I/E ratio -  inhalation/exhalation ratio, or the relative length of inhalation (breathing in) compared to exhalation (breathing out).

IAQ - indoor air quality.

Idiopathic hypersomnia - a disorder of excessive sleepiness in which the affected individual sleeps longer than normal (greater than 10 hours), is excessively sleepy, falls asleep at inappropriate times, and frequently takes naps. Its exact cause is unknown.

IH - Idiopathic Hypersomnia 

IHD - Ischaemic heart disease 

Imidazopyridines - New class of compounds inducing sleepiness. Zolpidem, trade name Ambien, is in this class.

Immune system -  the body's defense system that protects us against infections and foreign substances.

Inappropriate Sleep Episodes – unplanned sleep periods often occuring in an unsafe situation (i.e., while driving). These episodes are always due to sleep deprivation.

Incentive spirometer -  a device that encourages deep inspiration to expand the lungs and improve cough effectiveness.

Indication - reason to use.

Inflammation - a response in the body that may include swelling and redness.

Inhaler -  See metered dose inhaler (MDI)

Insomnia - the inability to sleep applied to the general complaint of having trouble falling or staying asleep; insomnia is a symptom usually caused by underlying problems. See also transient, short-term, chronic, and sleep onset insomnia.

Inspiration - breathing air into the lungs; inhalation.

Inspiratory Phase - part of the breathing cycle in which air is inhaled. 

Intubation -  placing a tube in the trachea (wind pipe) to enable artificial breathing; can be a lifesaving procedure.

Invasive - Refers to a medical procedure in which a bodily orifice or the skin must be penetrated for the purpose of collecting data, or for diagnosing or treating a disorder.

IPAP - Inspiratory Positive Airway Pressure. The pressure prescribed by a physician for the inspiratory phase on a Bi-level CPAP device, used in the treatment of OSA.

IPFM - Integral pulse frequency modulated 

IPPB - abbr. intermittent positive pressure breathing

IRM - Institute of Respiratory Medicine

Irritant - substance that is not an allergen (see allergen) but can cause a reaction in the airways or damage the lungs.

ITS - I'm Tired Syndrome