Breathing mechanics, dead space and gas exchange in the extremely obese, breathing spontaneously and during anaesthesia with intermittent positive pressure ventilation

Abstract

Breathing mechanics and gas exchange were studied in 10 extremely obese subjects (average weight 138 kg) prior to and during anaesthesia with mechanical ventilation. Breathing mechanics were analysed from measurements of transpulmonary pressure (during anaesthesia, trans-chest wall pressure as well) inspiratory gas flow and tidal volume. Gas exchange was studied by analysing inspired and from the Bohr equation, and the division into anatomical and alveolar dead space was arrived at by capnography. The patients were anaesthetised with neuroltpt agents and ventilated with an air-oxygen mixture. Lung compliance during spontaneous breathing was below normal and decreased further during artificial ventilation. Chest wall compliance measured during anaesthesia was within normal limits. Lung resistance was above normal during spontaneous breathing and increased further during mechanical ventilation. Total dead space was normal during spontaneous breathing and increased moderately during artificial ventilation, the increment coming mainly from alveolar dead space. A moderate hypoxaemia was recorded during spontaneous breathing, and the alveolar-arterial oxygen tension difference was slightly elevated. During anaesthesia this difference was markedly greater. It is concluded that the most probable reason for the relative hypoxaemia is right-to-left shunting.