Continuous Positive Airway Pressure (CPAP) is the primary treatment method prescribed for managing Obstructive Sleep Apnea (OSA). This device delivers pressurized air through a mask to prevent the upper airway from collapsing during sleep. The effectiveness of CPAP therapy is measured by monitoring the frequency of breathing disturbances, referred to as “events per hour.” Checking this number regularly confirms the treatment is working as intended, ensuring continuous, restorative sleep and mitigating the health risks associated with OSA.
Understanding Sleep Apnea Event Metrics
The measurement of sleep disturbance is quantified using the Apnea-Hypopnea Index (AHI), which represents the total number of specific breathing events divided by the hours slept. The two types of events counted are apneas and hypopneas. An apnea is a complete cessation of airflow lasting at least ten seconds, typically due to the total collapse of the airway during sleep.
A hypopnea is a partial obstruction where airflow is significantly reduced for at least ten seconds. The American Academy of Sleep Medicine (AASM) defines a hypopnea as a reduction in airflow of at least 30% associated with either a drop in blood oxygen saturation of 3% or more, or an arousal from sleep. The AHI consolidates both complete and partial breathing disturbances into a single metric that reflects the overall quality of nocturnal respiration.
Establishing the Baseline: AHI Severity Without Treatment
The AHI score classifies the severity of sleep apnea before treatment begins, providing a necessary baseline for comparison. For adults, an AHI of less than 5 events per hour is considered normal. This minimal level of events is common even in healthy individuals and is not typically a cause for concern.
A diagnosis of mild sleep apnea is given when the AHI falls between 5 and 15 events per hour. Moderate sleep apnea is classified when the number of events increases to between 15 and 30 per hour. Severe classification applies when a person experiences 30 or more breathing events every hour of sleep. These untreated scores highlight the extent of the breathing problem the CPAP machine is intended to correct.
The Target AHI Range for Effective CPAP Therapy
The primary goal of CPAP treatment is to reduce the AHI from the patient’s baseline score to a level considered normal for a healthy adult. Effective CPAP therapy is defined as achieving an AHI of less than 5 events per hour. This target indicates the machine successfully maintains an open airway and eliminates the vast majority of obstructive events.
Many sleep specialists aim for an even lower residual AHI, often encouraging patients to target a score below 2 or even 1 event per hour. While an AHI of zero is rarely achieved or expected, brief respiratory events can still occur even with optimal pressure settings. The remaining events, known as the residual AHI, are typically not associated with significant symptoms or adverse health effects once the score is below the clinical threshold of 5. A consistently low AHI confirms the prescribed air pressure is adequate and the patient receives the full therapeutic benefit.
Common Reasons for Elevated AHI During CPAP Use
If a patient is diligently using their CPAP machine but their AHI remains above the target range, the cause is often related to equipment or compliance issues rather than a treatment failure.
One of the most frequent problems is a mask leak, which occurs when the seal around the nose or mouth is compromised. A poor fit allows pressurized air to escape, meaning the necessary pressure is not delivered to the airway. This can cause the machine to register false events or fail to prevent real ones.
Another common factor is that the prescribed air pressure may simply be too low to overcome the severity of the patient’s airway collapse. Sleep apnea severity can fluctuate due to weight changes, medication use, or other factors. This means the original pressure setting may no longer be sufficient to keep the airway open throughout the night.
An elevated AHI can also be caused by positional apnea, where breathing events occur predominantly when the patient sleeps on their back. When these positional events are not accounted for, the overall AHI can remain high despite otherwise successful therapy.
Patients should first troubleshoot their mask fit and usage habits. If the AHI remains persistently high, it signifies the need to contact a sleep physician. A specialist may need to adjust the pressure settings or investigate other potential medical causes for the residual breathing events.