With regards to the use of high-flow nasal oxygen therapy in adults:
a) Describe the mechanisms by which high flow nasal oxygen therapy is believed to exert its beneficial effects.
b) List two potential adverse effects associated with the use of high-flow nasal oxygen therapy.
c) List two relative contraindications to the use of high-flow nasal oxygen therapy.
- Precise mechanism of benefit is yet to be elucidated.
- Increased FiO2:
- Gas inlet flow limits secondary room air entrainment.
- Provides anatomic oxygen reservoir in nasopharynx and oropharynx.
- Rinsing of airway deadspace with oxygen.
- CPAP effect:
- decreases atelectasis and improves V-Q matching.
- improves compliance.
- decreases work of breathing by counteracting intrinsic PEEP.
- Greater comfort:
- Warmed and humidified oxygen can be better tolerated, therefore better compliance from patients.
- Pressure areas in the nose. Epistaxis.
- Possible gastric distension.
- Nasal fracture.
- Upper airway haemorrhage. Base of skull fracture.
- Recent upper airway or aerodigestive tract surgery.
There is a great article available (Ricard et al, 2012) which dissects this oxygen delivery system.
Describe the mechanisms by which high flow nasal oxygen therapy is believed to exert its beneficial effects.
- PEEP effect
- Though it seems to only be about 3cm H2O with 60L/min flow, when the mouth is open
- If it works, then it has all the benefits of PEEP - recruitment of atelectatic lungs, decreased work of breathing, and so forth.
- On top of that, it is supposed to overcome the "nasopharyngeal resistance" of obese OSA patients
- In fact the benefits seem to be most pronounced in the obese patients- and the degree of improvement in gas exchange tends to be related to the degree of increase in end-expiratory lung volume, which suggests that there is a real alveolar recruitment effect.
- Increased FiO2 -
- The upper airways are "rinsed" with humidified oxygen; this is called the "pharyngeal dead space washout"; each breath drags more of this oxygenated air from this anatomical dead space and into the lungs.
- The delivery of high flow oxygen at a high concentration cannot be accomplished reliably by oher low-flow means, such as the Venturi mask.
- Increased comfort -
- Apparently, its comofortable. The main reason for this is the fact that the mouth is left alsone, unlike most forms of CPAP.
- Addtionally, the humidification of oxygen tends to decrease the nasty side effects of oxygen therapy, such as raw stripped mucosa
List two potential adverse effects associated with the use of high-flow nasal oxygen therapy.
- Overdistension of the alveoli, and barotrauma
- Discomfort associated with the device, its flow or the high temperature/humidity
- Nasal mucosal damage due to high flow
- Pressure areas due to the device
- Failure to achieve the desired effect because of mouth-breathing
- Overabundance of secretions (Velasco et al, 2014) - though some might view this as a desired effect
- Time-wasting (delaying the inevitable intubation)
- Aspiration of food or upper airway secretions
- Aspiration of circuit condensation water (there's no evidence that this causes pneumonia, but people complained about it in a survey of paediatric ICUs conducted by Manley et al, 2012)
List two relative contraindications to the use of high-flow nasal oxygen therapy.