CPAP Terms Explained: Complete Sleep Apnea Glossary From an RRT

You just got your sleep study results. Your doctor said you have obstructive sleep apnea and need CPAP. Then they handed you a pamphlet full of terms like AHI, titration, IPAP, and hypopnea — and sent you home.

I've been a Licensed Respiratory Therapist for over a decade. I've set up CPAP in ICUs, emergency departments, and living rooms across Georgia. Most patients leave their first CPAP appointment more confused than when they arrived.

This glossary is the resource I wish every new CPAP user had. Plain English. Clinical accuracy. Written by someone who uses these machines every single day — not a content farm.

If at any point you want a live walkthrough of your specific results or equipment, you can book a 30-minute RT Consultation for $49.99 and I'll review everything with you directly.

A

Adaptive Servo-Ventilation (ASV)

ASV is an advanced PAP mode designed for central sleep apnea and complex sleep apnea — cases where your airway isn't blocked, but your brain's breathing signal is unstable. Unlike standard CPAP machines, ASV monitors your breathing pattern in real time and adjusts pressure breath-by-breath to stabilize it. It's significantly more sophisticated than fixed or auto CPAP. According to the American Academy of Sleep Medicine (AASM), ASV is the preferred modality for complex and treatment-emergent central sleep apnea.

⚕️ RRT Note: ASV is contraindicated in patients with heart failure with reduced ejection fraction (HFrEF). This is exactly why a proper clinical evaluation matters before choosing equipment. If you're unsure what therapy is right for you, book an RT consultation.

Apnea

From the Greek for "without breath." Clinically, an apnea is a complete cessation of airflow lasting 10 seconds or more during sleep. Ten seconds sounds short — try holding your breath right now and count. That's happening dozens, sometimes hundreds of times per night with untreated severe sleep apnea. Each event drops blood oxygen, spikes cortisol, and fragments your sleep architecture.

The National Heart, Lung, and Blood Institute estimates over 30 million Americans have sleep apnea — the majority undiagnosed. If you suspect you have it, ask about our home sleep study option.

Apnea-Hypopnea Index (AHI)

AHI is the most important number from your sleep study. It counts how many apneas and hypopneas occur per hour of sleep on average. Your AHI determines your diagnosis severity and the urgency of treatment.

⚕️ RRT Note: An AHI of 14 (mild) can cause just as much daytime exhaustion as an AHI of 40 depending on your sleep architecture and oxygen desaturation depth. Don't dismiss a "mild" diagnosis — treat it. Our CPAP machine collection includes options for every severity level.

APAP (Auto-Titrating CPAP / AutoPAP)

APAP machines automatically adjust pressure breath-by-breath throughout the night based on real-time airway resistance. This means you get higher pressure when you're on your back in REM sleep (when apneas are worst) and lower pressure when you're on your side (when less is needed). Most modern machines operate in APAP mode by default.

Our most popular APAP machines include the ResMed AirSense 11 AutoSet and the ResMed AirSense 10 AutoSet (Certified Refurbished). Both are RT-inspected before shipping from our Augusta, GA facility.

B

BiPAP / BiLevel / BPAP

BiPAP (Bilevel Positive Airway Pressure) delivers two separate pressure levels: a higher pressure during inhalation (IPAP) and a lower pressure during exhalation (EPAP). This pressure drop during exhalation is the critical difference — it makes breathing out feel natural instead of like exhaling against a wall.

BiPAP is typically prescribed for patients with COPD, obesity hypoventilation syndrome, neuromuscular disease, or for OSA patients who cannot tolerate standard CPAP pressure. Insurance often requires a documented CPAP failure before approving BiPAP.

Browse our BiPAP machine collection, including the Certified Refurbished ResMed AirCurve 10 VAuto BiPAP and the ResMed S9 BiPAP. ⚕️ RRT Note: If you feel like you're "fighting" your CPAP when you exhale, you're a classic BiPAP candidate — book a consultation before assuming CPAP just isn't for you.

Body Position & Sleep Apnea

Sleeping supine (on your back) is the most dangerous position for OSA — gravity collapses the tongue and soft palate directly into the airway. Many patients have positional OSA, meaning their AHI is 5 while side-sleeping and 35 while on their back. Your sleep study records body position throughout the night. If your apneas are predominantly positional, your prescribed pressure may be conservative — discuss this with your RT.

C

Central Sleep Apnea (CSA)

In CSA, breathing stops not because the airway is blocked, but because the brain temporarily fails to send the breathing signal. The airway is open — the lungs simply aren't getting the instruction. CSA is more prevalent in patients with heart failure, stroke history, or those on opioid medications. Standard CPAP often worsens CSA; ASV is typically the preferred treatment. Per AASM guidelines, CSA requires specialized diagnosis and treatment planning.

CPAP Compliance

Compliance means using your CPAP consistently enough for therapeutic benefit and — critically — for insurance and legal documentation. The accepted standard is:

⚕️ RRT Note: Medicare requires this threshold to continue covering your equipment. CDL truck drivers must document this compliance annually per FMCSA regulations to maintain their commercial medical certificate. We provide formal CPAP compliance letters for CDL drivers — contact us to order one.

Complex Sleep Apnea (CompSA)

CompSA occurs when a patient has both OSA and CSA simultaneously. Sometimes the central component was always present but masked by obstructive events. Other times, starting CPAP unmasks central apneas previously hidden. CompSA usually resolves with time and proper pressure management — occasionally ASV therapy is needed. This is one of the reasons CPAP data review matters: a rising central apnea count on your data report is a clinical red flag worth discussing with your provider or your RT.

CPAP (Continuous Positive Airway Pressure)

CPAP is the gold-standard, first-line treatment for obstructive sleep apnea, recommended by the AASM, the American Thoracic Society, and the American Academy of Otolaryngology. The machine generates a steady stream of pressurized air delivered through a mask to act as a pneumatic splint — physically holding the upper airway open throughout the night.

"CPAP" is often used loosely to describe all PAP therapy (including APAP and BiPAP). Technically CPAP delivers one fixed pressure, while APAP auto-adjusts. Browse our full selection of CPAP and APAP machines — new and certified refurbished, all RT-inspected.

CPAP Pressure (cmH₂O)

CPAP pressure is measured in centimeters of water pressure. Most patients are prescribed between 6 and 15 cmH₂O. Your pressure comes from your sleep study and should only be changed by a clinician. Too low and apneas continue. Too high and you risk aerophagia (air swallowing), mask leak, or treatment-emergent central apneas. If your pressure feels wrong, book an RT consultation before adjusting it yourself.

CPAP Titration

Titration is finding your optimal CPAP pressure. It happens two ways: (1) in-lab titration, where a sleep technologist manually adjusts pressure while monitoring you overnight, or (2) auto-titration at home using an APAP machine over several nights. Modern APAP has largely replaced in-lab titration for straightforward OSA — it's more convenient and often equally effective for uncomplicated cases.

D

Data Download / CPAP Therapy Data

Every modern CPAP machine stores nightly therapy data — AHI, leak rate, usage hours, pressure percentiles, and apnea subtypes. This data is accessible via SD card, the machine's display, or a connected smartphone app (ResMed uses MyAir). Reviewing your data monthly is one of the highest-value habits a CPAP user can build. If you don't know how to interpret your numbers, a 30-minute RT consultation will walk you through every line of it.

E

EPAP (Expiratory Positive Airway Pressure)

EPAP is the pressure delivered during exhalation in BiPAP therapy. It is always set lower than IPAP, making exhalation feel natural. The gap between IPAP and EPAP is called Pressure Support (PS) — a larger PS means more active ventilatory assistance from the machine per breath.

EPR (Expiratory Pressure Relief)

EPR is ResMed's brand name for its exhalation comfort feature on the AirSense 11 and AirSense 10. When enabled at level 1, 2, or 3, the machine drops pressure by that many cmH₂O during exhalation. Philips Respironics calls their version C-Flex. ⚕️ RRT Note: Start with EPR at 1 or 2 — it significantly improves comfort without materially reducing therapy effectiveness.

Events (Sleep Apnea Events)

A sleep "event" is any scored apnea or hypopnea. Your total event count divided by sleep hours equals your AHI. Events are subcategorized as obstructive apnea, central apnea, mixed apnea, or hypopnea — each reveals something different about what your airway and nervous system are doing during sleep and helps direct the right treatment approach.

F

Flow Limitation

Flow limitation is a partial upper airway obstruction — not severe enough to score as a hypopnea, but enough to flatten the airflow waveform and cause micro-arousals. Your APAP machine detects flow limitation and automatically increases pressure to prevent it from progressing to a full apnea. Patients with high flow limitation can have elevated RDI even with a "controlled" AHI — a nuance often missed without clinical data review.

Full Face Mask

A CPAP mask style that seals over both the nose and mouth. Full face masks are essential for mouth-breathers and patients with chronic nasal congestion. They require precise fit to avoid leak at the cheeks or forehead. Browse our CPAP mask collection including full face, nasal, and nasal pillow options. ⚕️ RRT Note: Full face masks have higher average leak rates — if your data shows persistent large leak with a full face mask, sizing down or switching styles often solves it.

H

Home Sleep Apnea Test (HST)

An HST is a self-administered diagnostic test you take at home overnight. It records breathing effort, airflow, oxygen saturation, and heart rate. HSTs are appropriate for uncomplicated OSA screening and significantly more convenient than in-lab polysomnography. We offer home sleep study options — contact us to learn more or use our STOP-BANG screening tool to assess your risk first.

Humidification / Heated Humidifier

CPAP air is dry. Pressurized room air delivered directly into your airway will desiccate nasal passages and throat — especially at pressures above 10 cmH₂O. A heated humidifier adds moisture before the air reaches your mask. Both the ResMed AirSense 11 and AirSense 10 include integrated heated humidifiers. I recommend using humidification with every patient — it dramatically improves long-term compliance.

Hypopnea

A hypopnea is a partial reduction in airflow — defined by the AASM as ≥30% drop in airflow for ≥10 seconds, accompanied by either ≥3% oxygen desaturation or an arousal. Hypopneas are counted alongside apneas in your AHI. Many patients have far more hypopneas than complete apneas — the ratio tells a clinician a lot about obstruction severity and upper airway anatomy.

I

IPAP (Inspiratory Positive Airway Pressure)

IPAP is the pressure delivered during inhalation in BiPAP therapy. It is always higher than EPAP. In a BiPAP prescribed at IPAP 16 / EPAP 10, the pressure support is 6 — meaning the machine is actively assisting each inhale significantly. Higher IPAP is used when patients have respiratory muscle weakness or hypoventilation and need the machine to do more of the breathing work.

L

Large Leak / Unintentional Leak

All CPAP masks have intentional leak through the exhalation port — that's normal design. Large leak is unintentional air escape from a poor seal: at the nose bridge, around the cheeks, or through the mouth when wearing a nasal mask.

⚕️ RRT Note: High leak is the single most common reason CPAP therapy fails. Before you give up on CPAP, check your leak number in the MyAir app or on your machine's display. A different mask style or size often solves the entire problem. Book a mask-fit consultation if you need guidance.

M

Mixed Sleep Apnea

A mixed apnea starts as a central event (no breathing effort) then transitions into an obstructive pattern (effort present, airway blocked) within the same event. Mixed apneas are common in complex sleep apnea and frequently resolve once CPAP pressure is optimized and the patient adapts to therapy over the first 4–8 weeks.

N

Nasal Mask

A nasal mask covers the nose from bridge to upper lip, offering better pressure delivery than nasal pillows at higher pressures. It's not appropriate for mouth-breathers unless a chin strap is added. Browse our nasal mask options — we carry multiple fit profiles for different face shapes.

Nasal Pillow Mask

Nasal pillows are the smallest CPAP mask type — small silicone inserts seal directly at the nostrils with no frame over the nose bridge. Excellent for claustrophobic patients, glasses wearers, and side sleepers. Generally not ideal above 12–13 cmH₂O. These are often my first recommendation for new CPAP users due to their minimal footprint and lower leak profile. Find yours in our mask collection.

O

Obstructive Sleep Apnea (OSA)

OSA is the most common form of sleep apnea, affecting an estimated 26% of adults ages 30–70 according to published epidemiological data. In OSA, upper airway muscles relax during sleep, causing the tongue and soft palate to collapse backward and block airflow. The result: obstruction → oxygen drop → arousal → re-opening → repeat, dozens or hundreds of times per night.

CPAP therapy directly prevents this cycle by maintaining airway patency with pressurized air. It's been validated by decades of clinical research as the most effective treatment for moderate-to-severe OSA.

Oxygen Saturation (SpO₂)

SpO₂ is the percentage of hemoglobin carrying oxygen in your blood. Normal is 95–100%. During apnea events, saturation drops — severe OSA patients can desaturate into the 70s or 60s during sleep, causing real cardiovascular and neurological strain over time. Your sleep study records your lowest SpO₂, average overnight saturation, and percentage of time below 88%.

If your physician prescribes supplemental oxygen in addition to CPAP, we carry home and portable options including the VH-2 Home Oxygen Concentrator, the VH-3 Home Oxygen Concentrator (1–7 L/min), the Inogen One G4 Portable, and the Oxlife Liberty 2 Portable (10 L).

P

PAP (Positive Airway Pressure)

PAP is the umbrella term for all pressurized airway therapy — CPAP, APAP, BiPAP, and ASV are all forms of PAP therapy. The word "CPAP" is often used loosely to refer to all of them, but each works differently and is indicated for different conditions and severity levels.

Polysomnography (PSG)

A PSG is a full in-lab sleep study — the gold standard for diagnosing complex sleep disorders. It simultaneously records brain waves (EEG), eye movements, muscle activity, heart rhythm, respiratory effort, airflow, oxygen saturation, and body position. A sleep technologist monitors you throughout the night. PSG captures what a home sleep test cannot: central apneas, REM behavior disorder, narcolepsy, periodic limb movements, and precise sleep architecture staging. Contact us if you need guidance on which test is appropriate for your situation.

Pressure Support (PS)

In BiPAP therapy, Pressure Support equals IPAP minus EPAP. A PS of 4 provides moderate ventilatory assistance. A PS of 8–12 provides significant support — used in patients with neuromuscular disease, OHS, or COPD with CO₂ retention. Higher PS = more work the machine does per breath on your behalf.

R

Rainout

Rainout is condensation inside your CPAP tubing — warm humidified air meets a cooler tube and water droplets form. Solutions: heated tubing (the ResMed AirSense 11 includes ClimateLineAir heated tube), an insulating tube sleeve, or slightly lowering your humidifier temperature setting.

Ramp

The ramp feature starts your CPAP machine at a low, comfortable pressure and gradually increases to your prescribed therapeutic pressure over a set time (10–45 minutes). This eases you to sleep before full pressure activates. ⚕️ RRT Note: Ramp sounds comfortable but means you're not in full treatment mode until it completes. Once you're adapted to CPAP, consider disabling ramp — you're leaving therapy time on the table every night.

RDI (Respiratory Disturbance Index)

RDI is broader than AHI. It includes apneas + hypopneas + respiratory effort-related arousals (RERAs) — brief disruptions caused by increased breathing effort that don't quite meet the hypopnea threshold. A patient can have an AHI of 4 (no OSA diagnosis) but an RDI of 22, indicating significant sleep disruption standard scoring misses. If you feel exhausted despite a "normal" AHI, ask your provider about RDI — or book a consultation with me to review your study together.

Residual AHI

Residual AHI is your AHI while using CPAP. A well-controlled residual AHI should be under 5 events/hour. Consistently above 5 suggests suboptimal pressure, significant mask leak, or an underlying condition like CSA that your current therapy isn't fully addressing. Your residual AHI is visible in the ResMed MyAir app or on your machine's SD card data — let me help you interpret it.

S

Sleep Architecture

Sleep architecture is the structural pattern of your sleep — the cycling between light sleep (N1, N2), deep slow-wave sleep (N3), and REM. A healthy adult cycles through these stages every 90–110 minutes. Untreated OSA devastates this architecture with hundreds of micro-arousals that prevent adequate time in slow-wave and REM sleep. This is why OSA patients feel exhausted after 8 hours in bed — they're spending most of the night in shallow N1/N2 sleep, never reaching restorative depth. Effective CPAP therapy restores normal sleep architecture, often within the first week.

Sleep Efficiency

Sleep efficiency is the percentage of time in bed you're actually asleep. Healthy is 85%+. Severe untreated OSA can drop efficiency to 55–65%. Effective CPAP treatment consistently improves sleep efficiency — this is one of the first measurable improvements patients report within the first 2–4 weeks of consistent therapy.

T

Titration Study

A titration study is a sleep lab procedure where a technologist manually adjusts your CPAP pressure throughout the night to find your optimal setting. Split-night studies combine diagnostic PSG and titration in a single overnight session. Auto-titration at home using an APAP machine has largely replaced in-lab titration for uncomplicated OSA — and it's what most patients we work with use.

Travel CPAP

Travel CPAPs are compact versions of standard machines designed for portability. Most are FAA-approved for in-flight use, operate on universal voltage, and weigh under 1 lb. Key considerations: Does it maintain the same APAP algorithms as your home device? Does it have battery capability for off-grid use? View our full CPAP lineup — and if you're a CDL driver needing CPAP in your truck cab, see our CDL compliance letter service.

U

Upper Airway Resistance Syndrome (UARS)

UARS is a condition where increased upper airway resistance causes arousals and daytime fatigue without meeting the AHI threshold for an OSA diagnosis. Patients with UARS often have an AHI under 5 but a significantly elevated RDI. They are frequently told their sleep is "normal" when it clearly isn't. UARS is best detected with full PSG — home sleep tests often miss it. If this sounds like you, book a consultation to discuss your study results in detail.

A Note From Your RT

This glossary is updated regularly based on questions from patients and MyRespCo customers. If there's a term you've encountered that isn't here, send it to us and I'll add it with a clinical explanation.

Sleep apnea is a serious but highly treatable condition. Understanding your therapy puts you in control of it. Every piece of equipment we carry at MyRespCo has been personally selected by a working RRT — not a purchasing department.

— Yashil Bhatt, RRT · My Respiratory Company · Augusta, GA · 706-772-0263