CPAP therapy relies on analyzing collected data, presented as metrics that track breathing events. While the overall Apnea-Hypopnea Index (AHI) is well-known, the Central Apnea Index (CAI) is a separate, important metric often confusing to users. Understanding the CAI is crucial because it reveals a distinct type of breathing event that standard CPAP may not treat. The CAI measures how often the brain’s signals to breathe are interrupted, signifying a neurological rather than a mechanical issue.
Defining the Central Apnea Index
The Central Apnea Index (CAI) quantifies the frequency of central apnea events occurring during CPAP use. It is calculated by dividing the total number of central apnea events recorded by the total hours the device was in use, resulting in the average number of central apneas per hour. A central apnea event is defined as a complete cessation of airflow lasting ten seconds or more. The key characteristic is the absence of any respiratory effort during the pause. A CAI of five or more events per hour is generally considered significant. The goal when reviewing CPAP data is a CAI well below this threshold, indicating the event type is controlled or absent.
The Physiology of Central Apnea
A central apnea event stems from a temporary failure in the brain’s respiratory control center, located in the brainstem. The brain fails to send the necessary signal to the diaphragm and respiratory muscles, meaning the patient makes no attempt to inhale. The airway remains fully open, distinguishing it from mechanical breathing issues. This signaling lapse is often caused by an unstable ventilatory control system, known as “high loop gain.” Breathing is regulated by the balance of carbon dioxide (\(\text{CO}_2\)) in the blood. If the \(\text{CO}_2\) level drops too low, crossing the “apneic threshold,” the brain pauses its signal, resulting in a central apnea.
Distinguishing Central from Obstructive Events
The CAI is isolated from the total Apnea-Hypopnea Index (AHI) to distinguish it from the Obstructive Apnea Index (OAI), as the two event types have different physiological origins. Obstructive Apnea (OA) occurs when throat muscles relax, causing the airway to physically collapse and block airflow despite the patient’s effort. The OAI measures the frequency of these physical blockage events per hour. Central Apnea (CA) involves a complete lack of respiratory muscle effort, indicating a neurological lapse in the drive to breathe. The overall AHI sums obstructive events, central events, and hypopneas. Separating the CAI is crucial because standard CPAP, designed to keep the airway open, cannot correct a problem rooted in the brain’s signaling.
Addressing Elevated CAI Readings
A consistently elevated CAI (five or more events per hour) suggests the patient may have underlying Central Sleep Apnea (CSA) or Treatment-Emergent Central Sleep Apnea (TECSA). TECSA, also called Complex Sleep Apnea (CompSAS), occurs when CPAP eliminates obstructions but unmasks or induces central events by changing the \(\text{CO}_2\) balance and increasing ventilatory instability. If the CAI remains high despite optimal CPAP pressure, the standard machine is ineffective for the central component. The user must consult a sleep physician, as this requires a change to a more advanced therapy. This often involves Adaptive Servo-Ventilation (ASV), which monitors breathing and automatically adjusts pressure support to stabilize the respiratory pattern.