It's also amazing just how many males are found dead on the commode by first responsers. The cause of death in the cases I've seen has been attributed to heart attacks and I've never discussed the issue with our medical examiners, but now I'm starting to wonder about the physiology involved in a simple BM.
The vasovagal response is one thing that can happen as a result of it...that is likely to result in simply feeling terrible or fainting.
The other is that straining to pass stool can raise pressure (if you have a mirror in bathroom you can see if really cranking some abdominal and pelvic muscles makes the blood vessels in your neck pop out - fun in the bathroom!) and create more work for the heart - this is why heart attack and heart surgery patients are always put on stool softeners, to reduce the need to strain - hence the heart attack.