Types of Respiratory Events
1. Obstructive apnea occurs when there is ventilatory effort without airflow and the cessation of airflow is for 10 seconds or more.
2. Central apnea occurs from a loss of central nervous system drive to respiratory motor neurons. In central apnea there is no ventilatory effort present and arousal may not occur.
3. Mixed apnea has an initial central apnea followed by an obstructive apnea and arousal. Because mixed apneas are successfully treated by elimination of upper airway obstruction, they are considered a variant of obstructive apneas.
4. Hypopnea has a 50% decrease in airflow. It may be obstructive, central or a combination or both. A combination of decreased airflow, a 2-4% oxyhemoglobin desaturation, and an electroencephalographic (EEG) arousal can typically identify hypopneas measured on standard polysomnography.
Apnea Index (AI) refers to the number of apnea events per hour of sleep.
The Respiratory Disturbance Index (RDI) includes both the numbers of apnea plus the number of hypopneas.
Respiratory Disturbance Index can better measure obstructive sleep apnea severity. However multiple variables including age and gender can influence what is considered normal. It is unknown if there is any threshold of respiratory disturbance that defines disease.
General consensus accepts that a Respiratory Disturbance Index of less than 20 events per hour is likely mild apnea, 20 to 40 events per hour is moderate apnea, and a Respiratory Disturbance Index of greater than 40 events per hour is severe obstructive sleep apnea syndrome. For any given individual, the threshold of disease varies according to age and coexisting medical conditions.