Physics of Airway Obstruction
20.2.1 Resistance to Airflow
Poiseuille’s law (Box 20.1) explains changes that occur as fluid (liquid or gas) passes through a tube. It describes the relationship between pressure and flow rate when the diameter of the tube is varied. Even though the equation does not completely fit for the respiratory tract, the principles remain true.
Minor reductions in airway diameter result in dramatic increases in airway resistance.
The law demonstrates this as the radius is altered to the power of 4. This principle is pivotal to understanding the effects of reducing the diameter of a tube.
Box 20.1 Poiseuille’s Law
where R is resistance, μ is viscosity, l is length of pipe, and r is radius.
20.2.2 Laminar and Turbulent Flow
In laminar flow, the fluid moves in parallel waves, in contrast with the irregular fluctuations that characterize turbulent flow ( ▶ Fig. 20.1). The healthy flow of air within the airway depends on laminar flow as the resistance is least in this situation.
In the presence of a stricture, the diameter is reduced, so to maintain flow through that stricture, the flow rate at that point must increase and hence turbulence may occur.
Fig. 20.1 Laminar versus turbulent flow.
Progressively reducing the airway radius eventually results in a critical radius that brings about turbulent flow. The critical point at which turbulent flow occurs is indicated by the Reynold’s number (see Box 20.2). The use of heliox (helium 80% plus oxygen 20%) exploits knowledge of the Reynold’s number. Heliox reduces stridor as its density is lower than that of air (nitrogen 79% plus oxygen 21%). Heliox improves the clinical condition because it reduces the work of breathing. The lower density of heliox means a lower Reynold’s number and hence lower probability of turbulence/stridor. 2
Box 20.2 Factors Affecting the Critical Point (Reynold’s Number, Re) at which Airflow Becomes Turbulent
where Re is the Reynolds number, d is the diameter of the pipe, v is the mean velocity, μ is the dynamic viscosity, and ρ is the density.
20.2.3 The Bernoulli Principle
This indicates that in order for the fluid/gas to pass through a narrowed point in a tube, it must speed up. Consequently, the pressure at the point of narrowing drops. It is this reduction of pressure plus the increased flow rate that results in the vibration of the walls of the airway and column of air that we hear as a stridor. 1 The point at which stridor occurs within a given airway is at the change from laminar to turbulent flow (i.e., the Reynolds number; ▶ Fig. 20.2). What does this mean in real clinical terms?
The resistance to airflow rises with a reduction in tracheal diameter. When the diameter reduces by approximately 70%, there is a significant change in resistance. This means that when a child has stridor, even at the onset and even if the stridor is very mild, a significant narrowing of the airway is present.
Fig. 20.2 Stridor in relation to the Reynold’s number. (a) Normal caliber airway, laminar flow. (b) Narrowed airway. As air passes through the narrowed segment, laminar flow becomes turbulent, that is, the threshold for laminar flow (Reynold’s number) is exceeded.
The Venturi effect is the name given to the phenomenon that results in a narrowed collapsible tube collapsing further as air passes through it. This is because the pressure exerted by a gas within a tube is equal in all directions except when there is flow in a forward linear direction. Linear flow results in a reduction of the vector of force exerted outward against the walls of the tube. This allows some collapse of the tube, as demonstrated in clinical practice with bilateral vocal cord palsy when the vocal cords (sitting within the flow of air) are seen to prolapse toward each other during inspiration. This is also the effect that causes supraglottic collapse and stridor in laryngomalacia.
The collapsible nature of the intrathoracic airway is additionally important as an obstruction at the bronchi (e.g., foreign body causing a ball valve effect) may mean the child requires considerable extra effort to exhale. The intrathoracic pressure has to be greater than atmospheric pressure to permit exhalation of air; hence, the airway diameter may reduce, thus increasing the difficulty of breathing. If there is an obstruction within the airway outside the thorax, then forced inspiration will result in an intrathoracic low pressure to generate a pressure gradient enough to “suck” air in. This may result in the extrathoracic trachea collapsing by the Venturi effect.
Changes in intrathoracic pressure may be one of the factors that precipitates gastroesophageal reflux as the low pressure causes a pressure gradient across the gastroesophageal junction and stomach contents enter the thoracic esophagus.
20.3 Assessment of the Airway
The history and examination will direct the otolaryngologist’s attention to particular anatomical regions of the airway. There may be synchronous pathology, for example, laryngomalacia plus bronchial stenosis. Understanding severity aids the management of the compromised child and dictates the urgency of intervention.
20.3.1 Clinical Assessment by History
Stertor is a relatively low-pitched sound that originates at the level of the nasopharynx or oropharynx.
It is typically due to the partial collapse of pharyngeal wall structures into the airway. The most frequent causes of stertor include adenotonsillar hypertrophy. However, tongue position and placement are also important in children with relative macroglossia or with hypotonia due to neurologic conditions.
Stridor is upper airway noise arising from the large diameter airway, that is, the larynx and trachea. The noun “stridor” is derived from the Latin. The verb “stridere” means to make a harsh sound (squeak or screech). Stridor is a clinical term reserved characteristically for a noise generated from the large diameter airways. Smaller diameter airways give rise to wheeze, as with asthma (and this is expiratory). This chapter will concentrate on stridor.
Stridor can occur from birth. The commonest cause of congenital stridor is laryngomalacia, followed by congenital vocal cord palsy, which is a significantly less frequent second cause. Both these conditions will be present at birth but may take a couple of weeks to develop. Stridor that occurs 3 months after birth may be due to a subglottic hemangioma. A history of previous intubation (particularly with prematurity) is a risk for subglottic cyst formation.
A history of recurrent croup is often associated with subglottic stenosis. It is believed that many different virus types cause croup. These viruses cause some degree of subglottic edema. If the subglottis has a preexisting subglottic narrowing, the croup becomes more clinically important. The same viruses may cause subglottic edema in children without subglottic narrowing. However, they may not induce enough edema to reduce the tracheal diameter by the approximately 70% needed to result in a clinically audible stridor.
With a history of tracheoesophageal fistulas (TOFs), some element of tracheomalacia is to be expected as this condition is due to a defect in the development of the trachea and esophagus, and therefore weakness of the tracheal wall is present. If the stridor changes with a change in position of the infant, this indicates the site of pathology, for example, lying down on the left side may reduce the stridor caused by left-sided vocal cord palsy. This sign is not reliable.
In a stridulous child, fever, hoarseness, and a rapid onset may indicate an infective etiology. The history of airways symptoms is an important determinant of management.
A rapid onset indicates infection or foreign body whereas a gradual history implies cysts, stenosis, hemangiomas, or recurrent papillomatosis.
A prodromal upper respiratory tract infection may signify croup, tracheitis, or the now rare epiglottitis. If previous endotracheal (ET) intubation has occurred, consider cysts or stenosis. With altered voice or hoarseness one should exclude respiratory papillomatosis, a glottic web and foreign body. “Singer’s” or “Screamer’s” nodules in children do not cause airway compromise. For children with feeding difficulty, coughing, choking, or cyanosis, TOF, a vascular ring, or an inhaled foreign body may be the cause.
If a parent expresses concern about the work of breathing of their child, they are usually correct and there is a diagnosis that can be made.
20.3.2 Clinical Assessment by Examination
It is important to recognize that the airway starts at the nose and mouth and these must be included in any airway assessment. A cold spatula can identify a patent nasal airway bilaterally. It is not unusual to find a unilateral choanal atresia presenting in later childhood. Even with bilaterally patent posterior nasal apertures, a baby can still develop nasal airway obstruction, for example, if there is an intranasal mass, severe rhinitis, or the rare congenital nasal pyriform aperture stenosis (see ▶ 16). The size and position of the jaw is important as retrognathia can cause supraglottic crowding and obstructive symptoms. Typically, obstruction at this level will cause stertor rather than stridor, but parents and referring clinicians may be uncertain and the history cannot be relied upon completely.
Stridor has the following three basic forms that help determine the level of airway obstruction:
Inspiratory.
Expiratory.
Biphasic.
Inspiratory stridor usually occurs when there is an obstruction above the level of the vocal cords, for example, laryngomalacia causes inspiratory stridor because the epiglottis or arytenoid mucosa prolapses into the laryngeal airway with inspiration but the inlet opens again with expiration. Be cautious as occasionally severe tracheomalacia can also result in inspiratory stridor.
Biphasic stridor is always a significant clinical sign, as it signifies a severe obstruction and requires early airway endoscopy. Stridor can be biphasic in any severe laryngotracheal obstruction but is classically described in either vocal cord palsy or severe subglottic stenosis.
Expiratory stridor is relatively uncommon. It may occur in tracheomalacia, when the noise is largely expiratory due to tracheal collapse as the child breathes out, but can also arise due to a prolapsing subglottic mass. During inspiration, the mass is drawn in. On expiration, the mass may partially obstruct the inferior aspect of the vocal cords (this ball valve effect can be seen with respiratory papillomatosis; however, these children are additionally hoarse).
Assessing the nature of the stridor is the first part of the assessment of a child’s airway. It is normally this that alerts the parents to a problem.
Additional features of airway obstruction include poor feeding, and a distressed and unhappy child with the following:
Tracheal tug.
Sternal recession.
Subcostal recession.
This order approximately indicates the increasing severity of the obstruction. Additional signs include the use of accessory muscles of respiration. A drooling child with quiet stridor and bracing him/herself holding the head upright is a classical image of acute epiglottitis. In an infant, the extended head position, nasal flaring, and head bobbing are also significant.
Rate of breathing is also a reliable and relatively objective sign that can be appreciated readily by parents, carers, nurses, and clinicians. Tachypnea is more likely with increased work of breathing. Perhaps unsurprisingly, as the child tires, breathing becomes less effective and the volume of breathing reduces.
This reduced breath volume results in a quietening of stridor and is deceptively dangerous (Box 20.3).
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