Continuous Positive Airway Pressure (CPAP) therapy is a widely used treatment for obstructive sleep apnea, providing pressurized air to keep the airway open during sleep. Modern CPAP devices record and display various data points each night to ensure the treatment is working effectively. These metrics offer a detailed picture of how well the machine prevents breathing disturbances and how consistently it is used. Understanding these numbers helps users monitor their treatment success and communicate effectively with healthcare providers regarding necessary adjustments.
The Apnea-Hypopnea Index
The Apnea-Hypopnea Index (AHI) is the most direct measurement of therapy effectiveness, representing the clinical outcome of CPAP use. It is the average count of apnea and hypopnea events experienced per hour of sleep. An apnea is a complete cessation of airflow lasting at least ten seconds, while a hypopnea is a partial reduction in airflow for the same duration, often accompanied by a drop in blood oxygen levels.
The AHI score is calculated by dividing the total number of breathing events by the total hours the machine was used. Before treatment, an AHI below 5 events per hour is considered normal, 5 to 14.9 is mild, and 30 or more is severe. The goal of effective CPAP therapy is to reduce this number below 5 events per hour, though many specialists aim for 1 or 2 for optimal health benefits. A consistently high AHI suggests the treatment is not working as intended, possibly due to incorrect pressure settings or an ill-fitting mask.
Interpreting Your Prescribed Pressure
The pressure number displayed on your CPAP device is measured in centimeters of water pressure (\(\text{cm H}_2\text{O}\)). This measurement indicates the force of the air delivered to your airway to prevent soft tissues from collapsing. Pressure is typically set within a range of 4 to \(20 \text{ cm H}_2\text{O}\), with the average prescribed setting often around \(10 \text{ cm H}_2\text{O}\).
If you use a standard CPAP machine, the displayed number is the single, fixed pressure setting determined by your sleep study, delivered constantly throughout the night. Auto-adjusting CPAP (APAP) machines operate differently, delivering pressure within a minimum and maximum range. APAP devices use algorithms to automatically increase or decrease pressure as needed to clear obstructions. This pressure setting represents the machine’s input, designed to achieve the desirable low AHI output.
Tracking Compliance and Equipment Performance
CPAP machines track metrics related to user adherence and equipment integrity, in addition to AHI and pressure. Usage hours measure the total time the machine was used during a sleep period. This metric is monitored by healthcare providers and insurance companies to confirm adherence, often requiring a minimum threshold to maintain device coverage.
Compliance is typically defined as using the machine for at least four hours per night for 70% of nights within a 30-day window. Consistent use for at least four hours is associated with health improvements, including reduced daytime sleepiness. The CPAP’s leak rate, measured in liters per minute (\(\text{L/min}\)), assesses the mask seal performance.
An acceptable leak rate is generally considered to be below \(24 \text{ L/min}\). If the leak rate consistently exceeds this threshold, it suggests a problem with the mask fit, such as a loose strap or worn cushion. A high leak rate compromises therapy effectiveness because the machine struggles to maintain the prescribed pressure level, potentially leading to a higher AHI.