A 60-year-old woman comes to the emergency department due to shortness of breath and productive cough. She has a 30-pack-year smoking history. Oxygen saturation is 88% on room air. On physical examination, she appears uncomfortable and uses her accessory respiratory muscles to breathe. Expiratory wheezes are heard throughout the lungs. The patient is started on high-flow oxygen supplementation. Shortly afterward, she becomes increasingly lethargic and confused. This patient's clinical decline is most likely attributable to an increase in which of the following parameters?
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