A new hybrid anaesthetic circuit for a low-flow rebreathing technique

The coaxial Mapleson D (Bain) circuit is close to an ideal anaesthetic breathing circuit but it requires a high fresh gas flow (VF). The CO2 absorber circuit can be used with a low VF but the patient end is clumsy and hyperventilation is common unless a small tidal volume (VT) and a low respiratory frequency (f) is used. To overcome these problems a new hybrid anaesthetic circuit has been constructed. The new system has a slender coaxial patient end, connected to a Y-piece and a CO2 absorber circuit with unidirectional valves. The VF supply runs inside the coaxial tube to the patient end, similar to the Bain circuit. The new circuit was tested with simulated controlled ventilation in a lung model and compared to an absorber circuit, an absorber circuit with an added 300 ml dead space (VD) and a Bain circuit. The fraction of end tidal CO2 (FECO2) was measured as a function of VF. With VF 11 min-1, VT 10 ml.kg-1 and f 12 breaths min-1 a simulated 70 kg patient had an FECO2 of 4.8% and a 40 kg patient 5.5% with VF 21.min-1. At zero VF the new system was equal to an absorber circuit with 300 ml VD. With increasing VF the FECO2 in the new circuit decrease towards the absorber circuit values. The new system combines the benefits of the Bain circuit with those of a low flow CO2 absorber circuit and offers flexibility in the choice of combinations of VT, f and VF.