• H R Wexler, A Aberman, A A Scott, and J D Cooper.
• Can Anaesth Soc J. 1975 Jul 1; 22 (4): 417-31.

AbstractDirect measurement of the inspired tracheal oxygen concentration was made in patients breathing through standard aerosol face masks. Factors affecting the tracheal FIO2 were analyzed using both mechanical and mathematical models. When oxygen is delivered to the face mask at low flow rates, there is considerable patient variation in the measured tracheal FIO2. Delivery of oxygen at higher flow rates (15 litres per minute or greater), reduces such variation. Furthermore, turbulent air currents within and around the face mask reduce the measured FIO2 and contribute to fluctuations in the FIO2. This effect may be virtually eliminated by placing shields around the mask orifices as described. Using the shielded mask, it is possible to deliver the desired oxygen concentration to the patient more accurately and to maintain humidification of the delivered gases. Changes desired in inspired oxygen concentration are accomplished by changing the concentration of the incoming gas mixture, and not by merely changing the flow rate of oxygen delivered to the system. Using the shielded mask, it is possible to deliver an inspired oxygen concentration of 100 per cent. This is not true with most other commonly used face masks, and, therefore, caution should be used to avoid administration of unnecessary high inspired oxygen concentrations with this type of mask.

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