The increase of dead space appears to be linked to the increase of the positive pressure within the alveoli (9). In the supine position under general anaesthesia, tidal volume is directed primarily towards the lung regions not dependent, incomplete alveolar gas mixing with areas of increased alveolar ventilation/perfusion (V̇/Q) mismatching within terminal respiratory units and the preferential spread of distribution of ventilation to areas of less perfusion, increases alveolar dead space (VDalv) (8).

Dead space is the portion of ventilation that is not participating in gas exchange, because it does not come in contact with the pulmonary capillary blood flow (5-7). Gas exchange is impaired by the development of atelectasis, resulting shunts, ventilation/perfusion mismatching (1) and lung overdistension can be characterized by an increased physiologic dead space (2-4). Respiratory function changes during general anaesthesia with intermittent positive pressure ventilation (IPPV) in the supine position.

Anatomic and physiologic dead space