Vegetable. That is how one ends up with it. Very bad news.
Oxygen. Oxygen is one of the fuels we need for normal cell and organ function. Lack of oxygen will cause damage to cells and metabolic functions which is called ischemia. If brief, it can be reversed, but prolonged ischemia will lead to cellular death.
Hypoxia is the cause. Ischemia means lack of oxygen perfusion to a target organ -- and hypoxia means reduced oxygen saturation and can be the cause of ischemia.