The arterial CO2 tension is the primary index of respiratory regulation available on pulmonary function reports. The anatomic sites of the cell clusters that establish a person’s tidal volume and respiratory rate are located in the brainstem. Inputs from both higher brain centers and peripheral (vagal) stretch receptors modify the respiratory pattern. The occlusion pressure, a pressure measurement representing central drive to breathe is made from an occluded airway and often used to guide decision-making and to establish prognosis in intubated intensive care unit patients. Use of this index requires familiarity with the basic concepts of respiratory regulation.