OK. So there is this guy at the pool where I swim who is a retired Navy doctor. He used to take care of divers back in the 1950s. He says that death is caused in shallow water blackout because the low oxygen causes the heart to spasm and the person dies of heart failure. I told him , no, they usually just suffocate. But him being the doctor "expert" (50 years ago) won't listen to me. So what is the real mechanism of death during shallow water blackout - suffocation or heart failure, or something else?