Control of end-tidal carbon dioxide during IPPV. Variable functional apparatus deadspace.

We have described a method for control of end-tidal carbon dioxide tension during intermittent positive pressure ventilation in a model lung and in 19 adults during general anaesthesia supplemented by central or peripheral neural blockade. The inspiratory and expiratory limbs of an open or a circle anaesthesia system were interconnected and ventilated simultaneously in a variable manner during inspiration. The flow of mixed-expired gas, normally one-way, became to-and-fro (variable functional apparatus deadspace, or "virtual' deadspace). At minute volume ventilation > or = 100 ml.kg-1.min-1 (patients), the value of end-tidal carbon dioxide tension was varied reproducibly within the range 4.1-6.5 (SD 0.1)kPa independently of fresh gas flow or other prescribed patterns of ventilation. At a steady state, stable nominated values of end-tidal carbon dioxide tension within the range were attained. By how much any given intra-operative value of end-tidal carbon dioxide tension may be said to affect peri-operative outcome is debatable, but during surgery any change in a nominated value may usefully indicate a change in the steady state.