While patients in a persistent vegetative state typically maintain instinctive reflexes such as breathing, heart rate, and sleep cycle, it’s not unusual for them to have more variable patterns of heart rate and blood pressure compared to healthy adults. There is also a condition that can affect those in a persistent vegetative state called “dysautonomia” in which the nervous systems controlling heart rate and blood pressure are “off-balance” with each other. Pain can also cause an elevated heart rate, and possible sources of pain should be considered given your sister’s inability to communicate in other ways.