A low‐flow to‐and‐fro system

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Abstract
The insertion of a long deadspace tube between an anaesthetic breathing system and a ventilator produces only imperfect separation of the breathing system gas from the driving gas. This laboratory study has investigated different connecting tubes to establish the maximum tidal volume possible before ventilator gas contaminates the gas in a low-flow to-and-fro system (fresh gas flow 1 litre/minute). A larger volume tube enables the use of larger tidal volumes, and plastic corrugated tubes are slightly better than black rubber corrugated tubes in this respect. The maximum tidal volume possible without contamination decreases as ventilatory rate increases, but the maximum minute volume is increased. A 22 mm plastic corrugated tube of internal volume 1.5 litres should be adequate for clinical use with the to-and-fro system described in this study at a fresh gas flow of 1 litre/minute.