Respiratory Volume-Pressure Relationships During Anaesthesia

Abstract

RESPIRATORY VOLUME-PRESSURE RELATIONSHIPS (AV/AP) are quantified in terms of compliance. The effect of general anaesthesia on the compliance of the total respiratory system (lungs plus chest wall) remains in dispute. Most workers have reported a reduction in compliance during anaesthesia, i-s However, others have reported compliance to be increased, 9 or unchanged. 1°,11 One obstacle to the interpretation of these previous studies is that methods used to obtain control preanaesthetic measurements were different from those used in the anaesthetized subject. For example, measurements obtained by attempts at voluntary relaxation of respiratory muscles in conscious subjects have been compared with values obtained by passive inflation of the respiratory system in anaesthetized individuals. In this study, we have measured volume-pressure relationships for the total respiratory system in the conscious subject and again under several conditions following induction of general anaesthesia using identical methods under all circumstances. In addition, a new method for representation of respiratory volume-pressure data in logarithmic form is used to facilitate presentation, analysis and interpretation of results. METHODSRespiratory compliance was measured using the weighted spirometer technique of Cherniak and Brown} 2 The subjects of the study were 16 consenting adults scheduled for elective operations. All were free of systemic disease, did not smoke and ranged in age from 19 to 55 years. Seven were anaesthetized with halothane with oxygen and nine with neurolept-analgesia~ with oxygen.The experimental apparatus is represented in the diagram in Figure 1. All subjects were studied in the supine position and breathed either from a tightly fitting face mask or through a cuffed tracheal tube. Pressure within the airway relative to atmospheric was measured from the side-arm of the mask adapter using a Sanborn 270 transducer calibrated against a water manometer and recorded on a Sanborn 954 recorder. Respiratory volume changes were recorded on the spirometer kymograph (Collins, 9L). All volume changes were corrected for gas compression and water displacement in the spirometer circuit, and were converted to B.T.P.S.In the halothane group, the subjects were studied in the conscious state and