Fresh gas flow and carbon dioxide rebreathing in a low pressure semi-open anaesthesia system

Abstract

We have constructed a simple system for field anaesthesia by using a Farman entrainer and a semi-open circuit to convert a draw-over apparatus to a continuous flow air/O2 system. Compressed O2 was the driving gas for the entrainer; fresh gas (FG) delivered to the semi-open circuit was a mixture of O2, entrained air and anaesthetic vapour. The purpose of this study was to examine FG flow rate and CO2 rebreathing during intermittent positive pressure ventilation (IPPV). A nonrebreathing inflation valve (Laerdal) placed at the end of the expiratory (efferent) limb of the circuit vented both expiratory gas and excess FG. Ambient air IPPV was applied through the Laerdal valve from a self-inflating bag or ventilator. Since this circuit is functionally similar to a T-piece, the gas from the efferent limb (340 ml, containing FG) entered the lungs first. If tidal volume was larger than 340 ml the balance was ambient air. Minute ventilation of the lungs with efferent limb gas was defined as Veff. Respiratory gas was sampled at the endotracheal tube and the CO2 tension was measured with a NIHON-KOHDEN CO2 analyzer. Thirty-seven adult patients having intra-abdominal or pelvic surgery under general tracheal anaesthesia were studied. Four FG flow rates (5.7, 8.0, 9.3, and 10.4 L · mm-1), corresponding to driving gas pressures of 40, 60, 80, and 100 mmHg, were introduced in random order. Although inspired CO2 was detected at FG flow rates of 5.7-9.3 L · min-1, there were no differences in PetCO2 among the four groups. We conclude that clinically important rebreathing does not occur when VFG is 1.67 times Veff For general use we recommend a VFG of more than twice the Veff which for this circuit configuration would be about 8 L · min-1. This requires a minimum O2 flow of 1.6 L · min-1 or driving gas pressure of 60 mmHg. © 1993 Canadian Anaesthesiologists.