The distribution of ventilation, diffusion, and pulmonary blood flow was studied in 5 normal subjects and 10 obese patients in terms of 3 lung compartments by the method of King and Briscoe.

Normal subjects and obese patients with normal arterial oxygen tension and normal or low arterial carbon dioxide tension had a relatively well ventilated and underperfused slow compartment. The ratio of alveolar ventilation to perfusion for the less well ventilated space was larger than that of the well ventilated compartment in all 5 normal subjects and in 3 of 4 obese patients with normal arterial oxygen tension. There was an uneven distribution of diffusion in relation to blood flow. The smaller flow compartment received an average of 1.5 per cent of the total diffusion. Obese patients with low arterial oxygen tension and normal carbon dioxide tension had normal alveolar ventilation, but abnormal distribution. Ventilation to the slow space was markedly decreased. The diffusion in relation to blood flow was more evenly distributed and did not contribute significantly to the low arterial oxygen tension. A large anatomic shunt and a low ratio of alveolar ventilation to perfusion in the less well ventilated space accounted for the hypoxemia. Obese patients with hypoxemia and chronic hypercapnia (Pickwickians) had low over-all alveolar ventilation with normal distribution. Blood flow to the slow space was significantly increased at the expense of the fast space. This shift in blood flow resulted in a relatively high ratio of alveolar ventilation to perfusion in the well ventilated space, and a very low ratio of alveolar ventilation to perfusion in the less well ventilated space. A large anatomic shunt was also present. These factors explained the hypoxemia and hypercapnia in terms of 3 lung compartments.