Shortfall of Apnea Hypopnea Index (AHI)

Obstructive Sleep Apnea Traits:
During wakefulness, the airway is held open by the high activity of the numerous upper airway dilator muscles, but after the onset of sleep, when muscle activity is reduced, the airway collapses.

Collapse of the upper airway occurs if the negative upper airway pressure generated by inspiratory pump muscles exceeds the dilating force of these upper airway muscles.

Obstructive sleep apnoea is a common disorder of repetitive pharyngeal collapse during sleep. Pharyngeal collapse could be complete (causing apnoea) or partial (causing hypopnea).

The best test for obstructive sleep apnoea is overnight polysomnography in a laboratory with the primary outcome measure of apnoea- hypopnea index (number of apnoeas plus hypopnoeas per h or sleep)
This test involves concurrent monitoring of both sleep and respiration.

Despite the quality of neurophysiological signals obtained during an overnight PSG, most of the data collected is ignored and treatment decisions rely heavily on the AHI.

AHI has several limitations:
For example, a patient with very long respiratory events may experience substantial hypoxemia but have a relatively low AHI.
Another patient may have more frequent events and therefore a much higher AHI, but minimal exposure to hypoxemia.

In addition, non-apneic respiratory events that do not meet the scoring criteria for a hypopnea are associated with heart rate changes and increased respiratory pharyngeal resistance.
Furthermore, the total AHI correlates poorly with the key causes and consequences of the disease.

Traditionally, obstructive sleep apnoea was considered to primarily be a problem of upper airway anatomy, where craniofacial structure or body fat decreased the size of the pharyngeal airway lumen, leading to an increased likelihood of pharyngeal collapse.

Anatomical:
The upper airway can collapse at one or multiple sites. The pharyngeal structures that can contribute to airway crowding and collapse include the dilator muscles such as genioglossus, soft palate, lateral pharyngeal walls, and the epiglottis.

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