Is it possible for a person to flatline for a couple seconds and watch it on a HR monitor?

My neighbor just told me how a bee stung him in the mouth and how he was rushed to the ER. He had 3 epi shots and the nurse told him to "relax" so the swelling in his mouth would go down. He said he felt himself relaxing and almost falling asleep when he looked over at the HR machine and saw it flatline. He said he freaked out and jumped up and pointed to the machine and his friend who was with him called the nurse. When she saw the printed out sheet it showed he flatlined for 3 seconds.

Is this possible? How does something like this happen?
Answer:
A few second's sinus arrest isn't that unusual. Also, when we use adenosine to treat tachyarrhythmias people routinely get to see that, along with experiencing some other minor side-effects like crushing substernal chest pain and a feeling of impending doom.

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