Sleep apnea is a common disorder characterized by repeated pauses in breathing or periods of shallow breathing during sleep. These breathing disruptions prevent the body from getting enough oxygen and fragment sleep, leading to daytime fatigue and other health issues. To accurately diagnose and gauge the severity of sleep apnea, specialists rely on various metrics, or indices, derived from an overnight sleep study called a polysomnography. The Central Apnea Index (CAI) is one such specific metric used to quantify a particular type of breathing event.
Defining the Central Apnea Index (CAI)
The Central Apnea Index (CAI) is a specific measurement that quantifies the frequency of central apneas during a sleep study. It is calculated by taking the total number of recorded central apnea events and dividing that sum by the total hours of sleep recorded.
A central apnea event is defined during polysomnography as a cessation of airflow for ten seconds or longer without any identifiable respiratory effort. This means the brain temporarily failed to send the necessary signal to the diaphragm and chest muscles to initiate a breath. This neurological failure distinguishes it from other types of breathing pauses where the body is still actively trying to breathe.
CAI Compared to AHI
The CAI is often evaluated in the context of the Apnea-Hypopnea Index (AHI), which is the most widely recognized metric for sleep apnea severity. While the CAI focuses exclusively on central apneas, the AHI provides a comprehensive summary of nearly all disruptive breathing events. The AHI is calculated by summing the total number of apneas and hypopneas—both central and obstructive—and dividing by the total sleep time.
An obstructive apnea is a pause in breathing lasting ten seconds or more where respiratory effort is present but airflow is blocked by a collapse of the upper airway. A hypopnea is a partial reduction in airflow, often associated with a drop in blood oxygen saturation or an arousal from sleep.
Because the AHI includes central apneas, obstructive apneas, and hypopneas, it represents the overall burden of sleep-disordered breathing. The CAI, however, isolates the events caused by a lack of respiratory drive from the brain, providing a specific look into the neurological component of the condition. For a diagnosis of Central Sleep Apnea (CSA), the CAI must typically be five or more events per hour, and these central events must make up more than 50% of the total AHI score.
Clinical Significance of the CAI Measurement
Tracking the Central Apnea Index is a foundational step in accurately diagnosing the specific type of sleep apnea a patient has. A high CAI score is the defining characteristic of Central Sleep Apnea (CSA), which is caused by a failure of the central nervous system to regulate breathing. This is distinct from Obstructive Sleep Apnea (OSA), where the airway physically collapses despite the brain’s signal to breathe.
Distinguishing between these two types is important because their treatments differ significantly. Standard continuous positive airway pressure (CPAP) therapy is the primary treatment for OSA but may be ineffective or even worsen central events in some patients. In fact, a condition known as treatment-emergent central sleep apnea can develop when CPAP is used to treat a predominantly obstructive disorder.
A high CAI reading often guides clinicians toward more specialized therapies, such as adaptive servo-ventilation (ASV) or specialized bi-level positive airway pressure (BiPAP) devices. These advanced devices can monitor the patient’s own breathing patterns and adjust pressure support to stabilize irregular breathing cycles. This targeted approach is particularly relevant for patients whose central sleep apnea is linked to underlying medical conditions like heart failure or certain neurological disorders.