An Analysis of the ObstructiveDefect

and would not affect its patency. In contrast, in the right lateral position, gravity caused the mass to protrude further into the airway of the left main-stem bronchus and thereby decrease ventilation to the left lower lobe. Conceivably, this led to hypoxemia based on low venUlation-perfusion matching in the left lower lobe. Because the patient’s symptoms developed immediately upon turning onto his right side, we believe that the mechanism of the tachypnea and dyspnea was mechanical rather than related to desaturation. In patients with unilateral lung disease, such as atelectasis or pneumonia, involving either the right or left lung, Remolina and colleagues4 noted that lower arterial oxygen tension occurred when the involved “sick” lung was dependent as opposed to the lateral position with the “good” lung dependent. Three ofthe nine patients had primary bronchogenic or metastatic carcinoma involving the lung. The authors postulated that these changes were due to ventilationperfusion mismatching associated with the gravitational influences on pulmonary blood flow.