OXYGEN AND MEDICAL GAS THERAPY

Oxygen Delivery Systems

Supplemental oxygen can be delivered by a variety of different systems that can be divided into variable-performance and fixed-performance systems. For variable-performance systems the inspired gas is a mixture of the delivered oxygen diluted with a variable amount of air. The amount of air depends on the patient's tidal volume and respiratory rate, thus the FIO₂ can only be estimated. In fixed-performance systems the FIO₂ is reliably known. Oxygen delivery systems can be further divided into low-flow, reservoir, and high-flow systems.^[5]

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Low-Flow Systems

Low-flow systems deliver oxygen at flows that are below the patient's inspired flow. The remaining inspired flow comes from atmospheric air, diluting the delivered oxygen. Thus these are variable-performance systems. The most commonly used low-flow system is the nasal cannula, which is a small-bore oxygen supply tube that is connected to two short prongs that are inserted into the nares of the patient. The delivered FIO₂ at different flows has been estimated ( Table 75-2 ), but the actual FIO₂ depends on the patient's minute ventilation. Flows above 6 to 8 L/minute can lead to patient discomfort, including nasal drying and bleeding.^[6] The Vapotherm (Vapotherm, Inc., Annapolis, MD) delivers heated and well-humidified oxygen via a nasal cannula with a slightly larger bore, which allows patients to tolerate flows as high as 60 L/minute. Currently there are no published data estimating the FIO₂ at various levels of high oxygen flow via the Vapotherm system.

Reservoir Systems

The oronasopharynx acts as a natural respiratory reservoir. The patient's exhaled gas remains within this space and is re-inspired during the next inhalation, thus contributing to the anatomic dead space. Delivered supplemental oxygen fills this anatomic reservoir between breaths, slightly increasing the FIO₂ with each breath beyond the set flow rate. Reservoir systems extend the anatomic reservoir, thus further increasing the FIO₂ .

The simple facemask ( Fig. 75-2A ) is a basic reservoir system. Oxygen is delivered to the facemask at a determined flow rate. Between breaths, oxygen fills the space within the mask (the reservoir). The next inspired breath has components from the oxygen source, oxygen from the reservoir, and air leaking in around the mask. Since the amount of entrained air is unknown, the simple facemask is a variable-performance system ( Table 75-2 ). The flow rate must exceed 5 L/minute to replace exhaled gas with fresh oxygen. Otherwise, the mask would add to the patient's dead space and lead to rebreathing of CO₂ . ^[7]

A partial rebreathing mask ( Fig. 75-2B ) extends the reservoir further by incorporating a reservoir bag. The initial exhaled gas enters the reservoir bag, and the remainder of the exhalation is eliminated out of the mask. This initial exhaled gas comes from the anatomic dead space and thus contains high O₂ and low CO₂ . The next inspired breath comes primarily from the reservoir bag, containing a mixture of supplied oxygen and the oxygen-rich exhaled gas (which is rebreathed). Partial rebreathing masks are variable-performance devices and can deliver a higher FIO₂ than simple facemasks (see Table 75-2 ).

Nonrebreather masks ( Fig. 75-2C ) incorporate a series of one-way valves to prevent any exhaled gas from entering the reservoir bag. The reservoir bag is fully supplied by the delivered oxygen and contains no CO₂ . If there were no leak around the mask, the entire inhaled breath would come from the reservoir bag, making this a fixed-performance oxygen delivery system. However, the standard nonrebreather typically has some air leakage; furthermore, the one-way valve from one of the two exhalation ports is usually removed to allow emergency air intake in case the oxygen supply fails. Thus, the standard nonrebreather mask is typically a variable-performance system, but it can deliver a high fraction of oxygen.

High-Flow Systems

High-flow systems deliver oxygen at flows that exceed the patient's minute ventilation by three to four times. The "Venturi mask" is an air entrainment mask that uses a high-flow jet of oxygen. The high oxygen flow produces a shearing force distal to the jet orifice that entrains air into the mask in a specific ratio. The size of the jet orifice can be adjusted, yielding a predictable entrainment of air, and thus the FIO₂ can be determined (fixed-performance). The Venturi mask can reliably deliver FIO₂ values of 0.24 to 0.4. The largest jet orifice provides the lowest oxygen velocity and thus the least air entrainment and highest FIO₂ (0.40).

Air entrainment nebulizers are based on the same principle and can be used to deliver humidified/heated air at a higher FIO₂ . This system is often used to deliver oxygen to patients with an endotracheal tube through a T-connection in which one large-bore tube is connected to the air entrainment nebulizer and the other limb serves as a reservoir tube. As long as flow from the air entrainment nebulizer is adequate to allow flow from the oxygen source through the reservoir tube throughout the respiratory cycle the patient does not inspire room air. A similar setup can be used with patients breathing through a facemask with large reservoir tubes ("whiskers") attached to the exhalation ports ( Fig. 75-2D ). High-flow fixed-performance blending systems mix high-pressure oxygen and air to the desired fraction of delivered oxygen. This mixture is delivered at high flows (>60 L/minute) ensuring that the FIO₂ is essentially equivalent to the delivered fraction of oxygen.