One relatively less known fact on NRM (Non-Rebreather Mask)

Non-rebreather masks are designed to capture the first 150ml of the exhaled breath into the reservoir bag for inhalation during the subsequent breath. 

 Clinical significance: This portion of the breath was initially delivered at the end of inhalation and was therefore delivered to the "deadspace" anatomy where gas exchange does not occur. Therefore, there would be no depletion of oxygen nor gain of carbon dioxide during the rebreathing component.