CPAP and Sleep Apnea Glossary

9

90 day trial period: Informal name for the first 3 months of Medicare PAP coverage, during which adherence must be shown and a clinician must re-evaluate you for coverage to continue.
95th percentile pressure: On auto machines, the pressure the machine stayed at or below for 95% of the night. Clinicians often use it when reviewing settings.

A

A-Flex: Philips Respironics' exhalation pressure relief for auto-CPAP mode. See Flex.
A4604: The HCPCS code for tubing with a heating element for a PAP device. Medicare usual maximum: 1 per 3 months.
A7030: The HCPCS code for a full face mask used with a PAP device. Medicare usual maximum: 1 per 3 months.
A7031: The HCPCS code for a replacement full face mask cushion. Medicare usual maximum: 1 per month.
A7032: The HCPCS code for a replacement nasal mask cushion. Medicare usual maximum: 2 per month.
A7033: The HCPCS code for replacement nasal pillows, billed per pair. Medicare usual maximum: 2 per month.
A7034: The HCPCS code for a nasal interface (mask or cannula type), with or without head strap. Medicare usual maximum: 1 per 3 months.
A7035: The HCPCS code for PAP headgear. Medicare usual maximum: 1 per 6 months.
A7037: The HCPCS code for standard PAP tubing. Medicare usual maximum: 1 per 3 months.
A7038: The HCPCS code for a disposable PAP filter. Medicare usual maximum: 2 per month.
A7046: The HCPCS code for a replacement humidifier water chamber. Medicare usual maximum: 1 per 6 months.
A9279: The HCPCS code for a monitoring feature or device. Medicare lists it as statutorily non-covered for PAP.
AHI (apnea-hypopnea index): The average number of apneas plus hypopneas per hour of sleep. It is the main number used to grade sleep apnea severity and, on a CPAP machine's report, to estimate how well therapy is controlling events. The AASM grades adult OSA as mild at an AHI of 5 to 15, moderate at 15 to 30 and severe above 30.
APAP (auto-adjusting positive airway pressure): A CPAP machine that adjusts pressure breath by breath within a minimum and maximum range set by your clinician, based on detected events such as apneas, flow limitation and snoring. Medicare codes it as E0601, the same as fixed CPAP.
ASV (adaptive servo-ventilation): A specialized bilevel device for central sleep apnea that adjusts pressure support breath by breath to stabilize breathing. In the SERVE-HF trial (NEJM, 2015), people with heart failure with an ejection fraction of 45% or less and predominantly central apnea had higher all-cause and cardiovascular mortality with ASV, so screening by a specialist matters.
AVAPS / iVAPS (volume assured pressure support): Bilevel modes that adjust pressure support to reach a target breathing volume (AVAPS is Philips' term, iVAPS ResMed's). They are used for hypoventilation conditions under specialist care.
Aerophagia: Swallowing air during PAP therapy, causing bloating, belching or stomach pain. NHLBI advises stopping the machine and contacting your provider if you have stomach discomfort or bloating; do not lower your prescribed pressure yourself.
Air Carrier Access Act: The US law under which DOT rules (14 CFR Part 382) govern how airlines handle passengers with disabilities and assistive devices such as CPAP machines.
Air filter: The inlet filter that keeps dust out of the machine. Medicare codes disposable filters as A7038 and non-disposable filters as A7039.
AirView: ResMed's cloud system that lets suppliers and clinicians review your machine's data remotely.
Airplane mode: A setting on connected machines that turns off wireless transmission for use in flight.
Anti-asphyxia valve: A valve in full face masks that opens to room air if airflow stops, so you can breathe if the machine stops.
Apnea: A pause in breathing. Medicare's PAP policy defines an apnea as cessation of airflow for at least 10 seconds.
Arousal: A brief shift to lighter sleep or wakefulness, often too short to remember. Repeated arousals from breathing events fragment sleep and cause daytime sleepiness.
AutoSet: ResMed's name for its auto-adjusting CPAP algorithm and mode.
AutoSet for Her: A ResMed auto mode designed for women with mild to moderate obstructive sleep apnea.

B

Backup rate: A timed breath rate on some bilevel devices (spontaneous/timed mode) that delivers a breath if you do not breathe on your own within a set time. Medicare codes bilevel devices with a backup rate as E0471.
Bacterial/viral filter: An inline filter sometimes added at the machine outlet. ResMed advises checking it regularly for moisture, which can increase breathing resistance.
Bi-Flex: Philips Respironics' pressure relief for bilevel devices, which Philips describes as adding a small amount of relief late in inspiration and during exhalation.
BiPAP / bilevel PAP (BPAP): A machine with two pressures: a higher one when you breathe in (IPAP) and a lower one when you breathe out (EPAP). BiPAP is a Philips trademark often used for all bilevel devices. It is used when CPAP is not tolerated or not effective, and for some breathing conditions beyond OSA.

C

C-Flex: Philips Respironics' exhalation pressure relief for fixed CPAP mode. See Flex.
CPAP (continuous positive airway pressure): A machine that delivers one steady pressure all night, set by your prescribing clinician. It is the standard treatment for obstructive sleep apnea.
CPAP-Check: A Philips DreamStation mode that delivers fixed CPAP but checks and adjusts pressure periodically within limits set by the provider.
Care Orchestrator: Philips Respironics' cloud system for suppliers and clinicians to review therapy data.
Central apnea: An apnea where the brain does not send the signal to breathe, so there is no breathing effort. CPAP pressure does not fix the cause. Frequent central apneas on your machine report are a reason to contact your clinician rather than change settings yourself.
Central sleep apnea (CSA): Sleep apnea caused by the brain not sending the signal to breathe, rather than by a blocked airway. It is treated differently from OSA and needs a clinician's evaluation.
Cheyne-Stokes respiration (CSR): A specific periodic breathing pattern where breathing gradually deepens and then fades into a central apnea, repeating in cycles. It is often linked with heart failure or stroke and is a reason to see your clinician. Some machines flag it in their data.
Chinstrap: A strap that supports the jaw to help keep the mouth closed. Medicare codes it as A7036.
Class I recall: The FDA's most serious recall classification, used where a product may cause serious injury or death.
Clear airway apnea: ResMed's label in its machine data for an apnea where the airway appeared open, which is often but not always a central event. The machine estimates this from airflow, so a sleep study is needed to confirm it.
Climate Control: ResMed's automatic humidity and tube temperature management on AirSense machines, available when the humidifier and ClimateLineAir tubing are used together.
ClimateLineAir: ResMed's heated tubing. The original ClimateLineAir fits AirSense 10 and AirCurve 10; ClimateLineAir 11 fits AirSense 11 and AirCurve 11. They are not interchangeable.
Clinical menu: The settings menu for providers, where pressure and modes are set. Changes there go through your prescribing clinician.
Coinsurance: The share of a covered cost you pay after the deductible, usually a percentage of the approved amount. Check your plan for the rate that applies to DME.
Complex sleep apnea: Also called treatment-emergent central sleep apnea: central apneas that appear or persist once obstructive events are treated with CPAP. Some cases settle over time; others need a different device, which is a clinician's decision.
Compliance (adherence): How consistently you use PAP. Medicare defines adherence as at least 4 hours per night on 70% of nights during a consecutive 30 day period in the first 3 months of use, plus a clinical re-evaluation.
Cushion: The soft part of the mask that seals against your face. It wears and is replaced more often than the frame.
cm H2O (centimeters of water): The unit used for CPAP pressure. One cm H2O is about 0.98 hPa, so machines often label ranges such as 4 to 20 cm H2O as 4 to 20 hPa.

D

DC/DC converter: A power adapter that runs a CPAP from a 12 V or 24 V DC source such as a battery or vehicle. ResMed says a converter is more efficient than an inverter.
DME (durable medical equipment): Medical equipment for repeated home use, such as CPAP machines. It is supplied by DME suppliers who bill insurance.
DME MAC: Medicare's regional contractors that process DME claims and write the local coverage policies for PAP devices.
DME supplier: The company that delivers your machine and supplies, bills your insurer, and often tracks your compliance.
DMEPOS: Durable Medical Equipment, Prosthetics, Orthotics and Supplies: Medicare's benefit category and supplier type for CPAP equipment.
Desensitization: Gradually getting used to wearing a mask and pressure, for example by wearing the mask while awake. Often used for claustrophobia.
Design life: The service life a manufacturer rates for a part. ResMed lists 5 years for the AirSense 11 device and 6 months for its air tubing and standard humidifier tub.
Detailed data vs summary data: Summary data is a nightly total (usage, AHI, leak). Detailed data records breath-by-breath flow and pressure, usually only on the SD card.
DreamMapper: Philips Respironics' patient app for viewing therapy data.

E

E0470: The HCPCS code for a bilevel respiratory assist device without a backup rate.
E0471: The HCPCS code for a bilevel respiratory assist device with a backup rate.
E0561: The HCPCS code for a non-heated humidifier used with a PAP device.
E0562: The HCPCS code for a heated humidifier used with a PAP device.
E0601: The HCPCS code for a continuous positive airway pressure device. Medicare uses it for both fixed CPAP and auto-adjusting single-level CPAP.
EPAP (expiratory positive airway pressure): On a bilevel machine, the pressure while you breathe out. It is the pressure mainly responsible for keeping the airway from collapsing.
EPR (Expiratory Pressure Relief): ResMed's comfort feature that lowers pressure a little when you breathe out, at level 1, 2 or 3 (1 to 3 cm H2O). It makes exhaling against the pressure easier. ResMed notes that on the AirSense 11 the delivered pressure will not drop below 4 cm H2O.
Elbow / swivel: The connector between the mask and the tubing, often able to rotate so the tube moves with you.
Epworth Sleepiness Scale: A short questionnaire scoring how likely you are to doze in everyday situations. It is a common way to track daytime sleepiness before and after treatment.
Excessive daytime sleepiness: Persistent sleepiness during the day, a common symptom of untreated sleep apnea and one of the symptoms Medicare's policy accepts for covering PAP at an AHI of 5 to 14.

F

FAA compliance label: A manufacturer's statement that a device meets FAA requirements for use on aircraft. ResMed states its AirSense 11 and AirMini meet FAA requirements for all phases of air travel.
Flex (C-Flex, A-Flex, Bi-Flex): Philips Respironics' family of exhalation pressure relief settings, named by mode (C-Flex for fixed CPAP, A-Flex for auto mode, Bi-Flex for bilevel). Philips describes Flex as giving pressure relief when you exhale, adjustable from level 1 to 3 when your provider has enabled it.
Flow generator: The technical name for the blower unit of a CPAP machine.
Flow limitation: A flattening of the inspiratory airflow curve that shows the airway is partly narrowed. Auto-adjusting machines respond to it, and OSCAR and ResScan can chart it.
Flow rate graph: A chart of airflow over time, the detailed data OSCAR and ResScan use to show events and flow limitation.
Full face mask: A mask covering the nose and mouth, used by mouth breathers and people whose mouth opens during sleep. Medicare codes it as A7030 and replacement cushions as A7031.

H

HCPCS code: The Healthcare Common Procedure Coding System code used to bill equipment and supplies. CPAP machines are E0601; masks and parts use A codes such as A7030 to A7039.
HME (heat and moisture exchanger): A waterless humidification insert that captures heat and moisture from your breath and returns it on the next breath. ResMed's HumidX is designed for AirMini and must be changed every 30 days.
Headgear: The straps that hold the mask in place. Medicare codes it as A7035, with a usual maximum of 1 per 6 months.
Heated humidifier: A humidifier with a heating plate, coded by Medicare as E0562. The heat increases how much moisture the air can carry.
Heated tube: Tubing with a heating wire that keeps air warm on the way to the mask, preventing rainout. Medicare codes it as A4604.
Home sleep apnea test (HSAT): A simpler sleep test done at home that records breathing, effort and oxygen. It cannot measure actual sleep time, so results are reported per hour of recording.
Hours of use (usage): How long the machine ran with the mask on each night. It is what insurers check for compliance.
HumidX: ResMed's HME for the AirMini, with HumidX Plus for dry climates and high altitude and HumidX F20 for F20 full face masks.
Humidifier: A water chamber with a heater plate that adds moisture to the air, reducing dryness and congestion. Most home machines have one built in or as an attachment.
Humidity level: The comfort setting for how much moisture the humidifier adds.
Hybrid (oral/nasal) mask: A two piece mask with separate oral and nasal parts. Medicare codes it as A7027.
Hypercapnia: High carbon dioxide in the blood, a sign of inadequate breathing.
Hypoallergenic filter: A finer filter for people sensitive to allergens. ResMed notes that its hypoallergenic filter slightly reduces pressure accuracy at high leaks.
Hypopnea: A partial reduction in breathing. Medicare's PAP policy defines it as an event of at least 10 seconds with at least a 30% reduction in airflow or chest and abdominal movement and at least a 4% drop in oxygen saturation. The AASM scoring manual also has a rule that accepts a 3% drop or an arousal, so the same night can produce different AHI numbers under different rules.
Hypoxemia: Low oxygen in the blood.
hPa (hectopascal): A metric pressure unit used on many machines and manuals. 1 hPa equals 1 millibar, and about 1.02 cm H2O.

I

IPAP (inspiratory positive airway pressure): On a bilevel machine, the pressure while you breathe in.
Implanted metallic device: Implants such as pacemakers, defibrillators, neurostimulators and some shunts or clips that magnets in certain masks can interfere with, according to FDA safety communications.
Insomnia: Difficulty falling or staying asleep. It can coexist with sleep apnea and make it harder to get used to CPAP.
Intentional leak: The designed airflow out of the mask vent that flushes exhaled carbon dioxide. Machines usually subtract it so reports show only unintentional leak.
Interface: The clinical and billing term for any mask or nasal device that connects you to the PAP machine.
Inverter: A device that converts DC battery power to AC household power. It works with CPAP but wastes more energy than a DC converter.

L

LCD L33718: Medicare's Local Coverage Determination for PAP devices for obstructive sleep apnea. It sets the coverage criteria, the adherence rule and the usual maximum replacement quantities for supplies.
Large leak: A leak big enough that the machine cannot reliably measure your breathing or deliver the set pressure. Philips machines report time in large leak; a lot of it makes the AHI and other data unreliable.
Leak rate: How much air escapes from the system beyond the mask's intended vent, measured in liters per minute. ResMed's AirSense 11 Mask Fit check treats under 24 L/min as a good seal.
Lithium ion battery: The rechargeable battery type used in most CPAP batteries. Spares must be carried in carry-on baggage with terminals protected.

M

Mask Fit (ResMed feature): A ResMed AirSense feature that runs the machine at pressure so you can adjust the mask; it shows a good seal when leak is under 24 L/min.
Mask fit: How well the mask seals. Overtightening often causes more leaks and pressure sores, not fewer.
Mask frame: The rigid part of the mask that holds the cushion and connects to the headgear and tubing.
Mask liner: A fabric layer between skin and cushion. Medicare's PAP policy article says liners are not interfaces and should not be billed as replacement cushions.
Mask resistance setting: A setting on some machines that tells the device which mask type is attached so it can compensate for the mask's resistance and vent flow.
Mask size: Most masks come in sizes such as small, medium, large and wide; sizing guides or a fitting with your supplier help you choose.
Mask vent: The small holes in the mask that let exhaled air out. They must never be blocked, because they flush out carbon dioxide.
Mask with magnets: Masks whose headgear clips use magnets. The FDA has issued safety communications, including on ResMed masks in January 2024, warning that the magnets can interfere with certain implanted metallic devices such as pacemakers, and advising people with those implants to switch to a non-magnetic mask.
Minimum pressure / maximum pressure: The lower and upper limits of an auto-adjusting machine's pressure range, set by your clinician.
Minute ventilation: The total volume of air breathed per minute (tidal volume times breathing rate).
Mixed apnea: An apnea that starts as a central event and ends as an obstructive one.
Modem / cellular connectivity: A built-in wireless connection that sends usage data to the manufacturer's cloud so your supplier and clinician can review it.
Mouth leak: Air escaping out of the mouth when using a nasal or pillows mask. It causes dry mouth and high leak readings. Common fixes are a chinstrap or a full face mask.
myAir: ResMed's patient app and website showing nightly usage, leak, events and a sleep score.

N

Nasal cradle mask: A nasal mask whose cushion sits under the nose rather than over the bridge, such as ResMed's AirFit N30.
Nasal mask: A mask that covers the nose but not the mouth. Medicare codes the full mask as A7034 and replacement cushions as A7032.
Nasal pillows mask: A small mask with soft inserts that sit at the entrance of the nostrils. Medicare codes replacement pillows as A7033, billed per pair.

O

ODI (oxygen desaturation index): The number of times per hour that blood oxygen drops by a set amount (commonly 3 or 4 percent) from baseline. It is reported by sleep tests and some oximeters, and tends to track AHI, but it is not the same measure.
OSCAR: Open Source CPAP Analysis Reporter: free, community-built software that reads therapy data from many CPAP machines' SD cards and charts events, leaks, pressure and flow.
Obesity hypoventilation syndrome (OHS): Inadequate breathing leading to high carbon dioxide levels in people with obesity, often alongside OSA. It is commonly treated with bilevel PAP under specialist care.
Obstructive apnea: An apnea where the airway closes but the body keeps trying to breathe. This is the event CPAP pressure is designed to prevent by holding the airway open.
Obstructive sleep apnea (OSA): Repeated narrowing or closure of the upper airway during sleep, causing pauses in breathing, drops in oxygen and broken sleep. PAP therapy is the most common treatment.
Oral mask: A mask that delivers pressure through the mouth only. Medicare codes an oral interface as A7044.
Oximetry: Measuring blood oxygen with a clip-on sensor, usually on a finger. Some CPAP machines and OSCAR can record oximetry alongside therapy data.
Oxygen saturation (SpO2): The percentage of hemoglobin carrying oxygen, measured by a pulse oximeter. Sleep tests record how often and how far it drops during sleep.
Ozone CPAP cleaner: A device that uses ozone gas to sanitize CPAP equipment. The FDA has not cleared or approved any ozone or UV device for cleaning CPAP machines, has received reports of asthma attacks, headaches and breathlessness after ozone cleaner use, and notes ozone can break down hoses and masks.

P

PAP (positive airway pressure): The family of treatments that use pressurized air delivered through a mask to keep the airway open. It includes CPAP, APAP and bilevel devices.
PE-PUR foam: Polyester-based polyurethane foam used in certain Philips Respironics machines to reduce sound and vibration. In June 2021 Philips recalled certain CPAP, BiPAP and ventilator devices because the foam can break down and particles or chemicals could be breathed in or swallowed.
Passover humidifier: A non-heated humidifier where air passes over the water surface. Medicare codes a non-heated humidifier as E0561.
Patient menu: The settings you can change yourself, such as ramp, humidity, tube temperature and EPR or Flex level.
Periodic breathing: A cycling pattern of breathing that waxes and wanes. It can be a sign of central breathing instability and is worth showing your clinician if it appears often.
Philips Respironics recall (2021): The June 2021 recall of certain Philips CPAP, BiPAP and ventilator devices over PE-PUR foam breakdown, which the FDA says affected 15 million devices worldwide.
Philips consent decree (2024): On April 9, 2024, a federal court entered a consent decree against Philips Respironics that, among other things, restricts the production and sale of new CPAP and BiPAP machines from several US facilities until certain requirements are met.
Policy article A52467: The Medicare policy article that accompanies LCD L33718, with coding guidance such as which HCPCS code applies to which mask part.
Power supply unit (PSU): The external power brick for a CPAP. The AirSense 11 uses a 65 W unit.
Prescribed pressure: The pressure or pressure range your clinician has set. Changing it is a clinical decision; comfort settings in the patient menu are separate.
Prescription (CPAP): In the US, CPAP machines are prescription devices. Many retailers also require a prescription for masks and heated tubing.
Pressure support: The difference between IPAP and EPAP on a bilevel machine, which helps move air in with each breath.
Prior authorization: Approval some insurers require before they will cover a machine or supplies.

R

RDI (respiratory disturbance index): A count of breathing events per hour. In sleep labs it usually adds RERAs to apneas and hypopneas. In Medicare's PAP policy, RDI means apneas plus hypopneas per hour of recording time, the measure used for home sleep tests that cannot measure actual sleep time.
RERA (respiratory effort related arousal): A stretch of increasing breathing effort or flattened airflow that ends in a brief arousal from sleep, without meeting the criteria for an apnea or hypopnea. Medicare's PAP policy does not count RERAs in the AHI.
Rainout: Water condensing inside the tubing or mask when warm humid air cools, causing gurgling or droplets on the face. Fixes include heated tubing, a tubing cover, a warmer room or a lower humidity setting.
Ramp: A comfort feature that starts at a lower pressure and gradually increases to your therapy pressure while you fall asleep. On the AirSense 11 the ramp time can be set to off, 5 to 45 minutes in 5 minute steps, or Auto.
Reasonable useful lifetime (RUL): Medicare's 5 year expected life for a PAP device. A replacement after 5 years needs an in-person evaluation showing you still use and benefit from it.
Rental (capped rental): A payment arrangement where the insurer pays a monthly rental for a set period instead of buying the machine outright. Ask your supplier how your plan handles it.
ResScan: ResMed's clinical software for reading detailed data from ResMed machines.
Residual AHI: The AHI your CPAP machine estimates while you are using it, shown in apps and reports. It is an estimate from airflow, not a sleep study result. A residual AHI that stays high, or that rises, is something to raise with your prescribing clinician.
Respiratory rate: Breaths per minute, reported by many machines and in OSCAR.
Resupply: Ordering replacement masks, cushions, filters and tubing on your insurer's schedule, often managed by your DME supplier.

S

SD card: A memory card in many CPAP machines that stores detailed therapy data that software such as OSCAR or ResScan can read.
Sleep efficiency: The percentage of time in bed spent asleep, reported by sleep studies.
Sleep specialist: A physician trained in sleep medicine who diagnoses and manages sleep disorders and sets PAP prescriptions.
Sleep study (polysomnography): An overnight test in a sleep lab that records breathing, oxygen, brain waves, heart rhythm and movement. It is the reference test for diagnosing sleep apnea and for in-lab titration.
SlimLine tubing: ResMed's lighter, narrower unheated tubing.
SmartRamp: A Philips DreamStation ramp mode that holds a constant lower pressure until the machine detects a need for more pressure, rather than increasing at a steady rate.
SmartStart: A ResMed feature that starts therapy automatically when you breathe into the mask; its companion SmartStop stops therapy automatically after you remove the mask.
Snore index: Some machines count vibration in the airflow as snoring and report it. Snoring on CPAP can mean the pressure, mask seal or position is not fully controlling the airway, which is worth discussing with your clinician.
Split-night study: A lab sleep study where the first part of the night diagnoses sleep apnea and the second part titrates CPAP pressure.
Spontaneous/timed (S/T) mode: A bilevel mode that follows your own breathing but adds timed breaths at a backup rate if needed.
Standard Written Order (SWO): The order a Medicare supplier must have before billing for PAP equipment and supplies.
Starting pressure: The pressure a machine begins at when you start therapy or a ramp. It is usually lower than therapy pressure to make falling asleep easier.
Supplemental oxygen bleed-in: Adding oxygen into the PAP circuit when prescribed. The AirSense 11 guide lists a maximum supplemental oxygen flow of 15 L/min.

T

Tidal volume: The volume of air in a normal breath, reported by some machines and OSCAR.
Titration: The process of finding the pressure that controls your breathing events, either in a sleep lab with a technician adjusting pressure, or at home using an auto-adjusting machine and its data.
Titration study: A lab sleep study focused on finding the right pressure for PAP therapy.
Top-of-head tube connection: A mask design where the tube connects at the top of the head, keeping the hose off the face. ResMed calls these tube-up designs.
Travel CPAP: A small, lightweight CPAP designed for travel, often without a water humidifier. The ResMed AirMini weighs about 300 g.
Treatment-emergent central sleep apnea: The clinical term for complex sleep apnea: central apneas that emerge during PAP treatment of obstructive sleep apnea.
Tube temperature: A comfort setting for heated tubing that sets how warm the air is kept. Raising it reduces rainout.
Tube type setting: A setting on some machines for which tubing is attached (for example slim versus standard), so pressure is compensated correctly.

U

UV CPAP cleaner: A device that uses ultraviolet light to sanitize CPAP parts. The FDA says it is not FDA cleared or approved for cleaning CPAP machines and that UV can break down accessories.
Under-nose full face mask: A full face mask that seals under the nose and around the mouth, leaving the nose bridge free, such as ResMed's AirFit F30 and F40.
Unintentional leak: Air escaping around the mask seal or out of the mouth, rather than through the vent. This is the leak number to watch.
Upper airway resistance syndrome (UARS): Sleep-disordered breathing with many RERAs and flow limitation but few full apneas or hypopneas.
Usual maximum quantity: The supply quantity Medicare considers reasonable and necessary in a period, for example 2 nasal cushions per month. Larger quantities are denied unless justified.

W

Water chamber (tub): The removable water tank of a humidifier. Medicare codes a replacement as A7046, with a usual maximum of 1 per 6 months.
Watt-hour (Wh) rating: A battery's energy capacity, found by multiplying volts by amp hours. The FAA limits lithium ion batteries to 100 Wh, or 101 to 160 Wh with airline approval.

CPAP and sleep apnea glossary

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