Reference

Glossary

Plain-language definitions for the oxygen therapy and sleep apnea terms used across this site — the same terms that show up on a prescription, a spec sheet, or a sleep study report.

Adaptive Support Ventilation (ASV)

A bi-level therapy mode that continuously adjusts pressure support breath-by-breath in response to a patient's own breathing pattern — commonly used for central and complex sleep apnea, where standard CPAP or fixed bi-level therapy is often insufficient.

AHI (Apnea-Hypopnea Index)

The average number of breathing interruptions per hour of sleep, combining full pauses (apneas) and partial airflow reductions (hypopneas) that also drop blood oxygen or trigger a brief arousal.

Ambulatory Oxygen

Oxygen therapy prescribed for use during activity and movement outside the home, as opposed to oxygen prescribed only for rest or sleep — the prescription basis for most portable concentrator purchases.

Auto-Titrating CPAP (APAP)

A CPAP device that continuously measures airflow resistance and adjusts pressure within a prescribed range in real time, rather than holding one fixed pressure all night — useful for anyone whose apnea severity varies with position, congestion, or alcohol intake.

Battery Capacity (Wh)

Battery energy capacity measured in watt-hours — relevant for portable oxygen concentrators and CPAP because airlines cap carry-on lithium battery capacity at 160 Wh without prior airline approval, which extended batteries on some devices can exceed.

BiPAP / VPAP (Bilevel Positive Airway Pressure)

A device that delivers two separate pressures — a higher one for inhaling (IPAP), a lower one for exhaling (EPAP) — rather than one fixed pressure. Prescribed when standard CPAP isn't sufficient or comfortable, or for central and complex sleep apnea.

Cannula / Tubing Length

The length of tubing connecting a nasal cannula to an oxygen source — home setups commonly use 25 to 50 feet to allow movement between rooms, while portable concentrator tubing is typically much shorter for use while carrying the device.

Cash-Pay / Self-Pay

A payment model where the buyer pays directly rather than the retailer billing insurance, Medicaid, or Medicare — the model CIRRUS operates under. It removes prior-authorization queues and claim-adjudication delays, in exchange for no insurance reimbursement.

Central Sleep Apnea

A form of sleep apnea where the brain intermittently fails to signal the body to breathe, even though the airway itself is open — a different mechanism from obstructive apnea, and one standard CPAP is often less effective for.

cmH₂O (Centimeters of Water Pressure)

The pressure unit used for CPAP and BiPAP prescriptions — the amount of air pressure, measured in centimeters of water displacement, delivered to keep the airway open.

Continuous Flow

An oxygen delivery method that provides a steady, uninterrupted stream regardless of breathing pattern — necessary for patients who breathe too shallowly or irregularly to reliably trigger a pulse-dose sensor.

CPAP (Continuous Positive Airway Pressure)

A device that delivers a constant, single pressure setting through a mask to keep the airway open during sleep — the standard first-line treatment for obstructive sleep apnea.

DO-160

RTCA/DO-160 is the environmental qualification standard used to test aviation electronics for electromagnetic interference, altitude performance, and battery safety under cabin-pressure conditions — the specific standard behind "FAA-approved" portable oxygen claims.

FAA-Approved (Portable Oxygen)

Documentation from a device manufacturer confirming a specific portable oxygen concentrator model meets DO-160 environmental and safety testing, which airlines accept as clearance for in-flight use.

FEV1 (Forced Expiratory Volume in 1 Second)

The volume of air a person can forcefully exhale in one second, measured by spirometry and compared to a predicted normal for their age, height, and sex — the primary number used to grade COPD severity.

Full Face Mask (CPAP)

A CPAP/BiPAP mask that covers both the nose and mouth, typically prescribed for mouth breathers or anyone who can't maintain a nasal-only seal through the night.

GOLD Stage (COPD Staging)

A four-grade COPD severity classification from the Global Initiative for Chronic Obstructive Lung Disease, based primarily on FEV1: Grade 1 (mild, ≥80% predicted), Grade 2 (moderate, 50–79%), Grade 3 (severe, 30–49%), Grade 4 (very severe, <30%).

Humidifier (CPAP)

A heated water chamber integrated into or attached to a CPAP/BiPAP device, adding moisture to the delivered air to reduce dryness and irritation in the nose and throat — most modern machines heat to a target dew point rather than a fixed temperature.

Hypoxemia

Below-normal blood oxygen levels — generally a reading under 88–90% SpO2 accompanied by symptoms is the threshold most pulmonologists treat as needing prompt medical attention, and is often the trigger for a supplemental oxygen prescription.

Leak Rate (CPAP)

The volume of air escaping around a mask seal rather than through its intended vent, reported by most modern CPAP/BiPAP machines and used to judge mask fit — a persistently high leak rate usually means a mask sizing or seal problem, not a pressure problem.

LPM (Liters Per Minute)

The unit used to measure continuous-flow oxygen output — how many liters of concentrated oxygen a device delivers per minute. A prescription might specify, for example, 2 LPM at rest and higher during exertion.

Nasal Cannula

A lightweight tube with two small prongs that rest just inside the nostrils, delivering oxygen from a concentrator or tank — the standard delivery method for supplemental oxygen outside of a CPAP/BiPAP mask context.

Nasal Pillow Mask

A minimal-contact CPAP/BiPAP mask that seals directly at the nostril opening rather than covering the outside of the nose — generally the least bulky mask style, often preferred by side sleepers and anyone bothered by a full-face mask.

Non-Invasive Ventilation (NIV)

Mechanical ventilation delivered through a mask rather than an invasive airway (endotracheal tube or tracheostomy) — BiPAP is technically a form of NIV, though the term usually refers to higher-acuity ventilator-class devices used for respiratory failure rather than standard sleep apnea therapy.

Obstructive Sleep Apnea

The most common form of sleep apnea, caused by the physical airway collapsing or narrowing during sleep despite a normal signal from the brain to breathe. CPAP treats it by using positive pressure to physically prop the airway open.

Oxygen Concentrator

A device that manufactures oxygen from room air on demand, using a pressure-swing adsorption process to filter out nitrogen and concentrate ambient air (roughly 21% oxygen) to 87–96% output — as opposed to a compressed tank, which stores a fixed, finite supply.

Oxygen Purity Percentage

The concentration of oxygen a concentrator outputs after filtering ambient air — most portable and stationary concentrators produce 87–96% purity, compared to the roughly 21% oxygen in normal room air.

POC (Portable Oxygen Concentrator)

Shorthand for a battery-powered oxygen concentrator small enough to carry — as opposed to a stationary concentrator built for continuous home use, plugged into wall power.

Prescription Verification

The process of confirming a valid, current prescription matches the device class and flow/pressure setting ordered — legally required for oxygen concentrators and CPAP/BiPAP devices under FDA regulation, regardless of payment method.

Pulse Dose

An oxygen delivery method that senses the start of an inhalation and delivers a short burst of oxygen timed to it, rather than a constant stream — this conserves oxygen and is what makes small portable concentrators possible.

Pulse Oximeter

A clip-on device, usually worn on a fingertip, that estimates blood oxygen saturation (SpO2) and pulse rate by shining light through the skin and measuring absorption differences between oxygenated and deoxygenated blood.

Ramp Setting (CPAP)

A comfort feature that starts therapy at a lower, easier-to-tolerate pressure and gradually increases to the full prescribed pressure over a set time (commonly 5–45 minutes) as the user falls asleep.

Refurbished (Medical Device)

A previously owned device that's been inspected, serviced, and factory-tested before resale, typically at a lower price than new — usually still carrying a manufacturer or seller warranty, though warranty terms are often shorter than on a new unit.

Residual AHI

The number of apnea/hypopnea events still occurring per hour despite CPAP or BiPAP treatment, logged by the device itself. A high residual AHI despite consistent nightly use usually points to a leak or pressure-setting issue worth adjusting.

Rx (Prescription) Required

Indicates a device is a regulated medical device under FDA rules, requiring a valid, verified prescription before it can be sold and shipped — applies to all oxygen concentrators and CPAP/BiPAP devices, regardless of insurance or payment method.

Sieve Bed (Oxygen Concentrator)

The zeolite-filled canister inside an oxygen concentrator that adsorbs nitrogen from compressed room air via pressure-swing adsorption, leaving concentrated oxygen behind — the component most warranties treat as a separate wear item from the concentrator itself.

SpO2 (Blood Oxygen Saturation)

An estimate of the percentage of hemoglobin in the blood carrying oxygen, measured non-invasively with a pulse oximeter. Healthy adults typically read 95–100% at sea level.

Tidal Volume

The volume of air moved in a single normal breath, measured in milliliters — a core setting on a mechanical ventilator, prescribed per kilogram of the patient's body weight rather than as a fixed number across all patients.

Titration Study (Sleep Study)

A monitored sleep study, in a lab or at home, where pressure is adjusted while a technician or algorithm watches for the setting that eliminates apneas and hypopneas — this is where the specific cmH₂O prescription number comes from.

Ventilator (Mechanical)

A device that fully or partially takes over the work of breathing for a patient, used in ICU, transport, and long-term care settings — distinct from CPAP/BiPAP, which support natural breathing rather than replace it. An institutional and clinical-buyer category, not a home consumer device.