Why can a person in a persistent vegetative state breathe without a ventilator?

A person in a persistent vegetative state can breathe without a ventilator because the brainstem remains functional.12

While a persistent vegetative state involves severe brain damage that results in a long-term loss of consciousness, it is distinct from brain death. The brainstem is the portion of the brain responsible for regulating vital, involuntary functions, including breathing, heart rate, and blood pressure.321

Specifically, networks of neurons located in the pons and medulla of the hindbrain generate the automatic, rhythmic signals necessary to drive the respiratory muscles. As long as these lower brainstem structures are intact and functioning, the body can maintain spontaneous respiration without mechanical assistance, even if the higher centers of the brain—which govern cognitive function and conscious awareness—are severely impaired.42