POSITIVE PRESSURE VENTILATION USING A SEMICLOSED SYSTEM: A REASSESSMENT BY

The use of a semidosed system (the Magill attachment) for positive pressure breathing was investigated. Various patterns of ventilation were used and their effects assessed by serial measurement of the arterial carbon dioxide. It was found that this could be kept within limits comparable to those found during normal sleep using a flow of fresh gases as low as 5 l./min. This result was most easily achieved if the tidal volume were large (more than 1 1. for the average adult). Other incidental findings are discussed and it is conduded that semidosed systems provide a safe and economical method for maintaining ventilation when special equipment is not available. The use of semidosed anaesthetic systems for intermittent positive pressure ventilation (LPPV) was investigated by Waters and Mapleson (1961) who concluded that the Magill attachment was the least efficient of the systems they examined. Sykes (1959) had also investigated the Magill attachment for IPPV and had concluded that impracticably high flows of fresh gases were re-quired and also very large minute volumes if carbon dioxide retention were to be prevented. Parkhouse (1963) and Hunter (1963) condemned its use for IPPV for these reasons. These views seemed at variance with those of many anaes-thetists who believe that they regularly and easily produce hypocapnic apnoea performing IPPV with this apparatus. It was decided to reassess the use of the Magill attachment for LPPV. METHOD The subjects were twenty patients undergoing carotid angiography during the investigation of neurological disease. None had cardiovascular or pulmonary disease that was likdy to interfere with ventilatory function. All were given general anaes-thetics and intubated. All but three were venti-lated by machine except for the experimental period. While waiting for the films to be devdoped and while the arterial needle was kept patent, ventila-tion was maintained manually and serial blood samples withdrawn for gas analysis. Minute and expired tidal volumes (Vm and V,) were measure