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>The demand valve has been promoted as an easy-to-use system that conserves the oxygen supply since it releases oxygen only when the user inhales. Recent research, however, found that the nonrebreather mask set at 15 liters per minute can provide better tissue oxygenation than the demand valve with an oronasal mask.1 Researchers suspected that poor mask fit led to the underperformance of the demand valve. This article will summarize an evaluation of different mask configurations to determine if they influence the oxygen delivery fraction when breathing with the demand valve.2
>The dead space volume, representing the volume of gas that will be breathed (or rebreathed) from a sealed mask, was measured prior to testing. Oxygenation was measured in three different ways: the fraction delivered to the mouth (inspired oxygen fraction (FiO2), the fraction reaching the back of the throat (nasopharynx) and tissue oxygen levels measured through skin transcutaneous oximetry. Sensors were taped on six standard sites on the arms and legs, the skin was warmed to ensure good blood flow, and the levels of tissue oxygen were recorded at regular intervals throughout the oxygen breathing periods. After the oxygen breathing, the participants rated each mask configuration on comfort, ease of breathing and ease of holding the device in place.
>Participants rated the pocket face mask as the most comfortable and the nose clip as very uncomfortable. They described the intraoral mask (like a scuba regulator mouthpiece) as the easiest to use since it rests in the mouth when the subjects are lying on their backs. Constant pressure was required to maintain a seal with the other two masks. The intraoral mask had a very small dead space volume (14 milliliters), meaning little gas would be rebreathed. The dead space was eight times larger for the pocket face mask and 14 times larger for the silicone oronasal mask.
>Divers are accustomed to breathing through a mouthpiece, so the intraoral mask is easy to use, and facial hair and face shape do not interfere with the fit. It is easy to hold in place and continued to maintain a good seal under a wide range of conditions.
>The large dead space volume of the oronasal mask has both positive and negative effects. While the large volume can serve as a reservoir for the oxygen supply, it also means that there is more rebreathing of expired gas, diluting the fresh oxygen supply with expired carbon dioxide. Fidgeting with the mask can introduce more surrounding air that can dilute the inspired oxygen fraction. There is much less expired gas rebreathing or ambient air dilution with the intraoral mask.
>The intraoral and pocket face masks come with a filter and one-way valve that allows the masks to be used for mouth-to-mouth resuscitation in a nonbreathing diver. The filter and one-way valve assembly need to be removed before attaching these masks to the demand valve.
>References
>1. Blake DF, Naidoo P, Brown LH, Young D, Lippmann J. A comparison of the tissue oxygenation achieved using different oxygen delivery devices and flow rates. Diving Hyperb. Med. 2015; 45(2):79-83.
>2. Blake DF, Crowe M, Lindsay D, Brouff A, Mitchell SJ, Pollock NW. Comparison of tissue oxygenation achieved breathing oxygen from a demand valve with four different mask configurations. Diving Hyperb. Med. 2018; 48(4):209-17.
>© Alert Diver — Q4 Fall 2019