What Is the Goal AHI With CPAP Treatment of Sleep Apnea?

Monitoring the AHI Can Guide Sleep Apnea Therapy and Maximize Benefits

woman with sleep apnea mask
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Continuous positive airway pressure (CPAP) is commonly prescribed to treat sleep apnea. The goal is to improve breathing at night, but how do you know if the CPAP is working well enough? The apnea-hypopnea index (AHI) can be a helpful measure to track the effectiveness of your treatment.

What does the AHI reading mean on a sleep study or CPAP machine? What is considered an event? If the AHI number is elevated, you may ask, "How do I adjust my CPAP machine?"

Learn what your goal AHI should be to maximize the benefits of using CPAP for optimal therapy and how the pressures are determined and should be adjusted.

What Is AHI?

First, it is important to understand what the AHI reading means, both with sleep studies and on a CPAP machine.

This measurement is often presented within the context of a sleep study report. It is the number of times per hour of sleep that your upper airway partially or completely collapses, leading to a brief arousal (or awakening) from sleep or a drop in blood oxygen levels. The partial collapse of the airway is called a hypopnea. The complete absence of airflow through the nose and mouth, despite an effort to breathe as measured at the chest and abdomen, is called an apnea event.

The AHI is used to classify the severity of sleep apnea. This same classification is used to assess how well the CPAP machine is working. The following guidelines are used in adults:

  • Normal: Fewer than 5 breathing events per hour of sleep
  • Mild sleep apnea: 5 to 15 breathing events per hour of sleep
  • Moderate sleep apnea: 15 to 30 breathing events per hour of sleep
  • Severe sleep apnea: Greater than 30 breathing events per hour of sleep

Children’s sleep is analyzed with stricter criteria and more than one event per hour of sleep is considered to be abnormal on sleep testing.

What Is the Goal AHI to Optimize Therapy?

What should your goal AHI be? First, understand that there can be night-to-night variability in this measure. As noted, sleep apnea may be worsened by sleeping more on your back, having more REM sleep, or even by drinking more alcohol near bedtime. Therefore, it is not useful to chase a daily number. Rather, these variations should be averaged out over 30 to 90 days.

In general, the AHI should be kept at fewer than 5 events per hour, which is within the normal range. Some sleep specialists will target an AHI of 1 or 2 with the thinking that fewer events will be less disruptive to sleep. If the baseline AHI on the sleep study is quite high, such as 100 events per hour, even 10 events per hour may represent a significant improvement.

The optimal goal for you may depend on the severity and nature of your initial condition. It may be tempered by your compliance to treatment, with lower pressures allowed to improve comfort. The best pressure setting for you is best determined by your board-certified sleep specialist with the average AHI used in the context of your experience with the treatment.

How Do CPAP Machines Work for Treatment?

Modern CPAP and bilevel machines are able to track the residual number of breathing events occurring at your current pressure setting.

Each of these events could correlate with a brief awakening or a transient drop in the blood oxygen level.

You may believe that using your CPAP will prevent the condition entirely, but this is not necessarily the case. It depends, in part, on the pressure set by your sleep specialist.

Imagine trying to inflate a long floppy tube by blowing air into it. With too little air, the tube will not open and it will remain collapsed. Similarly, if the pressure is set too low on your CPAP machine, your upper airway can still collapse. This may result in either persistent hypopnea or apnea events. Moreover, your symptoms may persist because of inadequate treatment.

It is also possible that other variables will affect the amount of pressure that is needed to keep your airway open. These factors include sleep position (worse when sleeping on the back), sleep stage (worse in REM sleep), nasal congestion, and the use of alcohol or medications that act as muscle relaxants.

How a CPAP Machine Tracks Residual Sleep Apnea Events

Newer machines can track your residual abnormal breathing events and generate an AHI. This may be accessible on the device or via associated tracking web sites or apps. How is this accomplished?

Well, the short answer is that these methods are proprietary, confidential, and are not disclosed by the companies who make the devices. In simple terms, however, consider that the machine generates a constant pressure. It can also generate intermittent bursts of additional pressure. It then measures the resistance within the airway to this additional pressure.

If there is no clear difference in resistance between the lower and higher pressures, it is understood that the airway is open. However, if the airway is still partially (or even completely) collapsing, the additional pressure may encounter resistance. In “auto” machines, this will prompt the machine to turn up the pressure within the prescribed range to better support your airway.

Remember that this measurement is not as accurate as that which occurs in a formal sleep study. The measurement may be compromised by a high mask leak. If it remains elevated without a good explanation, it may require a repeat sleep study to assess your condition.

The AHI that the machine calculates is then recorded on the compliance data card. Your equipment provider or physician can then download this and generate a report to direct your treatment. It also may be displayed in the morning on the device’s screen or user interface. This information can also be shared to the cloud and provide you information about the effectiveness of your therapy with related programs.

How to Determine Pressure Needs and Adjust a CPAP

As noted above, there are a number of variables to consider when interpreting the AHI reading. It may not be resolved by simply turning the device's pressure up or down.

The needed amount of change may also be complicated. For example, someone with a high AHI on a sleep study may not necessarily require a high CPAP pressure to resolve the condition. Anatomy and other factors may have a role. If the device is turned up too high, it may cause central sleep apnea to occur. If the settings are too low, it may not work well enough to resolve the condition. 

If the AHI remains elevated, it is time to return to your board-certified sleep specialist for an assessment. This provider can interpret the AHI in the context of your baseline sleep study, anatomy, medications, change in health status, and other factors. This person should be the one to adjust your CPAP machine's settings. Though the settings can be changed by anyone, only your provider can make an informed decision.

If you have questions about whether your CPAP is working as well as it could be for you, contact your provider to discuss your status and the options available to optimize your therapy. Regular follow-up in clinic will ensure that your treatment is a success.

Sources:

"Common AHI Definitions." FAQ, Philips Respironics DreamMapper.

Kryger, M.H. et al. "Principles and Practice of Sleep Medicine." ExpertConsult, 6th edition. 2017.

"Why does my apnea-hypopnea index (AHI) change?" ResMed.

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