You have been called to a home for an 18-year-old male who informs you that he experienced a sudden onset of shortness of breath and back pain while watching television. He also informs you that he has a history of spontaneous pneumothorax, and the current symptoms he is experiencing are identical to those he felt with a previous pneumothorax. Assessment reveals the patient to be slightly dyspneic with breath sounds clear and intact bilaterally. During transport, what is most critical to continually monitor on this patient?