Continuous Positive Airway Pressure (CPAP) therapy is the standard method for managing obstructive sleep apnea. To gauge treatment success, CPAP machines monitor a specific metric known as Events Per Hour (EPH) or the Apnea-Hypopnea Index (AHI). This score is the primary indicator of treatment efficacy, reflecting how often breathing disturbances occur while the device is in use. Understanding this score is fundamental for individuals managing their sleep apnea and for healthcare providers assessing the machine’s effectiveness.
What is Events Per Hour (EPH)?
Events Per Hour (EPH), often referred to by clinicians as the Apnea-Hypopnea Index (AHI), measures the average number of breathing interruptions recorded per hour of therapy use. The EPH score is calculated by adding the total number of apneas and hypopneas detected during a sleep session and dividing that sum by the total hours the CPAP machine was running. It provides an average rate of respiratory events.
This metric directly measures how successfully the positive air pressure prevents the collapse of the upper airway. A lower EPH number signifies that the airway remained open and stable throughout the night. The EPH displayed on a machine serves as a day-to-day guide for tracking the consistency and effectiveness of the prescribed pressure setting.
The Specific Events That Are Counted
The EPH score counts two distinct types of breathing disturbances: apneas and hypopneas. These events are measured by the CPAP device’s internal algorithm, which detects changes in airflow and pressure. Both event types must last for a minimum of ten seconds to be included in the hourly calculation.
Apnea
An apnea is defined as a complete or nearly complete cessation of airflow, typically a reduction of 90% or more. Obstructive Apnea occurs when soft tissues in the throat physically collapse and block the airway. The machine also tracks Central Apneas, where the brain temporarily fails to send the necessary signal to the muscles that control breathing.
Hypopnea
A hypopnea represents a partial collapse of the airway, resulting in shallow or restricted breathing. For an event to be categorized as a hypopnea, the airflow must be reduced by 30% or more compared to the baseline breathing. This partial blockage must also be accompanied by a noticeable drop in blood oxygen levels or an arousal from sleep. Hypopneas disrupt sleep architecture and contribute to the overall EPH score.
Interpreting Your EPH Score
The EPH score provides a direct assessment of how well the CPAP device controls sleep-disordered breathing. The primary goal of CPAP therapy is to reduce the EPH to fewer than five events per hour, which is considered the normal, non-apneic range for adults. Many specialists encourage patients to aim for two events per hour or less for optimal therapeutic benefit.
The severity ranges used for diagnosis help frame the effectiveness of the treatment score. Untreated scores between 5 and 15 events per hour are classified as mild sleep apnea, 15 to 30 is moderate, and more than 30 indicates severe sleep apnea. If a treated EPH remains consistently above the goal of five, it suggests the therapy is not fully effective at the current pressure setting.
If the EPH score remains elevated (e.g., consistently above 5 or 10), the individual should contact their sleep medicine provider. A high number suggests that the prescribed pressure may be too low or that the mask fit is compromised by a leak. Adjusting the pressure settings or addressing mask fit issues are necessary steps to ensure the CPAP therapy achieves the desired therapeutic outcome.