Complex Apnea: Brain Signals, ASV Therapy, and Risks

Complex apnea is a form of sleep-disordered breathing in which central apneas appear or worsen specifically when a person with obstructive sleep apnea begins treatment with continuous positive airway pressure (CPAP). The formal name in current sleep medicine classification is treatment-emergent central sleep apnea, or TECSA. What makes it puzzling is the timing: the CPAP successfully eliminates the throat-collapse events it was prescribed for, yet a different kind of breathing pause, driven by the brain rather than a blocked airway, takes their place. The condition sits at an uncomfortable intersection of “treatment working” and “new problem emerging,” which is why it catches both patients and clinicians off guard.

How Complex Apnea Is Defined

The International Classification of Sleep Disorders, third edition, set out specific criteria for TECSA. A person must have primary obstructive sleep apnea on the initial diagnostic sleep study, show significant resolution of those obstructive events during CPAP titration, and then develop or retain central apneas at a rate of at least five per hour, with central events making up more than half of all remaining events. The central breathing pauses also cannot be better explained by another standalone central sleep apnea disorder, such as Cheyne-Stokes respiration tied to heart failure.¹ The term “complex sleep apnea syndrome” was coined earlier by researchers who noticed the phenomenon during titration studies, and you will still see “CompSAS” in older literature. The newer TECSA label reflects the field’s emphasis on the fact that the central events emerge as a consequence of treatment.

Why the Brain Stops Sending the Breathing Signal

To understand why CPAP can trigger central apneas, you need to know that during sleep, your drive to breathe is almost entirely controlled by carbon dioxide levels in the blood. When CO₂ rises, the brain says “breathe.” When it falls below a certain threshold, the brain briefly stops sending that signal, and you get a central apnea. In waking life, voluntary breathing keeps you going even if CO₂ dips. During sleep, that backup system is offline.

Research has shown that central apneas during sleep are consistently preceded by drops in CO₂. In one study, raising CO₂ levels by just a tiny amount, through either inhaling a CO₂-enriched gas or breathing through added dead space, virtually eliminated central apneas: the number of breathing pauses fell from over 40 per hour to fewer than 6.² This establishes that the apneas happen because CO₂ drops below the threshold the brain needs to keep triggering breaths.

CPAP can push a person past that threshold in a couple of ways. When CPAP opens the airway and ventilation suddenly improves, the lungs may blow off more CO₂ than before. If the pressure is set higher than needed, it can over-ventilate the patient, driving CO₂ even lower. At the same time, the inflation of the lungs at higher pressures can trigger stretch receptors that reflexively suppress the drive to breathe, a response sometimes called the Hering-Breuer reflex.³ The net result is the same: the brain’s breathing command goes quiet for several seconds at a time.

Another contributor is something sleep scientists call “loop gain,” a measure of how aggressively the breathing control system responds to disturbances. A person with high loop gain overreacts to small changes in CO₂: a tiny dip in CO₂ triggers a pause, the pause lets CO₂ build up, the buildup triggers an exaggerated breath, and the cycle repeats. The transitions between wakefulness and sleep, along with the responses of the throat muscles, effectively amplify this instability.⁴ People whose breathing control is already “twitchy” in this way are more likely to develop complex apnea once CPAP is introduced.

Who Is More Likely to Get It

Several factors raise the odds. Heart failure is one of the best-studied. Patients with heart failure who develop central sleep apnea tend to have a stronger-than-normal ventilatory response to CO₂. In one study, heart failure patients with sleep apnea had roughly two and a half times the CO₂ sensitivity of those without it, and sensitivity correlated with the number of breathing pauses per hour.⁵ That hair-trigger CO₂ response, layered on top of the ventilatory changes CPAP introduces, creates fertile ground for complex apnea.

Chronic opioid use is another well-recognized risk factor. Opioids depress the brainstem’s respiratory centers in ways that alter both the rhythm and depth of breathing during sleep. Patients on long-term opioids often show mixed patterns of central and obstructive apneas, and they are a population in which adaptive servoventilation has been specifically studied.⁶

Beyond individual physiology, there are modifiable technical factors. Mask leak during CPAP titration is associated with the development of central apneas. A study comparing patients who did and did not develop central events found that those with a central apnea index of five or more had significantly higher maximum mask leak rates. Among patients using a nasal mask, the difference in both average and maximum leak was even more pronounced.⁷ The likely explanation ties back to CO₂: a large leak lets exhaled CO₂ escape instead of being partially rebreathed, driving levels down and nudging the patient closer to the apneic threshold. This finding has practical implications: making sure the mask fits well and the leak is minimized could reduce the chance of complex apnea showing up during a titration night.

Altitude Makes It Worse

If you live in or travel to a high-altitude area, the risk of treatment-emergent central apneas goes up substantially. Central sleep apnea at altitude occurs because the low oxygen in thin air stimulates a strong ventilatory response, which blows off CO₂ and brings it closer to (or below) the apneic threshold. This happens in susceptible individuals above about 2,000 meters, and at very high elevations it affects most people.⁸

A study comparing CPAP titration outcomes across three sleep labs at different elevations in the Mountain West region of the United States found striking differences. At the lowest-elevation site, about 11% of patients developed a central apnea index above five during CPAP. At a moderately higher site, 22% did. At the highest site, nearly 39% met that threshold. The mean number of central apneas per hour on treatment ranged from fewer than 5 at the lowest site to over 19 at the highest. Titration quality was significantly worse at the highest altitude, and patients there were more likely to need repeat titrations.⁹ For anyone being started on CPAP above a few thousand feet, this is worth discussing with a sleep specialist, because a lab night at altitude may overestimate the severity of complex apnea compared to what would happen at sea level.

Does It Go Away on Its Own

One of the most reassuring aspects of complex apnea is that it often resolves spontaneously. The current understanding treats it as a dynamic process. In most patients, central apneas that emerge during early CPAP use fade over weeks to months of continued therapy. In a smaller subset, the events persist. And in a minority of patients, central apneas may appear for the first time only after weeks or months on CPAP, not on the initial titration night.¹⁰ Because of this transient nature in the majority, many sleep physicians adopt a wait-and-see approach, continuing CPAP for several weeks and re-evaluating before switching to a different device. Jumping to a more complex machine after one bad titration night may be premature, especially if mask leak or excessive pressure could have been contributing.

Adaptive Servoventilation and When It Helps

For patients whose complex apnea persists, adaptive servoventilation (ASV) is the most studied alternative. ASV devices adjust the amount of pressure support breath by breath: when the device detects that breathing is slowing or pausing, it increases support to maintain ventilation, and when the patient is breathing normally, it backs off. The goal is to smooth out the oscillation between overbreathing and not breathing at all.

The results in clinical studies have been strong for reducing apnea events. In one early efficacy study, ASV brought the average apnea-hypopnea index down to about 5 events per hour, a dramatic improvement compared to both baseline and CPAP alone.¹¹ A retrospective case series found that ASV virtually eliminated central apneas at optimal pressure settings, achieving an index of five or fewer events per hour in 80% of patients.¹²

A prospective randomized trial directly comparing ASV with CPAP in complex apnea patients found that at 90 days, about 90% of those on ASV had achieved a total apnea-hypopnea index below 10, compared with roughly 65% of those on CPAP. The central apnea index specifically was much lower in the ASV group.¹³ But that same trial also illustrated the dynamic nature of the condition: many CPAP patients improved too, reflecting the spontaneous resolution that happens in a significant fraction of cases.

A Critical Safety Warning for Heart Failure Patients

ASV works well for many people with complex apnea, but there is one population in which it can be dangerous. The SERVE-HF trial, a large randomized study, tested ASV in patients who had heart failure with reduced ejection fraction and predominantly central sleep apnea. While ASV improved the breathing metrics as expected, it increased both all-cause and cardiovascular mortality.¹⁴ The reasons are still debated. One hypothesis is that central apneas in severe heart failure may serve a compensatory role, giving the heart periodic rest, and overriding them with mechanical ventilation removes that protective effect. Whatever the mechanism, the clinical consequence was immediate: ASV is now contraindicated in patients with heart failure and a reduced ejection fraction. If you have heart failure and complex apnea, this is a conversation your sleep specialist and cardiologist need to have together before any treatment change.

Acetazolamide as a Drug Option

For patients who cannot use ASV or whose complex apnea is mild, the carbonic anhydrase inhibitor acetazolamide has shown promise. This medication works by causing a mild metabolic acidosis, which effectively raises the CO₂ set-point and makes the brain less likely to stop sending breathing signals. A systematic review and meta-analysis found that acetazolamide lowered the apnea-hypopnea index by roughly 38% compared with controls, with the effect being similar in obstructive and central sleep apnea. Higher doses, up to 500 milligrams per day, produced greater reductions. Acetazolamide also improved the lowest oxygen level during sleep and several measures of sleep quality.¹⁵ The evidence is mostly from short-term studies, so long-term data are limited, and the drug comes with side effects like tingling in the fingers, taste changes, and kidney stone risk. Still, it is a useful tool in the right patient, and it is sometimes used off-label for altitude-related central apnea as well.

Phrenic Nerve Stimulation for Central Apnea

An implantable device called the remedē System takes a completely different approach. Rather than delivering air pressure, it stimulates the phrenic nerve, the nerve that controls the diaphragm, with small electrical impulses timed to the patient’s breathing pattern. The device is implanted similarly to a cardiac pacemaker, with leads placed near the phrenic nerve through the venous system.

In the pivotal randomized trial, 51% of patients receiving active therapy met the primary outcome of at least a 50% reduction in the apnea-hypopnea index, compared with just 11% in the control group. About 91% had no serious adverse events over 12 months, and non-serious events like lead displacement were mostly resolved with device reprogramming.¹⁶ Five-year follow-up data showed sustained improvements, with the central apnea index dropping from a median of 23 events per hour at baseline to just 1 at five years. Serious adverse events related to the implant, device, or therapy occurred in 14% of patients, primarily during the first year, and none caused long-term harm.¹⁷

The device also improved nighttime oxygen levels. In the randomized trial, the median time spent with oxygen saturation below 90% fell from 33 minutes at baseline to 14 minutes at 6 months in the treatment group, a meaningful reduction in the hypoxic burden the body endures overnight.¹⁸ Post-hoc analysis has even suggested potential reductions in mortality and heart failure hospitalizations, though those findings come from reanalysis rather than a trial powered to detect them.¹⁹ Phrenic nerve stimulation is currently approved for moderate to severe central sleep apnea and represents a potentially important option for patients who cannot tolerate or safely use pressure-based therapies.

How Complex Apnea Differs from Cheyne-Stokes Breathing

People sometimes confuse complex apnea with Cheyne-Stokes respiration, and the overlap is real but the distinction matters clinically. Cheyne-Stokes respiration is a specific pattern of periodic breathing most commonly seen in heart failure, in which breathing gradually ramps up, hits a peak, then tapers off into a central apnea, only to ramp up again. The waxing-and-waning tidal volume creates a distinctive crescendo-decrescendo pattern on a sleep study. Patients with Cheyne-Stokes respiration are generally hypocapnic even before any treatment is applied, with their CO₂ already sitting close to the apneic threshold. The respiratory control instability that drives it is present at baseline, not triggered by therapy.

Complex apnea, by contrast, begins with a standard obstructive picture. The central apneas only show up after CPAP is introduced, and in many cases they were not present on the diagnostic study at all. The clinical management is also different: Cheyne-Stokes respiration in the setting of heart failure with reduced ejection fraction carries the ASV safety concern discussed above, whereas complex apnea in a patient without significant heart failure can often be treated with ASV safely and effectively. A sleep physician interpreting a titration study needs to know which pattern is present, because the treatment paths diverge.

Practical Steps If You Are Told You Have Complex Apnea

If your sleep lab tells you that central apneas showed up during your CPAP titration, the first question worth asking is whether anything modifiable contributed. Was there significant mask leak? Was the pressure set higher than necessary? Were you at altitude? Each of these factors can inflate central apnea numbers during a single night in ways that may not reflect your long-term response to therapy.

If the decision is to continue CPAP and monitor, that is a reasonable approach for many patients. Keeping your mask fitted properly and reporting any symptoms of disrupted sleep over the following weeks gives your sleep team the information they need to decide whether the problem is resolving. If central apneas persist and you are symptomatic, a trial of ASV or a discussion about acetazolamide may follow, depending on your underlying health. Heart failure patients should expect a more cautious workup before any device change, given the SERVE-HF findings. And for the subset of patients with severe, refractory central apnea who cannot use positive airway pressure devices, phrenic nerve stimulation offers a surgically implanted alternative with durable results over at least five years of follow-up.