Beginning at the nose and lips and proceeding down the airway of the infant or young child, several important anatomic differences are encountered as compared with the adult (Fig. 13.1). Many of these differences predispose the child to airway obstruction. For example, the tongue of the infant or young child is relatively large in proportion to the oral cavity. In addition, since the larynx has a more superior position in the neck of a pediatric patient compared with an adult, the tongue has a more rostral location in the hypopharynx (3). Consequently, a child lying supine may develop airway obstruction due solely to posterior displacement of the tongue, a problem that can often be alleviated with a pharyngeal airway. Infants are also obligate nasal breathers until approximately 3 to 5 months of age (3). As a result, swelling of the nasal mucosa, thick rhinorrhea, nasal foreign bodies, or choanal atresia may cause significant respiratory distress in the young infant. Perhaps the most obvious and important difference from the adult patient is that children have smaller corresponding airway luminal diameters. Because the resistance to air flow is inversely related to the fourth power of the radius (Poiseuille’s law), disease states that cause further narrowing of the airways (edema, secretions, etc.) disproportionately increase the work of breathing and the resulting oxygen demand in children compared with adults (Fig. 13.2). Further discussion of pediatric airway anatomy can be found in Chapters 14 and 16.

The oxygen consumption index of an infant is roughly twice that of an adult, due primarily to a higher rate of metabolism. As a result, pediatric patients develop hypoxemia more rapidly in response to inadequate oxygen delivery. Most practitioners have witnessed the precipitous decline in oxygen saturation that can occur when oxygen flow to a child in respiratory distress is briefly interrupted.

In general, the rate of oxygen flow delivered should be sufficient to maintain an oxygen saturation of 100% whenever possible. Although the concentration of oxygen delivered to the patient (F_iO₂) corresponds to the rate of flow, a linear relationship does not exist. A number of other important variables also contribute to the F_iO₂. These include the patient’s nasal and oropharyngeal resistance as well as factors that may affect the amount of atmospheric (room) air that mixes with the administered oxygen—inspiratory flow rate, minute ventilation, and tidal volume. As a rule, the greater the patient’s inspiratory flow rate (which is influenced by both minute ventilation and tidal volume), the greater the mixing of relatively oxygen-poor atmospheric air with administered oxygen and therefore the lower the F_iO₂ for any given oxygen flow rate. In other words, when a patient inhales with a greater velocity than the highest possible rate of oxygen flow, atmospheric air must enter the system to make up the difference.

For example, assuming a constant oxygen flow of 3 L/min, a larger patient with a minute ventilation of 12 L/min will breathe more atmospheric air than a smaller patient with a minute ventilation of 6 L/min. Although the rate of oxygen flow is the same, the F_iO₂ is greater for the patient with a lower minute ventilation. Thus, an older child will need a higher rate of oxygen flow to achieve the same F_iO₂ as a younger child. In addition, the specific delivery system used will influence the maximal oxygen concentration delivered to the patient. At a given oxygen flow, a nonrebreathing mask will provide a higher F_iO₂ than a standard mask, because the latter system allows for greater entry of atmospheric air.

There are several diseases affecting children that may result in hypoxemia and for which the administration of oxygen is indicated. The most common of these are asthma and bronchiolitis. In these two conditions, bronchoconstriction and airway inflammation increase lower airway resistance, contribute to atelectasis and ventilation/perfusion mismatching, and increase the patient’s work of breathing and oxygen consumption. The degree of hypoxemia may be exacerbated if the child has underlying chronic lung disease such as cystic fibrosis. Pneumonia and laryngotracheobronchitis (croup) also commonly affect children and may necessitate the administration of oxygen. In addition to primary pulmonary processes, other common indications for oxygen administration include status epilepticus and hypoventilation secondary to a medication overdose, such as with a sedative or narcotic. Hypercarbia and acidosis may be relatively well tolerated by a child, but even a short period of oxygen deprivation can lead to bradycardia or cardiac arrest (2). Once it is determined that the spontaneously breathing child with respiratory distress has a patent airway, oxygen should be administered empirically.

Cyanosis is the most definitive physical finding indicating hypoxemia. Typically, the perioral region and distal extremities are the first areas of the body to become cyanotic. Other signs of hypoxemia include agitation, lethargy, and bradycardia. Determination of oxygen saturation by pulse oximetry provides a means of quantifying the degree of hypoxemia (see Chapter 75). Unless adequacy of ventilation is a concern, there is
usually little need to perform a painful and time-consuming arterial puncture for blood gas analysis. Pulse oximetry offers a noninvasive, “real-time” modality that rapidly gives an indirect but generally reliable indication of the partial pressure of oxygen (PaO₂). In interpreting the pulse oximeter reading, it is important to remember that the patient will maintain an oxygen saturation over 90% as long as the PaO₂ is above 60 mm Hg, even if the PaO₂ is decreasing. Physiologically, this corresponds to the initial flat segment of the oxyhemoglobin dissociation curve (Fig. 13.3). Once the PaO₂ falls below 60 mm Hg, however, oxygen saturation will drop dramatically as the patient reaches the steep segment of the curve. Therefore, oxygen saturation of 90% is only minimally acceptable in most clinical situations. If an oxygen saturation of at least 90% cannot be maintained with a given method of delivery, a system capable of administering a higher concentration should be employed. In such cases, controlled or assisted ventilation may also prove necessary.

When administering oxygen, the clinician must first determine the concentration to be delivered and then select an appropriate delivery system. These decisions are affected by a number of patient variables, including age, degree of cooperation, respiratory effort, and extent of hypoxemia. For example, an older child in mild to moderate respiratory distress with minimal hypoxia may receive oxygen via a low-flow system such as a nasal cannula or simple mask. However, many younger children become extremely agitated and uncooperative when such devices are used, increasing oxygen consumption and potentially exacerbating hypoxemia. In such cases, it may be more effective to use a less efficient but better tolerated delivery method, such as a hollow plastic tube held close to the child’s face (“blow-by” method). For the child in greater distress, a nonrebreathing mask may be necessary to maintain adequate oxygen saturation. The clinician should be knowledgeable about the characteristics of each delivery system, the manner in which they are best used, and the advantages and disadvantages of each.