Canadian Journal of Anesthesia, Vol 22, 417-431, Copyright © 1975 by Canadian Anesthesiologists' Society

Measurement of Intratracheal Oxygen Concentrations During Face Mask Administration of Oxygen: A Modification for Improved Control

¹ Respiratory Care Unit of the Toronto General Hospital, the Department of Medicine, Mount Sinai Hospital, and the Departments of Surgery, Medicine, and Anaeshesia, of the University of Toronto

Direct measurement of the inspired tracheal oxygen concentration was made in patients breathing through standard aerosol face masks. Factors affecting the tracheal FI_OO2 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 FI_OO2. 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 FI_OO2 and contribute to fluctuations in the FI_OO2. 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.