Interpreting a CPAP Data Download
Have you been wondering what the heck all those numbers mean on your CPAP data download? CPAP data downloads, also known as CPAP compliance reports, contain a multitude of data that can tell you how effectively your sleep apnea is being treated. But all the numbers and jargon can be confusing. Dr. Krainin walks you through how to make sense of the numbers.
Average usage (total days) is really the most important metric of all. If you’re not really using your CPAP machine and all of your other numbers are perfect, you’re not going to benefit from it. If you’re using your CPAP machine consistently and your other numbers (leak, AHI, etc.) are out of whack, these can be optimized and your commitment to therapy will likely lead to success over time.
If average usage (total days) was a letter grade, this is what key intervals would correspond to:
- <4 hours: F. You’re really not getting significant benefit from your machine.
- 4-6 hours: C. If you’re a CDL holder, you’re meeting your DOT requirements but you’re not getting the most out of your therapy. You've probably noticed that you feel somewhat better than you used to prior to starting CPAP.
- 6-7 hours: B. This is the amount of usage that scientific research indicates is associated with the maximum health benefits beginning to kick in.
- >7 hours: A+. This is presuming that you’re actually sleeping > 7 hours as well, not just watching TV with the mask on for part of that time. The vast, vast majority of adults need at least 7 hours of sleep per day and need to use their CPAP machine during that entire sleep period to feel their very best.
Your CPAP machine provides calculated apnea hypopnea index (AHI) data. Based on my experience, here are some general rules about AHI statistics on your data report:
- >5: Your sleep apnea is not effectively being treated and you’ll remain at risk for the long-term health risks of sleep apnea.
- 3-5: Your therapy is working fairly well but it’s not perfect. You’ve probably noticed a significant improvement in your sleep quality and you might feel somewhat more alert during the day but you could still feel better.
- <3: The best outcomes are associated with this range of AHI.
Central (clear airway or CA) vs. Obstructive (obstructed airway or OA) vs. Hypopnea
- CAs are episodes where there was no breathing effort for at least 10 seconds. These are essentially "breath holding spells" - your brain did not tell your lungs to breath during this period of time.
- OAs are episodes where your airway completely collapsed (but you were trying to breathe) for at least 10 seconds.
- Hypopneas can be obstructive, central, or mixed. They are episodes of shallow or restricted breathing for at least 10 seconds associated that are associated with a significant oxygen drop (or alternatively by some definitions, terminated in a brain awakening). Obviously the CPAP machine is not recording your oxygen levels or your brain waves so this metric is at best a loose approximation based on airflow - how much air is moving through your airway - which the CPAP machine CAN calculate.
How to Interpret the Subcomponents of AHI
- Central: if >2, usually indicative of a clinically significant degree of central sleep apnea. If you had no significant central apnea on your diagnostic sleep study then this suggests “complex sleep apnea.” Consult with your sleep doctor about this.
- Obstructive: if >2, you probably need to increase your pressure EXCEPT if your leak is high and/or your centrals are also high.
- Hypopneas >2: this is less specific and could indicate that you need a pressure increase OR decrease depending on the entire context.
One thing to keep in mind when interpreting your AHI data is that when data downloads have been compared to sleep studies performed while using CPAP (the gold standard) the data downloads tend to UNDERREPRESENT the degree of residual sleep apnea. So if your PAP machine calculated AHI is 5, it’s actually probably significantly higher in reality. The PAP machine is not running an accurate diagnostic sleep study on you every night; the AHI data is best viewed as a good rough estimate of your residual breathing event index. It’s useful to me as a sleep doctor in that an AHI ≥3 indicates a “red flag.”
The “root of all evil” – high leak can cause:
- Your pressures to go too high, giving invalid pressure requirement data and possibly leading to your machine being set at an inappropriately high pressure. This can interfere with adherence to therapy.
- Your AHI to go up. I’ve consistently seen high leak cause increases in both obstructive and central events. The cause of higher levels of obstructive respiratory events is clear – the more air that escapes “out of the system,” the less that gets to the back of your throat, where it’s needed. The underlying cause of why high leak can lead to elevated central respiratory events is less clear.
- Worse leak! High leak can lead to a vicious cycle, compounding the already high leak: the majority of PAP machines “compensate” for high leak, i.e. if your leak starts going too high, the machine ramps up the pressure in response, trying to get more air to your airway. However, when your CPAP mask seal is faulty, the increased pressure may exacerbate the leak.
Different CPAP systems have different leak parameters.
- ResMed: <24 L/min is good; you will get a happy face on your display in the morning. If leak is >24 L/min, you will get a sad face. In my experience a 95% (avg) leak <12 is indicative of a really good mask seal.
- Philips provides leak data as an “average percentage of night spent in high leak.” <1% of time spent in high leak is the target.
- If your CPAP machine is on a fixed pressure, your pressure will remain constant. Your download will typically show a whole number for the set pressure.
- If your machine is on auto mode, you will see several pressure statistics that may including the following depending on the brand of CPAP you're using:
- Mean (average) pressure: this is the average pressure that kept your airway open for 50% of the usage period.
- P95 percentile (average) (ResMed) or P90 percentile (average) (Philips Respironics) pressure: this is the pressure that kept your airway open 95% or 90% (respectively) of the time in a given period. Sleep doctors usually want to set your baseline - or starting pressure once the ramp period is over - to this pressure. I tend to find ResMed's P95 percentile the more effective target so will usually go up 1-2 cwp above Philips' P90 percentile when setting these machines.
- Maximum percentile (average): the average pressure that was needed to keep your airway 100% open for the period in question.
Less Common Variables
Some CPAP data downloads provide information on the following:
- Flow Limitation Index: also known as FL, this is an estimate of the percent of time that your airway was partially closed. Higher levels of FL are associated with more subcortical or "micro" awakenings and poorer quality sleep.
- Vibratory Snoring (VS): is an estimate of how much residual snoring you're having while using your CPAP.
- Cheyne Stokes Respirations: this indicates a more severe form of central sleep apnea. Any time spent in Cheyne Stokes respirations is concerning and should be evaluated further by your sleep doctor.
- RERA index: respiratory effort-related arousals (RERAs) are subtle obstructive respiratory events that don't rise to the level of a hypopnea. Frequent RERAs cause a condition known as upper airway resistance syndrome or UARS.
- Periodic breathing: if your "percent of night in periodic breathing (PB)" is greater than 1%, you definitely have a significant component of this form of central sleep apnea. When the PB index is <.2% it is insignificant. A PB index between .2 and 1% may be clinically significant depending on your situation.
Bilateral Positive Airway Pressure (BPAP) Data DownloadsBPAP or BiPAP downloads will have additional information that may include the following:
- Respiratory rate: an normal adult respiratory rate is usually defined as 12-16 breaths-per-minute.
- Tidal volume (Vₜ or TV): a normal adult tidal volume is considered to be approximately 500 mL per inspiration.
- Minute ventilation: between 6 and 9.4 L is normal in adults.
- IPAP: inspiratory positive airway pressure (IPAP) is the pressure that the machine is giving you on inhalation.
- EPAP: expiratory positive airway pressure (EPAP) is the pressure delivered by the machine for exhalation after it's been reduced by a set amount from the IPAP.