High Flow Nasal Cannula

High Flow Nasal Cannulas' (also known as High Flow Nasal Prongs) are a relatively new invention in the medical field. It is a highly effective way to administer supplemental oxygen that is heated and humidified. 
Many clinical studies indicate that High Flow Nasal Cannulas' are a superior oxygen delivery device, that provide improved outcomes to unwell patients.

They are a form on non-invasive respiratory support. The effects of High Flow Nasal Cannula are similar to those of Continuous Positive Airway Pressure, CPAP, with a mask. 

This blog post will look at what High Flow Nasal Cannulas' are, and why we use them. 

What is High Flow Nasal Cannula?

High flow nasal cannula is similar to normal nasal cannula, as they are two prongs which sit in the nostril and provide oxygenation without the need for a mask, which some patients may not tolerate well. Normal nasal cannula can only deliver oxygen at flow rates of 1L/min up to 15L/min (although going about 4L/min is not recommended). This oxygen is dry and cold, which multiple studies indicate is bad for long term use. 
High flow nasal prongs allow the delivery of heated and humidified oxygen. This protects the nasal mucosa and improve mucocililary clearance of secretions.  As the name suggests, high flow nasal cannula can provide oxygen flow greater than 15L/min. Most units allow for delivery of oxygen up to 70L/min, depending on the manufacturer. 
Hight flow nasal cannula consist of an oxygen/air blender (allowing for titration and control over the Fi02), flow meter, active humidifier, single heated tube and nasal cannula. 

Why use it?

HFNC are becoming a popular choice in Intensive Care, Anaesthesia and Emergency care due to a number of reasons. It is considered to have a number of physiological advantages when compared to standard oxygenation techniques, including reduced anatomic dead space, PEEP, a consistent and measurable Fi02, and humidification. 
It can be used to support a diverse range of conditions, such as hypoxemic respiratory failure (Type 1 Respiratory Failure), exacerbation of chronic obstructive pulmonary disease, post extubation and pre-oxygenation prior to intubation, sleep apnoea, acute heart failure, and patients who are 'not for intubation'. The ability to titrate not only oxygen flow, but also Fi02 allows us to cater this therapy to each patients needs and to adjust when patients improve or deteriorate. 
HFNC has been shown to decrease work of breathing, allows for clearing of secretions, provides airway pressure and therefore preventing alveolar collapse, and preventing the need for respiratory support elevation. 
However, gaps in research are evident. There is not yet a confirmed list of 'indication', and it is currently assessment of a patient by patient basis. 

Benefits - 

Cold and dry air, such as the air delivered through standard oxygenation therapies, can lead to airway inflammation. This causes an increase in airway resistance and impairs mucociliary function, leading to decreased secretion clearance.  Heated and humidified air maintains effective mucociliary function, improves mucous clearance and reduces the caloric expenditure in acute respiratory failure. 
HFNC can provide extremely high gas flows, which is important in patients in acute respiratory failure. These patients are often tachypneic, and have significantly increased peak inspiratory flows. Often even masks such as a non-rebreather mask cannot match the patients peak inspiratory flows, in these circumstances HFNC are superior. 
HFNC increases the patients Functional Residual Capacity (FRC) by providing additional PEEP. It is approximated that for every 10L/min of flow, PEEP is increased by 1mmHg. This increase in PEEP also increased intrathroacic pressure therefore decreasing cardiac pre-load (hence why it can be used in acute heart failure). 
High flow nasal cannulas also 'washout' dead space. Often we rebreathe some of the expired air that is left in our upper airways. HFNC washout this expired air, and fill it with fresh gas. In healthy people, this isn't important. However in unwell patients, this also leads to less carbon dioxide being inhaled. 
Lastly, HFNC are often more tolerated than other forms of non-invasive positive pressure ventilation (CPAP or BiPAP). Therefore we increase our patients ability to be compliant. 

Special Care Required - 
HFNC is contraindicated in patients who's arterial blood gas shows significantly elevated PaC02, mid maxillary facial trauma or patients with suspected pneumothorax. 
As with all acutely ill patients, patients of high flow oxygenation need to monitored closely. Nurses and other staff should also be competent and comfortable in the use of high flow oxygenation devices. 
Remember to explain to the patient what the device is, and how it will help them. 
The use of HFNC needs to be constantly reviewed, and the settings should also be reviewed to ensure that they are suited to the patient. 

And remember, if your patient looks like they are deteriorating and may be a good candidate for HFNC, talk to the medical team. 
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Some Important Terms - 

• BiPAP - Bilevel Positive Airway Pressure - A way of giving respiratory support via a mask. 
• CPAP - Continuous Airway Pressure - A way of giving respiratory support via a mask. (CPAP & BiPAP will be discussed in a future post). 
• HFNC/HFNP - High Flow Nasal Cannula/High Flow Nasal Prongs.
• Fi02 - Fraction of Inspired Oxygen. 
• Sp02 - Peripheral tissue oxygenation measured via pulse oximetry. 
• PEEP - Positive End Expiratory Pressure. 
• High Flow - Oxygen flows exceeding 15L/min. 
• Low Flow - Oxygen flows up to 15L/min. 

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