Oxygen administration can be delivered via low-flow or high-flow systems, with humidity or not, and with a reservoir or not.
The standard nasal cannula delivers an FiO2 of 24-44% at supply flows ranging from 1-8 liters per minute (LPM). The formula is FiO2 = 20% + (4 × oxygen liter flow). The FiO2 is influenced by breath rate, tidal volume and pathophysiology. The slower the inspiratory flow, the higher the FiO2; the faster the inspiratory flow, the lower the FiO2. Since the delivered oxygen percentage is very inconsistent during respiratory distress, a nasal cannula is not recommended for acute severe hypoxemia or patients that breathe on a hypoxic drive where too high of an oxygen concretion may lead to respiratory depression. A nasal cannula utilizes no external reservoir of oxygen and relies on the patient’s upper airway as an oxygen reservoir. A humidification device is recommended for flows greater than 4 LPM to insure humidification of the dry inspired gas. Even with humidity, added flows 6-8 LPM can cause nasal dryness and bleeding. The best clinical indications for the nasal cannula are for patients who have a relatively stable respiratory pattern, who require low oxygen percentage, or who need supplemental oxygen during an operative or diagnostic procedure, or for chronic home care.
The volume of the facemask is approximately 100-300 cm3 depending on size. It can deliver an FiO2 of 40-60% at 5-10 liters.8 The FiO2 is influenced by breath rate, tidal volume and pathology. A flow rate of greater than 5 LPM must be set to ensure the washout of exhaled gas and carbon dioxide retention.
The non-rebreathing facemask is indicated when an FiO2 >40% is desired and for acute desaturation. It may deliver an FiO2 up to 90% at flow settings greater than 10 liters. Oxygen flows into the reservoir at 8-15 liters, washing the patient with a high concentration of oxygen. Its major drawback is that the mask must be tightly sealed on the face, which is uncomfortable and drying. There is also a risk of CO2 retention if the mask reservoir bag is allowed to collapse on inspiration. Humidification is difficult with this device, because of the high-flow required and the possibility of the humidifier popping off. This device is best utilized in acute cardiopulmonary emergencies where high FiO2 is necessary. Its duration should be less than four hours, secondary to inadequate humidity delivery and to variable FiO2 for patients who require a precise high oxygen percentage.
A Venturi mask mixes oxygen with room air, creating high-flow enriched oxygen of a desired concentration. It provides an accurate and constant FiO2 despite varied respiratory rates and tidal volumes. FiO2 delivery settings are typically set at 24, 28, 31, 35 and 40% oxygen. The Venturi mask is often employed when the clinician has a concern about CO2 retention or when respiratory drive is inconsistent. The addition of humidification is not necessary with this device, secondary to the large amount of ambient entrainment that occurs to ensure the exact FiO2 is delivered.10 The Venturi mask is often utilized in the COPD patient population where the risk of knocking out the patient’s hypoxic drive is of concern.
When oxygen delivery is anticipated to be prolonged, it should be humidified, whenever possible, to prevent dried secretions from obstructing smaller airways.
