If a patient experiences a rise in Paco2, one must determine whether there has been an increase in CO2 production or a decrease in alveolar ventilation. In response to questions, the resident explained that the patient was afebrile, had experienced no change in tube feedings, had no evidence of ischemic tissue, and had no evidence of thyrotoxicosis, all conditions that could lead to an increase in CO2 production. Total ventilation, or minute ventilation, is the sum of gas going to perfused alveoli (alveolar ventilation) and the gas going to airways and nonperfused alveoli (dead space ventilation). The resident explained that the patient was sedated and being supported with pressure control ventilation; the respiratory rate was set at 20 breaths/min, and the driving pressure was set at 25 cm H2O with a positive end-expiratory pressure at 5 cm H2O. The patient’s respiratory rate was the same as the rate set by the machine (ie, the patient was not “overbreathing the ventilator”). The resident explained that because the total ventilation had not changed, there must be an increase in the dead space ventilation to account for the rise in Paco2.