Abstract
The main purpose of the upper respiratory tract is to conduct air from the atmosphere to the lower respiratory tract. However, the upper airways serve a number of additional functions.
What are the components and functions of the upper respiratory tract?
The upper respiratory tract refers to the air passages that lie above the larynx, outside the thorax, and include:
Nose, nasal cavity and paranasal sinuses;
Mouth;
Pharynx, which consists of the nasopharynx, oropharynx and laryngopharynx.
The main purpose of the upper respiratory tract is to conduct air from the atmosphere to the lower respiratory tract. However, the upper airways serve a number of additional functions:
Nasal hairs filter any large inhaled particles.
The superior, middle and inferior nasal turbinates (conchae) within the nasal cavity direct the inspired air over the warm, moist mucosa, promoting humidification. The epithelium of the posterior nasal cavity is covered in a thin mucous layer, which traps finer inhaled particles. Cilia then propel this mucus to the pharynx to be swallowed.
The function of the four air-filled paranasal sinuses is of debate. They decrease the weight of the skull and protect the intracranial contents by acting as a ‘crumple zone’. They may also have a role in air humidification, immunological defence and speech resonance.
Olfactory receptors are located in the posterior nasal cavity. The proximal location of the olfactory receptors means that potentially harmful gases can be sensed by rapid, short inspiration (i.e. sniffing) before being inhaled into the lungs. They also play a major role in taste.
The pharynx is a complex organ whose functions include the conduction of air, phonation and swallowing. The muscles of the upper airway are arranged to facilitate its multiple functions:
– Pharyngeal constrictors: inferior, middle and superior constrictor muscles. During swallowing, these muscles contract to propel food into the oesophagus.
– Pharyngeal dilators: these muscles contract to maintain patency of the pharynx, so that air can flow to the lungs.
How does the upper airway remain patent during breathing?
During normal breathing, contraction of the diaphragm increases intrathoracic volume, which results in a negative airway pressure (see Chapter 7). Within the large airways, collapse is prevented by cartilaginous support. In contrast, the pharynx is largely unsupported and is therefore liable to collapse during inspiration. There are three groups of muscles responsible for maintaining upper airway patency:
Genioglossus, the main dilator muscle of the pharynx, which causes the tongue to protrude forward and away from the pharyngeal wall.
Palatal muscles control the stiffness and position of the palate, tongue and pharynx, as well as the shape of the uvula.
Muscles influencing the position of the hyoid, such as geniohyoid, exhibit phasic activity. This means that their activity is increased during inspiration, thus stiffening and dilating the upper airway, counteracting the influence of negative airway pressure. When conscious, the airway will remain patent, even in the presence of intrathoracic pressures as low as –60 cmH2O.
What happens to the upper airway during sleep?
During wakefulness, the activity of the pharyngeal dilator muscles is tightly controlled to maintain upper airway patency. Once an individual is asleep, the tone of the pharyngeal dilator muscles decreases significantly, leading to a reduced pharyngeal diameter. The greatest loss of pharyngeal muscle tone is associated with stage 3 non-rapid eye movement (NREM) sleep, the stage of sleep that is the most physically restorative.
In the majority of the population, upper airway patency is maintained during sleep. Susceptible individuals may experience pharyngeal obstruction:
Partial obstruction of the pharynx results in turbulent airflow during breathing, resulting in the characteristic noise of snoring, an affliction that affects approximately 30% of the population (and their bed-partners!).
Complete obstruction of the pharynx, as may occur in obstructive sleep apnoea (OSA).
What is OSA?
OSA is a sleep disorder characterised by recurrent episodes of complete upper airway obstruction during deep sleep. The pharyngeal collapse results in cessation of airflow despite the presence of diaphragmatic breathing effort. Each apnoeic period typically lasts 20–40 seconds, during which time hypoxia and hypercapnoea develop. The resulting chemoreceptor activation (see Chapter 22) rouses the individual from sleep sufficiently to restore pharyngeal muscle tone and therefore airway patency. A short period of hyperventilation occurs, until sleep deepens and airway obstruction recurs. This repeated cycle of sleep interruption (loss of stage 3 NREM and rapid eye movement sleep) and hypoxaemia is associated with the following problems:
Neuropsychiatric: daytime sleepiness, poor concentration, irritability, anxiety, depression.
Endocrinological: impaired glucose tolerance, dyslipidaemia, increased adrenocorticotropic hormone and cortisol levels.
Cardiovascular: hypertension, atrial fibrillation, myocardial infarction, stroke.
OSA affects approximately 5–10% of the general population, but the prevalence is thought to be much higher in the surgical population. Risk factors for the development of OSA include:
Anatomical factors: craniofacial abnormalities (such as Pierre Robin and Down’s syndromes) and tonsillar and adenoidal hypertrophy (the major cause of OSA in children).
Obesity, probably as a result of fat deposition around the pharynx. Abdominal obesity also decreases functional residual capacity (FRC), which exacerbates the hypoxaemia experienced during apnoeas.
Male gender, possibly as a result of a relatively increased amount of fat deposition around the pharynx.
The effective treatment options are lifestyle modification (smoking cessation, alcohol reduction and weight loss), mouth devices and nasal continuous positive airway pressure (nCPAP). Overnight nCPAP set at between +5 and +20 cmH2O probably works by acting as a pneumatic splint to maintain upper airway patency and has the effect of reducing daytime sleepiness and atrial fibrillation. It also improves mood, cognitive function and blood pressure control.
