Ventilation-perfusion distribution during inhalation anaesthesia. Effects of spontaneous breathing, mechanical ventilation and positive end-expiratory pressure.

Ventilation-perfusion (VA/Q) ratios were studied by means of an inert gas elimination technique in healthy subjects with an average age of 51 years in the supine posture (a) when awake, (b) during inhalational anaesthesia, spontaneously breathing, (c) during mechanical ventilation, and (d) when a positive end-expiratory pressure (PEEP) was applied. In the awake subject a bimodal distribution of VA/Q was recovered in most patients, one mode centered around the ratio of 1 and another, smaller mode, within low VA/Q-regions. Any shunt was less than 3% of cardiac output. With anaesthesia and spontaneous breathing, the low VA/Q mode was reduced and the shunt increased to an average of 6.2%. With mechanical ventilation, the major VA/Q mode was widened while the shunt was further increased in 4 of 10 subjects (mean 8.6%). With PEEP, the shunt was reduced and a new mode within high VA/Q-regions appeared. The shunt and low VA/Q-regions appeared. The shunt and low VA/Q-regions may be explained in terms of airway closure while the high VA/Q mode with PEEP may be attributed to the development of a zone I.