Ventilation‐Perfusion Distribution During Inhalation Anaesthesia

Abstract

Ventilation-perfusion (.ovrhdot.VA/.ovrhdot.Q) ratios were studied by means of an inert gas elimination technique in healthy [human] subjects with an average age of 51 yr in the supine posture when awake, during inhalational anesthesia, spontaneously breathing, during mechanical ventilation, and when a positive end-expiratory pressure (PEEP) was applied. In the awake subject a bimodal distribution of .ovrhdot.VA/.ovrhdot.Q was recovered in most patients, 1 mode centered around the ratio of 1 and another, smaller mode, within low .ovrhdot.VA/.ovrhdot.Q regions. Any shunt was < 3% of cardiac output. With anesthesia and spontaneous breathing, the low .ovrhdot.VA/.ovrhdot.Q mode was reduced and the shunt increased to an average of 6.2%. With mechanical ventilation, the major .ovrhdot.VA/.ovrhdot.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 .ovrhdot.VA/.ovrhdot.Q regions appeared. The shunt and low .ovrhdot.VA/.ovrhdot.Q regions may be explained in terms of airway closure while the high .ovrhdot.VA/.ovrhdot.Q mode with PEEP may be attributed to the development of a zone I. [Implications for studying the impairment of pulmonary gas exchange during anesthesia were discussed.].