Your patient is a 32 year old female who was thrown from a horse and struck her head on the ground. She was not wearing a helmet. She does not respond verbally or open her eyes to painful stimuli, but responds to painful stimuli with decerebrate posturing. Her right pupil is dilated and does not react to light. Her respirations are 6 per minute and irregular, pulse 56, blood pressure 188/98. You should ventilate the patient at a rate of?