Question 2

Highest mark

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.

College Answer

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.

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
- in fact, in neonates this may lead to pneumothorax
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
Epistaxis
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.

Nasal fracture, or any other sort of nasal injury
Recent nasal surgery
Base of skull fracture - you might induce a pneumocephalus and god knows what else
Decreased level of consciousness (or for that matter, anything else which might mandate intubation)

Groves, Nicole, and Antony Tobin. "High flow nasal oxygen generates positive airway pressure in adult volunteers." Australian Critical Care 20.4 (2007): 126-131.

Ricard, J. D. "High flow nasal oxygen in acute respiratory failure." Minerva Anestesiol 78.7 (2012): 836-841.

O’Brien, Bj, J. V. Rosenfeld, and J. E. Elder. "Tension pneumo‐orbitus and pneumocephalus induced by a nasal oxygen cannula: Report on two paediatric cases." Journal of paediatrics and child health 36.5 (2000): 511-514.