Supplemental Oxygen
Calvin A. Brown III
Steven C. Carleton
Robert F. Reardon
Neuronal and myocardial tissue undergo time-dependent, irreparable damage when deprived of oxygen for relatively brief periods of time, and may function abnormally in low oxygen states. In the context of emergency airway management, the principal purpose of supplemental oxygen is to avoid injury to, or dysfunction of, oxygen-sensitive tissues. Supplemental oxygen also is used to create a high-concentration oxygen reservoir in an effort to maximize the safe apnea period during rapid sequence intubation (RSI) (see Chapter 19). Many misconceptions exist about oxygen therapy, such as the effectiveness of the “100% non-rebreather mask,” a device which supplies, at best, 60% to 70% inspired oxygen. This chapter will address the principles of supplemental oxygen delivery and clarify common misunderstandings about this vital drug.
Ventilation and perfusion (V/Q) mismatches or impaired oxygen diffusion across the alveolar membrane reduce systemic arterial oxygen tension by a variety of mechanisms. In cases of reduced pulmonary arterial blood flow, as in pulmonary embolus, oxygen-filled alveoli are not perfused and cannot participate in gas exchange. In conditions with significant airspace disease, such as lobar pneumonia, perfused areas of poorly oxygenated lung fail to oxygenate the pulmonary circulation. Pulmonary edema causes accumulation of airspace fluid and also may impede effective oxygen transport across the alveolar membrane. The most effective way to improve oxygen diffusion is to increase alveolar oxygenation by providing higher oxygen concentrations, often augmented by positive pressure, thereby improving the diffusion gradient and allowing more effective pulmonary arterial oxygenation. This chapter focuses on ambient pressure oxygen supplementation. Positive-pressure oxygenation and ventilation is discussed in Chapter 6.
SUPPLEMENTAL OXYGEN
FiO2 refers to the fractional concentration of inspired oxygen. Room air FiO2 is 0.21, corresponding to 21% oxygen concentration. Correct terminology expresses FiO2 as a fractional number (e.g. 0.4), but common (mis)use is to express it as a percentage (e.g. 40%). For simplicity, we will use the percentage format for FiO2, as this represents the most common clinical usage. A variety of oxygen delivery systems provide a wide range of supplemental oxygen. Delivery systems are broadly categorized as either low-flow or high-flow, depending both on the inspiratory flow rate provided by each setup and whether inspired oxygen is fixed or variable. High-flow systems provide the entire inspiratory volume with a fixed, but adjustable, concentration of inspired oxygen. Low-flow systems require that the patient breathe variable proportions of room air and supplied oxygen with each breath. The most common oxygen delivery systems in the emergency department (ED) are low-flow types and include nasal cannulas, simple face masks, and face masks with oxygen reservoirs. The most common high-flow device is the Venturi mask. Oxygen supplementation is often delivered in an incremental fashion starting with low-flow systems and subsequently adjusted based on patient need. As a general rule, the least invasive mode of oxygen delivery, and the lowest flow rate needed to maintain normal saturation, should be used.
Low-flow Oxygen Delivery Systems
Nasal cannula
Nasal cannula is a low-flow oxygen delivery system in which oxygen can be administered through small-caliber tubing and narrow nasal prongs. Nasal cannula is often the first line of supplementation and is appropriate in patients with mild oxygen debt. Nasal cannula oxygen creates a high-concentration oxygen reservoir in the nose and posterior nasopharynx, which mixes with room air during inspiration, resulting in a marginal rise in FiO2. Common initial flow rates are 2 to 4 L per minute. At 6 L per minute, the nasal and nasopharyngeal O2 reservoirs are full and higher flow rates result in minimal, if any, increases in inspired oxygen. Maximal FiO2 from nasal cannula is approximately 40%. Other common low-flow systems and their expected maximum achievable FiO2 can be found in Table 5-1. The effect of nasal cannula oxygen may be tempered by nasal congestion or obstruction, or when used by obligate mouth breathers.
TABLE 5-1 Estimates of Maximum FiO2 with Various Low-flow Oxygen Delivery Systems | ||||||||||||||||||
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Simple face mask
If nasal cannula oxygen does not provide sufficient oxygen saturations, increasing the size of the upper airway reservoir by including the mouth and oropharynx can be achieved by simple face mask oxygen. Additionally, reservoir volume is contained within the mask (approximately 100 to 150 ml). A simple face mask is a non-form-fitting plastic mask that covers the entirety of the nose and mouth. It does not have an external bag reservoir and is attached to small-caliber oxygen tubing similar to that of nasal cannula. Typical flow rates range between 4 and 10 L per minute. The FiO2
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