CAI stands for Central Apnea Index, and it measures how many times per hour your breathing completely stops during sleep due to your brain temporarily failing to signal your breathing muscles. A CAI of 5 or more per hour is the threshold used to diagnose central sleep apnea. Unlike the more commonly referenced AHI (Apnea-Hypopnea Index), which counts all types of breathing disruptions, the CAI zeroes in on one specific kind: pauses that originate in the brain rather than from a physical blockage in your airway.
How CAI Is Calculated
During a sleep study (polysomnography), sensors track your breathing effort, airflow, oxygen levels, and brain activity throughout the night. Technicians review the data and classify each breathing pause by type. A central apnea is one where airflow stops but there’s no evidence your chest or abdomen were trying to breathe against a blocked airway. Your brain simply didn’t send the signal.
The CAI is the total number of these central apnea events divided by the total hours of sleep. So if you had 40 central apneas over an 8-hour study, your CAI would be 5 per hour, right at the diagnostic cutoff. For a formal diagnosis of central sleep apnea, clinicians also look for at least 50% of all apnea events to be central in origin, not obstructive.
CAI vs. AHI
The AHI is the broader number most people see on their sleep study results. It counts every apnea (complete breathing pause) and every hypopnea (partial reduction in airflow that drops your oxygen or briefly wakes you) per hour of sleep. An AHI under 5 is considered normal. The AHI doesn’t distinguish between obstructive events, where your airway physically collapses, and central events, where your brain pauses its breathing signals.
That distinction matters because the two types have different causes and different treatments. You could have a high AHI driven almost entirely by obstructive events, entirely by central events, or a mix of both. The CAI pulls out just the central component, giving a clearer picture of what’s actually happening. Two people with the same AHI of 30 could have very different treatment paths depending on their CAI.
What Causes a High CAI
Central apneas happen when the brainstem, which controls automatic breathing, temporarily loses its rhythm. Several conditions make this more likely:
- Heart failure: The most common medical cause. Fluid shifts and unstable blood gas levels create a breathing pattern called Cheyne-Stokes respiration, where breathing gradually speeds up, slows down, and stops in a repeating cycle.
- Stroke or brain injury: Damage to the brainstem or nearby areas can directly impair the breathing control center.
- Opioid use: Prescription painkillers suppress the brainstem’s drive to breathe, particularly during sleep.
- High altitude: Lower oxygen levels at elevation can destabilize breathing patterns, producing central apneas even in otherwise healthy people.
A small number of central apneas during sleep is normal. Healthy adults commonly experience a few per night, particularly during the transition from wakefulness to sleep. It only becomes clinically significant when the CAI reaches 5 or higher and symptoms like excessive daytime sleepiness, frequent awakenings, or morning headaches are present.
Treatment-Emergent Central Apnea
One of the more confusing situations involving CAI happens after someone starts CPAP therapy for obstructive sleep apnea. In some patients, the obstructive events resolve with CPAP, but new central apneas appear that weren’t there before. This is called treatment-emergent central sleep apnea, sometimes referred to as complex sleep apnea.
The exact mechanism isn’t fully understood, but the prevailing theory involves how CPAP changes pressure dynamics in the airway and chest. For some people, the positive pressure overshoots slightly, lowering carbon dioxide levels below the threshold that triggers the brain to initiate a breath. The brain essentially “forgets” to breathe for a few seconds until CO2 builds back up.
This is why monitoring CAI on CPAP is important, not just AHI. Your overall AHI might look acceptable on a CPAP report, but if the remaining events are predominantly central, the standard treatment may be part of the problem. In many cases, treatment-emergent central apneas resolve on their own within a few weeks to months of continued CPAP use. When they don’t, a different type of therapy is usually needed.
Treatment for a Persistently High CAI
Standard CPAP works well for obstructive sleep apnea but is often ineffective, and sometimes counterproductive, for central apneas. In a study of patients with central, mixed, and complex sleep apnea, CPAP titration still left the average AHI at about 34 events per hour, barely better than baseline.
Two alternatives perform significantly better. Bilevel positive airway pressure (often called BiPAP) with a backup breathing rate provides two different pressure levels and can trigger a breath if you haven’t taken one within a set time window. Adaptive servo-ventilation (ASV) goes a step further by continuously monitoring your breathing pattern and adjusting pressure breath by breath. In the same study, ASV reduced the average AHI from over 50 down to less than 1 event per hour, outperforming both CPAP and bilevel therapy.
There is one critical exception: ASV is not recommended for people whose central sleep apnea is caused by heart failure with a reduced pumping ability (low ejection fraction). In that population, ASV has been associated with worse outcomes. Your sleep specialist will factor in your cardiac health when choosing the right device.
Checking Your Own CAI
If you already use a CPAP or bilevel machine, many modern devices track central apneas separately from obstructive events. Apps like OSCAR (open-source CPAP analysis software) or manufacturer apps like myAir and DreamMapper can display your CAI alongside your overall AHI. A consistently elevated CAI on these reports, particularly above 5, is worth bringing up with your sleep specialist, as it may indicate that your current therapy settings need adjustment or that a different device would serve you better.