Providing supplemental oxygen
If breathing room air, a patient’s FiO2 will be 0.21. FiO2 is the fraction of inspired oxygen (0.21 at sea level as well as on Mount Everest) – frequently confused with the percentage of oxygen (21%) – frequently muddled with the partial pres-sure of oxygen (about 150 mmHg at sea level and much less on Mount Everest). We have several devices to increase the spontaneously breathing patient’s FiO2 (Fig. 10.5):
· Nasal cannula increases FiO2 about 1–2% per liter (thus 2L delivers about 28% oxygen). Flow rates above 5 L irritate the nose without further increasing the FiO2.
· A loosely fitting oxygen mask with an oxygen flow rate of 6–8 L/min may bring the inspired oxygen percentage to 60−80%.
· A non-rebreathing face mask can deliver ∼95% oxygen (the bag should be inflated . . . unlike what the flight attendants tell us).
For each device, we need an oxygen cylinder,10 a reducing valve to bring the high pressure of a full cylinder to a manageable 40 psi (12 atm), and a flow meter that lets us select a flow rate anywhere from about 100 mL/min to 10 000 mL/min. The actual inspired concentration of oxygen will depend on the flow rate of oxygen, as well as the patient’s peak inspiratory flow rate.