The parallel Lack anaesthetic breathing system
The parallel Lack system is a new modification of the Mapleson A system comprising separate inspiratory and expiratory tubes. To determine that the function of the system was that anticipated of a Mapleson A, the fresh gas flow requirements to prevent rebreathing during spontaneous ventilation were assessed in three situations: (I) a lung model (2) conscious volunteers and (3) anaesthetised patients. Two sets of criteria to define rebreathing were used; (A) those based on changes in ventilation or end‐expired carbon dioxide tension and (B) minimum inspired carbon dioxide tension. Using A, rebreathing occurred at afresh gas flow to minute ventilation ratio (Vfjve) of 0.75 for the lung model, and 0.73 for conscious volunteers. These results were comparable to those obtained for a Magill attachment. They were also close to the point at which mechanical dead space began to increase in the lung model. Criteria B gave much lower values for the onset of rebreathing. Rebreathing was present by criteria A in five of the six anaesthetised patients at afresh gas flow of 60 ml.kg ‘.min’ (Vfjvf of 0.78). The results confirm that the parallel Lack behaves as a Mapleson A system. The resistance to breathing posed by the parallel Lack was also comparable to the Magill system. Copyright © 1993, Wiley Blackwell. All rights reserved