Figure 6-20 Comparison of PaCO₂ in anesthetized, spontaneously breathing patients relative to preoperative forced expiratory volume in 1 second (FEV_1.0 ). Patients with chronic obstructive pulmonary disease (COPD) did not exhibit carbon dioxide retention before anesthesia, but the degree of alveolar hypoventilation is much greater in the more severely obstructed patients. (Adapted from Pietak S, Weenig CS, Hickey RG, et al: Anesthetic effects on ventilation in patients with chronic obstructive pulmonary disease. Anesthesiology 42:160, 1975.)