FRESH GAS-FLOW AND CARBON-DIOXIDE REBREATHING IN A LOW-PRESSURE SEMIOPEN ANESTHESIA SYSTEM

We have constructed a simple system for field anaesthesia by using a F arman entrainer and a semi-open circuit to convert a draw-over apparat us to a continuous flow air/O2 system. Compressed 02 was the driving g as for the entrainer, fresh gas (FG) delivered to the semi-open circui t was a mixture of 02, entrained air and anaesthetic vapour. The purpo se of this study was to examine FG flow rate and CO2 rebreathing durin g intermittent positive pressure ventilation (IPPV). A nonrebreathing inflation valve (Laerdal) placed at the end of the expiratory (efferen t) limb of the circuit vented both expiratory gas and excess FG. Ambie nt air IPPV was applied through the Laerdal valve from a self-inflatin g 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 wa s ambient air. Minute ventilation of the lungs with efferent limb gas was defined as V(eff). Respiratory gas was sampled at the endotracheal tube and the CO2 tension was measured with a NIHON-KOHDEN CO2 analyze r 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 . min-1), corresponding to driving gas pressu res of 40, 60, 80, and 100 mmHg, were introduced in random order. Alth ough inspired CO2 was detected at FG flow rates of 5.7-93 L - min - 1, there were no differences in PETCO2 among the four groups We conclude that clinically important rebreathing does not occur when V(FG) is 1. 67 times V(eff). For general use we recommend a V(FG) of more than twi ce the V(eff), which for this circuit configuration would be about 8 L . min-1. This requires a minimum 02 flow of 1.6 L . min - 1 or drivin g gas pressure of 60 mmHg.