Fixed-Performance (High-Flow) Equipment
The basic design follows that of the nonrebreathing reservoir mask but with more “capable” components. Self-inflating bags consist of a roughly 1.5 L bladder, usually with an oxygen inlet reservoir. Anesthesia bags are 1-, 2-, or 3-L non–self-inflating reservoirs with a tailpiece gas inlet. Masks are designed to pro-vide a comfortable leak-free seal for manual ventila-tion. The inspiratory/expiratory valve systems may vary. The flow to the reservoir should be kept high so that the bags do not deflate substantially. When using an anesthesia bag, operators may frequently have to adjust the oxygen flow and exhaust valve to respond to changing breathing patterns or demands, particularly when maintaining a complete seal between the mask and face is difficult.
The most common systems for disposable and permanent self-inflating resuscitation bags use a unidirectional gas flow. Although these devices offer the potential for a constant FIO2 greater than 0.9, tailpiece inlet valves will not open for a spon-taneously breathing patient. Opening the valves requires negative pressure bag recoil after compres-sion. If this situation is not recognized, clinicians might be misled into thinking the patient is receiv-ing a specific concentration of oxygen when this is not the case.
There are limits to the ability of each system to maintain its fixed-performance characteristics. Delivered FIO2 can approach 1.0 with either anesthe-sia or self-inflating bags. Spontaneously breathing patients are allowed to breathe only the contents of the system if the mask seal is tight and the reservoir is adequately maintained.
Failure to maintain an adequate oxygen sup-ply in the reservoir and inlet flow is a concern. The spring-loaded valve of anesthesia bags must be adjusted to prevent overdistention of the bag. Self-inflating bags look the same whether or not oxygen flow to the unit is adequate, and they will entrain room air into the bag, thus lowering the delivered FIO2.
The gas delivery approach with air-entraining masks is different than with an oxygen reservoir. The goal is to create an open system with high f low about the nose and mouth, with a fixed FIO2. Oxygen is directed by small-bore tubing to a mixing jet; the final oxygen concentration depends on the ratio of air drawn in through entrainment ports. Manu-facturers have developed both fixed and adjustable entrainment selections over an FIO2 range. Most provide instructions for the operator to set a mini-mum flow of oxygen. Table 57–3 identifies total flow at various inlet flows and FIO2.
Despite the high-flow concept, FIO2 can vary up to 6% from the anticipated setting. The air-entraining masks are a logical choice for patients who require greater FIO2 than can be provided by devices such as the nasal cannula. Patients with COPD who tend to hypoventilate with a moderate FIO2 are candidates for the Venturi mask. Clinicians providing oxygen therapy with Venturi masks should be aware of the previously mentioned problems involving the mask itself. FIO2 can increase if the air entrainment ports are obstructed by the patient’s hands, bed sheets, or water condensate. Clinicians should encourage the
patient and caregivers to keep the mask on the face continuously. Interruption of oxygen is a serious problem in unstable patients with hypoxemia and or hypercarbia.
Direct analysis of the FIO2 during air-entrain-ment mask breathing is difficult to perform accu-rately. Arterial blood gas analysis and the patient’s respiratory rate should guide clinicians as to whether the patient’s demands are being met by the mask’s flow. If that occurs, then inlet oxygen flows may need to be increased or an alternate device selected.
Large-volume, high-output or “all-purpose” nebu-lizers have been used in respiratory care for many years to provide mist therapy with some control of the FIO 2. These units are commonly placed on patients following extubation for their aerosol-pro-ducing properties. Like the air-entraining masks, nebulizers use a pneumatic jet and an adjustable orifice to vary entrained air for varying FIO2 lev-els. Many commercial devices have an inlet orifice diameter that maximally allows only 15 L/min when the source pressure is 50 psi. This means that on the 100% setting (no air entrainment) output flow is only 15 L/min. Only patients breathing at slow rates and small VT will receive 100% oxygen. This problem has been addressed by the development of high-flow, high-FIO2 nebulizers. For more common applications that use an FIO2 of 0.3–0.5, room air is entrained, reducing the FIO2 and increasing the total flow output to 40–50 L/min.
Knowledge of the air/oxygen ratio and the input flow rate of oxygen allows the total outflow to be calculated. Nebulizer systems can be applied to the patient with many different devices, including aerosol, tracheostomy dome/collar, face tent, and Τ-piece adapter. These appliances can all be attached via large-bore tubing to the nebulizer. This open system freely vents inspiratory and expiratory gases around the patient’s face or out a distal port of a Τ-piece adapter. Unfortunately, the lack of any valves allows patients to secondarily entrain room air. It is common practice to use either a reservoir bag before the Τ-piece or a reservoir tube on the distal side of the Τ-piece to provide a larger volume of gas than that coming from the nebulizer. A typical concern of those applying air-entrainment aerosol therapy with controlled oxygen concentration is whether the system will provide adequate flow. Clinicians should observe the mist like a tracer to determine adequacy of flow. When a Τ-piece is used and the visible mist (exiting the distal port) disappears during inspira-tion, the flow is inadequate.
Another concern in clinical practice is that excess water in the tubing collects and can obstruct gas flow completely or can offer increased resistance to flow. The latter may increase the FIO2 above the desired setting. Other complications include bron-chospasm or laryngospasm in some patients as a consequence of airway irritation from sterile water droplets (condensate of the aerosol). In such cir-cumstances, a heated (nonaerosol) humidification system should be substituted.
Dual air–oxygen flowmeters and air–oxygen blend-ers are commonly used for oxygen administration as well as freestanding continuous positive airway pres-sure (CPAP) and “add-on” ventilator systems. These systems differ from the air-entraining nebulizers, as their total output flows do not diminish at FIO2 greater than 0.4. With these high-flow systems, the total flow to the patient and FIO2 can be set indepen-dently to meet patient needs. This can be done using a large reservoir bag or constant flows in the range of 50 to more than 100 L/min. Clinicians can use a variety of appliances with these systems, including aerosol masks, face tents, or well-fitted nonrebreath-ing system masks with air–oxygen blenders. Face-sealing mask systems can also be constructed with a reservoir bag and a safety valve to allow breathing if the blender fails. The high flows of gas require use of heated humidifiers of the type commonly used on mechanical ventilators. Humidification offers an advantage for patients with reactive airways. Because of the high flows, such systems are used to apply CPAP or BIPAP for spontaneously breathing patients.
Although many of the devices previously described have pediatric-sized options, many infants and neonates will not tolerate facial appliances. Oxy-gen hoods cover only the head, allowing access to the child’s lower body while still permitting use of a standard incubator or radiant warmer. The hood is ideal for relatively short-term oxygen therapy for newborns and inactive infants. However, for mobile infants requiring longer term therapy, the nasal cannula, face mask, or full-bed enclosure allow for greater mobility.
Normally, oxygen and air are premixed by an air–oxygen blender and passed through a heated humidifier. Nebulizers should be avoided. Most pneumatic jet-type nebulizers create noise levels (>65 dB) that may cause newborn hearing loss, and cold gas can induce an increase in oxygen consump-tion. Hoods come in different sizes. Some are sim-ple Plexiglas boxes; others have elaborate systems for sealing the neck opening. There is no attempt to completely seal the system, as a constant flow of gas is needed to remove carbon dioxide (minimum flow >7 L/min). Hood inlet flows of 10–15 L/min are adequate for a majority of patients.
Helium–oxygen (heliox) mixtures have a notable, yet limited clinical role. In addition to its uses in industry and deep-sea diving, heliox has a number of medical applications. Helium is premixed with oxygen in several standard blends. The most popu-lar mixture is 79%/21% helium–oxygen, which has a density that is 40% that of pure oxygen. Helium– oxygen mixtures are available in large-sized com-pressed gas cylinders.
In anesthetic practice, pressures needed to ven-tilate patients with small-diameter tracheal tubes can be substantially reduced when the 79%/21% mixture is used. Heliox can provide patients with upper airway–obstructing lesions (eg, subglottic edema, foreign bodies, and tracheal tumors) with relief from acute distress until more definitive care can be delivered. The evidence is less convincing in treating lower airway obstruction from COPD or acute asthma. Helium mixtures may also be used as the driving gas for small-volume nebulizers in bron-chodilator therapy for asthma. However, with heliox, the nebulizer flow needs to be increased to 11 L/min versus the usual 6–8 L/min with oxygen. Patients’ work of breathing can be reduced with heliox as compared to a conventional oxygen/nitrogen gas mixture.
Hyperbaric oxygen therapy uses a pressurized chamber to expose the patient to oxygen tensions exceeding ambient barometric pressure (at sea level the ambient pressure is 760 mm Hg). With a one-person hyperbaric chamber, 100% oxygen is usually used to pressurize the chamber. Larger chambers allow for the simultaneous treatment of multiple patients and for the presence of medical personnel in the chamber with patients. Multi-place chambers use air to pressurize the chamber, whereas patients receive 100% oxygen by mask, hood, or tracheal tube. Common indications for hyperbaric oxygen include decompression sickness (the “bends”), certain forms of gas embolism, gas gangrene, carbon monoxide poisoning, and treat-ment of certain wounds.
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