Continuous Positive Airway Pressure (CPAP) therapy is the standard treatment for obstructive sleep apnea, delivering a constant stream of pressurized air to keep your airway open while you sleep. Modern CPAP machines are sophisticated devices that record detailed data every night. Understanding these collected numbers is essential for evaluating the effectiveness of your treatment and optimizing your sleep health with your healthcare provider.
Your Prescribed Pressure Setting
The most foundational number related to your therapy is the prescribed pressure setting, measured in centimeters of water (cm H2O). This measurement represents the force of air required to prevent the soft tissues in your throat from collapsing and causing an obstruction. This setting is determined through a titration study, which identifies the exact amount of air needed to eliminate breathing events. Standard CPAP machines use a single, fixed pressure, typically between 4 and 20 cm H2O, delivered constantly throughout the night. In contrast, an auto-adjusting positive airway pressure (APAP) machine uses a pressure range, such as 5 to 15 cm H2O, and automatically adjusts the pressure in real-time based on your breathing patterns.
Decoding the Apnea-Hypopnea Index (AHI)
The Apnea-Hypopnea Index (AHI) is the most important metric for gauging the success of CPAP therapy, as it measures the frequency of disturbed breathing. The AHI is calculated by dividing the total number of apnea and hypopnea events detected during the night by the total hours of sleep, providing the average number of respiratory events per hour. An apnea is a complete cessation of airflow lasting at least ten seconds, while a hypopnea is a partial reduction in airflow lasting ten seconds or more, associated with a drop in blood oxygen levels.
Untreated sleep apnea is classified based on the AHI score: fewer than five events per hour is normal; five to 15 is mild; 15 to 30 is moderate; and 30 or more is severe. The primary objective of CPAP treatment is to reduce the AHI score to below five events per hour, which is considered successfully treated sleep. Achieving an AHI consistently lower than two is an even better indication that the therapy is working effectively. Some machines also record whether an event was an obstructive apnea (OAI) or a central apnea (CAI), helping your provider understand the nature of any remaining events.
Tracking Usage Hours and Leak Rate
Beyond the clinical success reflected in your AHI, your CPAP machine tracks metrics related to the practical application and quality of therapy delivery, starting with usage hours. This number records the length of time you use the machine each night, a metric tracked by both your healthcare provider and your insurance company. For insurance compliance, usage often requires using the device for at least four hours per night on 70% of nights within a 30-day period. Consistent usage is necessary because health benefits, such as reduced daytime sleepiness, are directly linked to the total time the airway is kept open.
The machine also reports a leak rate, measured in liters per minute (L/min), which reflects the quality of the seal between your mask and your face. A certain amount of air leakage is expected, as masks have exhalation ports to vent carbon dioxide. However, an excessive leak rate compromises therapy effectiveness by preventing the full prescribed pressure from reaching your airway. A leak rate consistently below 24 L/min is generally considered acceptable. Monitoring this number helps ensure your mask fit is correct and your equipment is properly maintained for reliable therapy delivery.