Stridor is an important sign in pediatric patients and indicates airway pathology. Gaining an understanding of the causes of stridor and the management strategies is critical for otolaryngologists who encounter these patients.
KEY PHYSIOLOGY AND ANATOMY
The larynx (Figure 35–1) is subdivided into the following:
Supraglottis, which includes the epiglottis, aryepiglottic folds, and the false vocal cords (VC)
Glottis, which comprises the true VC
Subglottis, which is the area below the true VC, within the complete cartilaginous ring of the cricoid
Subdivisions of the larynx in coronal section viewed from posteriorly.
The cricoid lumen is particularly small in neonates, measuring as little as 4 to 5 mm in diameter.
The most important physiological principle to understand in pediatric airways is the Poiseuille equation (Figure 35–2).
Flow is proportional and therefore resistance is inversely proportional to the radius to the power of 4. The main implication of this is that narrowing of a small pediatric airway by edema does not result in a linear increase in resistance in proportion to the edema: in fact there is an exponential increase in resistance.
In a neonate with a 2-mm radius airway (4-mm diameter), a 1-mm radius reduction (50% of previous size) by edema results in a 16-time increase in airway resistance (1/0.54). This will manifest itself as serious airway obstruction. In an adolescent with a 4-mm radius airway, a 1-mm reduction (75% of previous size) results in just a 3-time increase in airway resistance (1/0.754).
Stridor is a high-pitched noise produced by turbulent airflow through a partially obstructed airway. It may be inspiratory, expiratory, or both (biphasic). The term stertor is used to describe a lower pitch, gurgling, or snoring type airway noise originating in the nose, nasopharynx, and oropharynx. As a general rule, inspiratory stridor originates from the supraglottis; biphasic stridor from the glottis, subglottis, and cervical trachea; and expiratory stridor from the intrathoracic trachea. This is due to the different flow dynamics at the different sites. In inspiratory stridor, the structures of the supraglottis are drawn into the airway on inspiration, resulting in stridor. On expiration, they are more open as a result of expiratory pressure. In the subglottis there is usually a fixed obstruction that does not change whether airflow is inspiratory or expiratory, so the stridor is biphasic. In the intrathoracic trachea, thoracic expansion on expiration leads to a relative lowering of the pressure within the thorax so the dynamic airway is more open, but due to elastic recoil of the chest on expiration, the pressure increases and narrowing of ...