Expired minute volume measurements during anaesthesia using non-rebreathing valves.

Non-rebreathing valves may cause leakage of anaesthetic gases from the inspiratory to the expiratory limb, leading to errors in measurement of exhaled gas volume and gas composition. The fresh gas flow (FGF 1 X min -1) leakage (gas-shunt) over a non-rebreathing valve (AMBU Paedi Anaesthesia System) was experimentally investigated in a conscious volunteer. The gas-shunt depended upon the tension within the breathing balloon, which is placed in the inspiratory limb. At low FGF's, i.e. with the breathing balloon close to collapse, no gas-shunting was registered. At higher FGF's, a large gas-shunt was found. Due to the negligible gas-shunt found at low FGF settings, this AMBU system might be used for analyses and measurements of exhaled gases during anaesthesia and spontaneous ventilation.