A 43-year-old woman with a history of Myasthenia Gravis presents to the ED with a chief complaint of generalized weakness. On exam, there are normal vitals except for a RR of 24, and you notice ptosis and dysphonia. You are concerned about impending respiratory failure. A respiratory therapist is called to the bedside to help with the decision of whether to intubate this patient. Which of these values can be helpful in guiding this decision?