When a person falls asleep, even someone who does not snore or have OSA will have collapse of the upper airway. It is simply a matter of degree. Factors that may contribute to the occurrence of snoring and/or OSA in a given individual can be divided into:
Intrinsic factors can be further categorized as:
Extrinsic factors can be categorized as follows:
Optimal treatment of snoring and OSA requires consideration of all these factors.
When air flows through a narrowed upper airway, there is increased turbulence. This causes pressure fluctuations which make the surrounding tissues vibrate. The soft palate, because it hangs loosely in the throat, usually vibrates the most. However, all of the tissues in the upper airway participate in making the snoring noise. This is why surgery to the soft palate alone often does not eliminate snoring.
The pressure fluctuations, and narrowing in one or more segments of the upper airway can result in an apnea or hypopnea when conditions are right. Loud snoring is strongly associated with apnea and hypopnea. However, not all patients with OSA snore.
The medical definition of apnea in adults is a stoppage in airflow that lasts longer than 10 seconds. Sleep specialists often try to make a distinction between Obstructive Apneas, Central Apneas, and Mixed Apneas. An Obstructive Apnea is a stoppage in airflow that is caused by a blockage of the air passage. A Central Apnea is a stoppage in airflow that is caused by a failure of the central nervous system to send the appropriate signals to the respiratory muscles. A Mixed Apnea has components of both. However, recent research suggests that many patients who are diagnosed to have Central Apnea on the basis of recordings of the respiratory effort made by external sensors are in fact having Obstructive Apnea. The best way to distinguish between the different types of apnea is to measure pressure changes in the chest using esophageal catheters. However, this is rather uncomfortable for the patient and is not routinely done.
If you looked at the upper airway (using a fiberoptic instrument) during an apnea, you would most often see that collapse is occurring at 2 or more levels. Also, the collapse does not always involve the same levels, even in the same patient.
Hypopnea is a drop in airflow; however, the precise definition of a hypopnea is somewhat arbitrary. The usual threshold is set at a drop in airflow to 30% of the baseline. However, the baseline can fluctuate during the course of a recording because the sensors used for detecting airflow are quite sensitive to position. Also, the response of the sensors may be non-linear. This means that the output of the sensor is not directly proportional to the airflow. Some experts also require that the decrease in airflow be accompanied by a drop in the oxygen saturation. However, different labs use thresholds ranging from 2 - 4 %. Whether or not a drop in oxygen saturation is detected also depends on the capabilities of the oxygen sensor and the electronics used to record the signal.
To complicate matters further, some experts describe a condition called Upper Airway Resistance Syndrome in which repeated partial collapses of the upper airway occur that do not result in measurable changes in airflow at the nose and mouth. Nevertheless, these partial collapses are believed to disrupt sleep in susceptible individuals.
In view of the above considerations, it may be time to replace the term OSA with Obstructive Sleep-Disordered Breathing or OSDB!
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