A 52-year-old woman on ICU is being ventilated with pressure support ventilation (PSV 12, PEEP 8, FiO2 0.35). She is awake, comfortable, and triggering the ventilator. The team notes that her minute ventilation is 12 L/min (high for her size). She is not anxious or in pain. What metabolic cause of hyperventilation should be excluded?