A Continuous Positive Airway Pressure (CPAP) machine is the standard treatment for sleep apnea, a disorder characterized by repeated interruptions to breathing during sleep. Modern CPAP devices continuously monitor breathing patterns and record occurrences known as “events.” These events represent instances where airflow is significantly reduced or completely stopped, signaling a breakdown in the user’s normal nocturnal breathing rhythm. Tracking these occurrences provides objective data that helps users and clinicians understand how well the pressurized air is preventing airway collapse and stabilizing breathing throughout the night.
Defining Respiratory Events
A respiratory event is a specific measured disruption to the normal flow of air into the lungs that lasts for a minimum of ten seconds. These disruptions are categorized into two primary types based on the degree of airflow reduction: apnea and hypopnea. An apnea is the more severe type, representing a near-complete cessation of breathing flow, typically defined as a reduction of airflow by 90% or more.
A hypopnea is a partial reduction in airflow. This event is defined as a reduction in breathing of at least 30%, which must also be accompanied by either a drop in the blood’s oxygen saturation or an arousal from sleep. Although not a total stop, a hypopnea indicates a period of shallow breathing that disrupts the restorative process of sleep. Reducing both apneas and hypopneas is the main goal of CPAP therapy.
The Classification of Event Types
While apnea and hypopnea describe the degree of airflow failure, events are further classified by their underlying cause. The most common cause is an Obstructive Apnea or Hypopnea, where the upper airway muscles relax, allowing tissue to collapse and physically block the passage of air despite continued effort to breathe. CPAP therapy is highly effective at treating this type by using pressurized air to act as an internal “pneumatic splint” to keep the airway open.
A different classification is the Central Apnea or Hypopnea, which occurs when the brain fails to send the correct signal to the breathing muscles. In this scenario, there is no physical blockage, and the body makes no effort to inhale or exhale, indicating a problem with the respiratory control center. A third category, the Mixed Event, involves characteristics of both types, often starting as a central failure and transitioning into an obstructive event. The CPAP machine distinguishes between these causes by monitoring the user’s breathing effort alongside the airflow.
Understanding the Apnea-Hypopnea Index (AHI)
The Apnea-Hypopnea Index, or AHI, is the most important metric for gauging the effectiveness of CPAP therapy. This index quantifies the total number of apneas and hypopneas that occur per hour of sleep, providing an average rate of respiratory disturbance. To calculate the AHI, the machine totals all recorded apnea and hypopnea events from a sleep session and divides that sum by the total hours the device was used.
The resulting AHI score is the primary number used to determine if sleep apnea is being adequately controlled. For an adult, an AHI of fewer than five events per hour is considered the therapeutic target. Scores are classified as mild (five to 14 events per hour), moderate (15 to 29), or severe (30 or more).
Monitoring the AHI allows patients and physicians to track progress and make necessary adjustments to the CPAP pressure settings. For example, a persistently high AHI, such as 18 events per hour, indicates that the current pressure is likely insufficient to prevent airway collapse. Conversely, a consistently low score confirms that the therapy is working as intended to maintain stable, uninterrupted breathing.