretty sure he s having SVT s. Regular monitoring of his BP, oxygen & bpm. My question is - is it safe to use the nitroglycerin?