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

Continuous positive airway pressure (CPAP) is a breathing device prescribed to treat sleep apnea. The purpose of a CPAP is to improve breathing at night. But how do you know if the treatment is working well enough?

The apnea-hypopnea index (AHI) is a helpful way to measure how severe your sleep apnea is. It can also help you see how well your body is responding to treatment.

This article explains how to use the AHI to optimize your CPAP machine. It also answers questions such as these:

• What does the AHI reading mean on a sleep study or CPAP machine?
• What is considered an event?
• If the AHI number is higher than usual, how can you adjust the CPAP machine?

What Is AHI?

First, it is important to understand what the apnea-hypopnea index (AHI) reading means.

The AHI is the average number of times your body has an apnea or a hypopnea event in one hour of sleep.

Apnea is when your breathing pauses while you sleep. The pause can be caused by your tongue or the soft palate at the back of your mouth closing off the airway.

Each time apnea happens, it can cause you to wake up briefly. It can also cause the amount of oxygen in your blood to drop. When air stops flowing through your nose and mouth, you are having an apnea event, even if your chest and abdomen are moving as you try to breathe.

Hypopnea refers to a period of shallow breathing. It is considered less severe than apnea. Researchers define hypopnea as a 30% drop in the amount of air flowing through your airways.

If you take shallow breaths, you may have hypoventilation, which means you're not taking in quite enough air to meet your body's oxygen needs. Hypoventilation can also happen if you have an unusually low respiratory rate. Your respiratory rate is the number of breaths you take in one minute.

Some researchers argue that hypopnea and apnea are almost identical events.

To qualify as an apnea or hypopnea event, the change in breathing must last at least 10 seconds.

The AHI counts both types of breathing disturbance. The number is used to rate the severity of sleep apnea. It is also used to measure how well treatments such as CPAP machines are working.

Where Your AHI is First Measured: A Sleep Study

The first place you may see an AHI is on a sleep study report.

To diagnose sleep apnea, specialists use a sleep study, also called a polysomnogram. These studies usually take place in a sleep disorder center. Sometimes sleep apnea is diagnosed using home sleep apnea testing.

These studies track your breathing patterns through the night. They rely on sensors such as these:

• Nasal cannula (or thermistor) with plastic prongs that sit in the nostrils
• Respiratory effort belts that stretch across the chest and/or stomach
• An oximeter clip that measures oxygen and pulse rate continuously by shining a laser light through a fingertip

The sensors show how many times you stop breathing or breathe shallowly during the night. They also can detect changes in the oxygen levels in your blood.

Some sleep facilities use other measures than the AHI. The respiratory-disturbance index (RDI) counts apnea and hypopnea events plus less severe breathing interruptions.

The oxygen-desaturation index (ODI) only counts the number of apnea or hypopnea events per hour that lead to an oxygen drop of at least 3%. This oxygen drop may raise your risk of long-term cardiovascular problems such as high blood pressure, heart attack, and heart failure. The oxygen drop could also lead to stroke, dementia, or other neurological problems.

If your sleep study does not contain these more specific measures, it's nothing to worry about.

What's a Good Goal for AHI?

Your breathing can change from one night to the next. For that reason, it probably isn't useful to try to reach a specific daily number. Sleep apnea may be worsened by:

• Sleeping more on your back
• Having more rapid eye movement (REM) sleep
• Using muscle relaxants
• Drinking more alcohol near bedtime
• Having nasal congestion or a stuffy nose due to a cold or allergies

Because your AHI can be affected by these factors, it may be best to calculate your average results over 30 to 90 days. The optimal goal for you may depend on how severe your condition is. Your goal could also depend on what's causing your breathing problem.

What Is an AHI Used For?

The AHI will be used to help choose the best treatment for you.

• CPAP machines are appropriate for mild, moderate, or severe sleep apnea.
• An oral appliance may be a better choice for mild or moderate sleep apnea. An oral appliance is a mouth guard a dentist makes for you. It holds down your tongue to keep it from blocking your airway.
• Surgery may be the best option if your apnea is related to your anatomy.
• Positional therapy may be helpful if your sleep apnea is worse when you sleep on your back.
• Other treatments may be recommended based on your AHI.

To interpret your AHI, your healthcare provider will consider the big picture: your baseline sleep study, anatomy, medications, change in health status, and other factors.

If you have surgery or you use an oral appliance, you may need another test to make sure the treatment has been effective in lowering the AHI.

How CPAP Machines Detect a Breathing Problem

The basic principle is that the machine pushes out air at a steady pressure. It can also send out short bursts of extra pressure. If your airway is partly blocked, the CPAP can measure how much the flow of air slows down.

In “auto” machines, when the machine senses that something is limiting the airflow, it turns up the pressure to better support your airway.

The measurements of CPAP machines used at home aren't always accurate. If readings stay high without a good explanation, you may need a new sleep study.

How CPAP Machines Treat Sleep Apnea

Modern CPAP machines track the number of breathing events occurring at your current pressure setting. Each event means that you're waking up briefly or having a short-term drop in blood oxygen level.

Using your CPAP will not necessarily prevent the sleep apnea entirely. 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. It will remain collapsed. If the pressure is set too low on your CPAP machine, your upper airway can still collapse. This could mean ongoing hypopnea or apnea events. Your symptoms could persist if your treatment doesn't go far enough.

Can You Have Apnea Events Even If Your CPAP Is Working?

You can continue to have sleep apnea events and other breathing problems even when you're using the CPAP machine. Newer machines can track those events and report them in an AHI.

The results may be visible on the device, or you may be able to check them using an app or website. This information can provide you with information about how effective your treatment is. When the CPAP calculates an AHI, your equipment provider or physician can also download this data and create a plan to improve your treatment.

Can Adjusting Your CPAP Pressure Change Your AHI?

A board-certified sleep specialist is the best person to decide which CPAP setting will work for you. The sleep specialist will consider the average AHI and your treatment experience. Though anyone can adjust the settings, your healthcare provider should make this informed decision.

Lots of factors can influence an AHI reading, and a high AHI doesn't necessarily require a high CPAP pressure. When deciding how much pressure you need, keep your own anatomy in mind.

If you have trouble adjusting to the treatment, lower pressure settings may make you more comfortable. If the settings are too low, however, it may not work well enough to resolve the condition.

If the device is turned up too high, it may cause central sleep apnea to occur. This is when your brain doesn't tell you to breathe during sleep.

A CPAP device can be adjusted to correct for different types of issues, including:

• Obstructive apnea (airway collapse)
• Central apnea (breath-holding episodes)
• Unknown issues (often related to leaky masks)

Different issues will need to be resolved in different ways. For example, the CPAP pressure may need to be turned up or down. Or the mask may need fitting or replacement.

Summary

An AHI is a measurement of how many sleep apnea or hypopnea events you experience during an hour of sleep. The term "apnea" refers to a pause in your breathing. Hypopnea refers to a decrease of 30% or more in the amount of air flowing in as you breathe.

During your initial sleep study, you'll receive a baseline AHI. That information will be used, along with other factors, to find the best treatment options for you.

If you use a CPAP device, it will continue to calculate AHI numbers as you sleep. Your sleep specialist can use this information to make sure you're getting the sleep treatments you need to protect your health.

A Word From Verywell

If you have questions about what AHI means to your condition and whether your CPAP is working as well as it should, contact your sleep specialist to discuss your options. Regular follow-up in a clinic will help ensure that your treatment is a success.