The importance of the expiratory pause

Abstract

SummaryA physical lung model simulating spontaneous respiration was used to investigate the influence of the respiratory pattern on the eficiency of the Mapleson A , C and D breathing systems. It is shown that the Mapleson A system is always the most efficient breathing system and that its performance is relatively independent of the respiratory pattern. When the expiratory pause is minimal, the Mapleson C systetn is almost as eficient as the Mapleson A , hut hecomes ever less eficient as the expiratory pause increases. The Mapleson D svstern is very ineficient when the expiratory pause is short. With a longer expiratory pause, this sys f em's eficiency approaches that qf the Mapleson A . The e.xperiniental results are compared with predictions generated by a niathetnutical niodel. There is good agreement between the two, oalidating the mathematics used. Key wordsEquipment; breathing systems, Mapleson ACD, lung model.A previous paper offered a theoretical analysis of the influence of the respiratory pattern on the efficiency of the Mapleson A, C and D breathing systems [I]. The analysis suggested that the principal determinant of the efficiency of the Mapleson C and D breathing systems is the length of the expiratory pause. A longer pause should render the C system less efficient and the D system more efficient.In this investigation, a physical lung model which simulates spontaneous respiration [2] was used to test these predictions. The effect of various changes in the respiratory pattern on the performance of the Mapleson A, C and D breathing systems was examined. For each case the respiratory pattern used was also analysed using the equations previously presented [I], allowing comparison between the predicted and actual results.As in the companion papers [I, 21, rebreathing is quantified as an increase in the ratio of deadspace ( V D ) to tidal volume ( VT), as described by Jaeger and Schultetus [3]. The physical deadspace within the model is designated VDpat and the total deadspace (including that added by the breathing system) is designated VDlOt. MethodThe model simulates spontaneous respiration and has been described previously [2]. It allows direct measurement of rebreathing in terms of VDtof/VT ratios for each breathing system. Briefly, the lung is represented by a bellows which is made to move via a mechanical link to a second, ventilated bellows. The deadspace is represented by corrugated tubing, a 9 mm ID tracheal tube (Portex) and a catheter mount. The inspired tidal volume is diverted through a gas splitter which allows the mean carbon dioxide concentration inspired into the 'alveoli' to be measured. The ratio of mean inspired to end-tidal (i.e. alveolar) carbon dioxide is the VDtOt/VT ratio. The respiratory pattern may be varied by altering the settings of the ventilator driving the model and/or the resistances in the ventilator circuit.The model was used to compare the Mapleson A, C and D breathing systems. In all experiments the respiratory rate was 10 breath.min-I, the tidal volume was kept con...