Breathing systems for use in anaesthesia. Evaluation using a physical lung model and classification
A lung model which measures effective alveolar ventilation is described and used to evaluate different breathing systems during simulated spontaneous ventilation. The model measures VD/VT ratios in relation to fresh gas flow and the characteristic shape of the resulting curve characterizes the particular breathing system. The various curves reflect the functional performance and, therefore, the flow requirements of the systems which have been reclassified accordingly. The classification described is based upon the structural arrangements which determine the flow patterns within the particular systems. The breathing systems compared during spontaneous ventilation were a circle 'C' absorber system, the circle 'C' without the absorber, the Mapleson A, B, C and D systems and a series of 'enclosed' and modified Mapleson A systems and the modified Mapleson D or Bain system.