Significant anatomical deviation...
http://www.emphysema.net/TTO_Q&A.htm

Increases in temperature may cause further enlargement of surrounding tissues and hence greater possibility of airway obstruction.

Other considerations for humidification devices
Hazards and complications associated with the use of humidification devices include
6.1 potential for electrical shock--heated humidifiers;(11-14)
6.2 hypothermia--HME or heated humidifiers; hyperthermia--heated humidifiers;(11-14)
6.3 thermal injury to the airway from heated humidifiers;(30) burns to the patient and tubing meltdown if heated-wire circuits are covered or circuits and humidifiers are incompatible;
6.4 underhydration and impaction of mucus secretions--HME or heated humidifiers;(1-7)
6.5 hypoventilation and/or alveolar gas trapping due to mucus plugging of airways--HME or heated humidifier;(1-7)
6.6 possible increased resistive work of breathing due to mucus plugging of airways--HME or heated humidifiers;(1-7)
6.7 possible increased resistive work of breathing through the humidifier--HME or heated humidifiers;(31-34)
6.8 possible hypoventilation due to increased dead space--HME;(8,15-25,26-30)
6.9 inadvertent overfilling resulting in unintentional tracheal lavage--heated reservoir humidifiers;(35)
6.10 the tact that when disconnected trom the patient, some ventilators generate a high flow through the patient circuit that may aerosolize contaminated condensate, putting both the patient and clinician at risk for nosocomial infection--heated humidifiers;(35)
6.11 potential for burns to caregivers from hot metal--heated humidifiers;

6.12 inadvertent tracheal lavage from pooled condensate in patient circuit--heated humidifiers;(35)
6.13 elevated airway pressures due to pooled condensation--heated humidifiers;
6.14 patient-ventilator dysynchrony and improper ventilator performance due to pooled condensation in the circuit--heated humidifiers;
6.15 ineffective low-pressure alarm during disconnection due to resistance through HME.(36)...
http://www.flightweb.com/forum/viewtopic.php?showtopic=24872

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Note added at 9 hrs (2007-02-01 05:53:18 GMT)
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this mentions some of the structures involved...
http://www.entcentrecclau.com.sg/info_obstructive_sleep.asp

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Note added at 1 day13 hrs (2007-02-02 10:06:45 GMT)
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People who create sleep apea devices (usually made of some type of acrylic or plastic) are assessed for suitability. Usually they are referred to a sleep doctor for assessment first. If they have anatomy which makes them borderline or poor candidates then
and intra-oral sleep apnea device is not warranted.
However the question hear pertains to a heated humidifier and when it should not be used. So we are either talking about
A) an intra-oral sleep apnea device or
B) some anatomical condition which precludes the humidifiers use in a heated mode.


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Note added at 1 day13 hrs (2007-02-02 10:24:23 GMT)
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I think I should have stated it as

significant deviation of supra-glottic airway anatomy

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Note added at 1 day23 hrs (2007-02-02 20:22:04 GMT)
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An HME is contraindicated under some circumstances.
5.1 Use of an HME is contraindicated for patients with thick, copious, or bloody secretions.(8,26-28)
5.2 Use of an HME is contraindicated for patients with an expired tidal volume less than 70% of the delivered tidal volume (eg, those with large bronchopleurocutaneous fistulas or incompetent or absent endotracheal tube cuffs).(15-25)
5.3 Use of an HME is contraindicated for patients with body temperatures less than 32°C.(8,29)
5.4 Use of an HME may be contraindicated for patients with high spontaneous minute volumes (> 10L/min).(8,26,29)
5.5 An HME must be removed from the patient circuit during aerosol treatments when the nebulizer is placed in the patient circuit.

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Note added at 1 day23 hrs (2007-02-02 20:32:15 GMT)
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Not for use with patients whose upper airways are bypassed...
http://global.respironics.com/UserGuides/UserGuideREMstarPro...


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Note added at 2 days1 min (2007-02-02 20:34:53 GMT)
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Contraindications for the Use of the HMEF* Table 1

http://www.chestjournal.org/cgi/content/full/120/4/1377/T1

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Note added at 2 days4 mins (2007-02-02 20:38:46 GMT)
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High-grade nasal obstruction...as contraindication to CPAP
http://www.treatment-options.com/article.cfm?PubID=NE03-1-1-...

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Note added at 2 days12 mins (2007-02-02 20:46:04 GMT)
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Bypassed Upper Airway....
http://www.cpapstation.com/catalog/partdocs/UserGuideMSeries...