It is also the stress of not knowing if you are doing it right, and a sense of panic. I had a company CPR class several years before.
My friend just exhailed and slid out of his chair mid sentance. It was surreal. I did get him back but didn’t know that when they hauled him out.
The guidelines keep changing too. Breaths to compressions.
One of my instructors told me, "You are working on a dead person, you can't make it worse." This is true but it doesn't reduce the stress.
Your friend owes you a beer. My son gave me a grandchild!