Re-evaluation of the Farman entrainer in a low-pressure system for field anaesthesia

The aim of this project was to develop a portable anaesthesia system that was compatible with modern anaesthesia practice under field conditions, when compressed gas supplies are limited. We assembled and evaluated a low-pressure plenum system, based upon the Farman entrainer, which was adaptable to spontaneous, assisted or intermittent positive pressure ventilation (IPPV). The entrainer was tested using a low flow of compressed gas, O2 at 1-3 L.min-1. We measured the fresh gas flow (FGF) and O2 concentrations (F1O2) delivered at various source gas flow rates (O2 flow), and with various breathing circuits. Entrainment ratio, FGF, and F1O2 were highly dependent upon resistance to flow in the different breathing circuits. With a wide bore T-piece the air/O2 entrainment ratio was 6:1, and the F1O2 was 0.3. When circuit resistance was higher, e.g., with the Bain circuit, air entrainment and FGF were reduced, but F1O2 was higher. Because it offered the lowest resistance, the T-piece circuit was selected for a clinical trial.