This is a common respiratory intervention in critical care, as well as a role in anaesthetic practice. It is usually delivered nasally, although some systems can be adapted to facemask use. Its use can overcome some of the challenges of delivering increased oxygen through other modalities, these problems being:
There are a variety of systems available. In general they will have the same key components:
Heated patient circuit
Delivery device e.g. nasal cannula
The systems allow separate adjustment of flow and oxygen concentration. A flow meter allows an appropriate flow rate to be selected, usually at a maximum of 40-60L/min. There is usually a dial to allow adjustment of the FiO2, up to 100%. The high flows will require a piped oxygen connection.
There is an active humidification/heating component to the system. This heats the air to the selected temperature, adjusted to patient comfort, usually between 33 and 43 degrees celsius. The air is also nearly fully humidified. A heated patient circuit prevents cooling (and subsequent condensation) of the gas as it is delivered to the patient.
The delivery system is usually through specially designed nasal cannula. These have an adjustable strap to fit them securely in place. The prongs themselves are soft to allow them to fit comfortably in the nose. The size can be selected for the patient.
Mechanism of Action
The mechanisms are not fully understood, with some myths being present about the exact effects.
The high oxygen flow means that the FiO2 actually reaching the alveoli is maintained, even and high work of breathing. Normally, the rapid inspiratory flow, especially when the patient is working hard in respiratory failure, would be more than the oxygen delivery and so air would also be entrained (essentially ‘diluting’ the FiO2). The high flow rates, which also create an oxygen reservoir in the airways, reduces this (although very high low rates can be reached in extreme respiratory exertion, up to 120 L/min).
The high flow rates also allow oxygenation to continue in the presence of apnoea. This is where the advantage arises in the airway management of patients in whom the oxygen reservoir of preoxygenation may still not provide an adequate safety net e.g. high BMI, pre-existing high oxygen demands. The high flows provide an ongoing oxygen gradient that is continually replenished, prolonging the period of oxygenation, even in the absence of the mass flow that occurs with ventilation.
The high flow rates also help with CO2 clearance. The rapid flow ‘washes out’ the airways and deadspace, maintaining the gradient that is needed for CO2 removal from the body. This can be clearly appreciated by the length of time that apnoea can be achieved.
The high flows are also described as developing some positive pressure in the airways. In some sources described to be quite high(3-7 cmH2O), but may be a bit lower at around 2.5 cmH2O. This is variably depending on the patient’s anatomy, especially if the have an open mouth (this may be common during respiratory distress with effortful mouth breathing) which would prevent any significant pressure generation. This low level of PAP is still described as being one contributing factor to the beneficial effects of NHFC. It may help reduce alveolar and airway collapse and so improve work of breathing and V/Q matching.
Normal breathing brings inspired air through the upper airways at comparatively lower flow rates (15 L/min) which allows time for warming and humidification. The nasal route achieves this more effectively than through the mouth (increase surface area achieved through the turbinates). With higher respiratory flows (such as in respiratory distress) this process is less efficient. The delivery of cold and dry supplementary oxygen exacerbates this. The heated and humidified air notably increases the comfort for the patient (compared with a NRB mask for example). There is also less physiological effort put into heating and humidifying the air. This is also translated into improved mucociliary function, as the drying effect can lead to impaired function and subsequent poor mucus flow.
It is important to be aware of these so that the systems are appropriately used. Their effectiveness in ‘making numbers better’ may, in some circumstances, lead to inappropriate use that only delays an appropriate respiratory intervention e.g. intubation and ventilation.
Uses include. Critical care
High oxygen demands
High risk airway
Hypoxic respiratory failure (non-cardiogenic)
Poor tolerance of other therapy
High risk airway
Apnoeic oxygenation intraoperatively
Base of skull fracture
These are usually a result of the high flow/pressure in the upper airway.
It is important to remember that inappropriate use may prevent timely intervention of more appropriate respiratory care (e.g. intubation) and thus lead to complications via this route.
This can be adjusted and should be adjusted to the patient’s work of breathing and comfort. A staged increase is recommended by some authors. An example regime (BJA Education):
25 L/min and 31 degrees - first 15 mins
Titration up of temperature and flows over first 30 mins
FLORALI This was a small French study. It compared NHFC to NIV to standard oxygen (NRBM) The results showed a reduction in 90 day mortality in the group receiving NHFC. There was a non-significant reduction in intubation rates in the group receiving NHFC.
THRIVE This was a small study (more of a case series) which looked at the use of NHFC for assisting in difficult intubations. It appeared to be effective at reducing the extent of severe desaturations and allow some impressively long periods of apnoea without ill effect. http://www.rapidsequence.org.uk/blog/apnoeic-oxygenation-and-thrive