There are many possibilities - my mom fainted and it turned out to be an electrical problem in her heart that required surgery. They couldn't identify the problem for a long time, and finally it was identified by a heart specialist. It can be blood sugar, blood pressure... and there are many, many possibilities. I wonder if this doctor did enough to investigate this? A blood test seems kind of like a start, but of course we don't know all of what the doc did with your dad. It might be wise to get a second opinion if your dad is willing. Hopefully it's nothing, but most folks don't faint when stressed, and so I wonder if there is a problem that has to be addressed. Maybe even one caused by stress.