What’s That Flow? – High Flow Nasal Cannula Devices

What Is HFNC

High flow nasal cannula (HFNC) blends oxygen and air to deliver a constant FiO2 to a patient. It also utilizes heated tubing combined with humidification connected to a two-prong nasal cannula to deliver a heated humidified gas mixture to the patient. HFNC works by filling the patient’s oropharynx with a reservoir of a controlled gas mixture. On inspiration, the patient inhales the gas, washing away anatomical dead space in the oropharynx and upper airways, delivering this gas to the lungs, which improves gas exchange.  This improvement in gas exchange improves the work of breathing.  The humidified gases delivered to the patient by the high flow cannula can decrease airway inflammation, maintain mucociliary function, and improve airway clearance.

Higher flow rates can be achieved with HFNC than with traditional nasal cannula. HFNC can deliver up to 60 L/min based on the device as compared to 6 L/min on a normal low flow nasal cannula. The higher flow rates help patients in distress because they have much higher peak inspiratory flow rate demands.  A patient in distress can easily pull 40 L/min.  A patient in distress with an increase in demand on a low-flow nasal cannula will entrain room air into the lungs, resulting in oxygen dilution. HFNC with higher flow rates will result in greater delivery of oxygen into the lungs, thus producing less entrainment of room air.  

Does HFNC Create PEEP?

According to Hyzy, HFNC has been shown in studies to produce a “PEEP effect.” It helps by decreasing work of breathing, improves oxygenation in patients with alveolar filling diseases, and unloads auto-PEEP. By increasing the airway pressure in the nasopharynx with increasing flow rates, the pressure at the end of expiration increases, similar to CPAP. While closed mouth breathing leads to approximately 0.8-1 of PEEP, open mouth breathing only produces about 0.4 of PEEP for each 10 L/min of flow. This result is limited and only provides mild PEEP at best.

Small Bore Cannula HFNC

One device that delivers HFNC utilizes a small-bore cannula to increase the velocity of the gas.  This type of HFNC was first introduced in 1999. This type distinguishes itself from other high-flow cannulas by using a high-velocity precision flow to deliver medical gases to the patient. These devices come with both a low flow and a high flow cartridge depending upon the patient’s age/size. The low flow cartridge provides 1-8 L/min while the high flow cartridge provides 8-40 L/min. The FiO2 is adjustable from 21% to 100%.  The device comes with the cartridge and tubing, the unit itself, two medical gas cords (one for air, the other for oxygen), and a variety of small-bore nasal cannulae to fit according to the patient population. The distributor describes the flow as “Placing your thumb over a running hose” thereby delivering medical gas at a quicker rate than others.  This device uses its high velocity to create a reservoir in the upper airways with highly oxygenated gases to treat hypoxic and hypercapnic patients.  This high velocity and reservoir reduce the patient’s work of breathing by filling the oropharynx (usually full of exhaled gas with higher quantities of CO2) with oxygenated gases decreasing a patient’s minute volume demand. This device uses membrane humidification to provide heated water droplets to humidify the gas delivered to the patient. The water can be seen flowing through the tubing of the machine when activated. This device flushes and fills gas faster in the upper airways due to its high velocity through the tubing and the small-bore nasal cannula.

Other benefits of this device include the ease of use, portability, ability to provide heated aerosol for trach patients, and ease of adding nebulizer treatments among numerous others. Portable units allow for the timely transport of a critical patient without the need of discontinuing treatment. This also allows for the early mobilization of patients to allow for quicker recovery. With the pre-assembled Aerosol Disposable Patient Circuit (ADPC), both continuous and intermittent nebulizer treatments can be added inline without interrupting the treatment. The ADPC is designed to work with the Aerogen Solo neb device. The trach adapter delivers heated humidification to possibly allow for quicker weaning and decannulation.  An added bonus is with the easy-to-control screen, its ease of use is irreplaceable.  

Large Bore Cannula HFNC

Large-bore HFNC distinguishes itself from small-bore HFNC by delivering flows from 2 to 60 L/min on the same unit without having to change cartridges. The FiO2 is adjustable from 21% to 100%.  Note, however, the upper O2 alarm is automatically set at 100%, which will cause the device to alarm if the delivered FiO2 is set at 100%.  You can fix this by setting the delivered FiO2 at 99%.  Unlike common small-bore devices, large-bore devices come with one medical gas cord (oxygen) that makes it easier to use when access to medical air is limited. This device comes with the unit itself, water chamber, o2 tubing, flow meter, O2 analyzer, and nasal cannulae varying in sizes specific to the patient population. This device utilizes entrained room air blending with dialed in oxygen by an attached flowmeter to create a delivered FIO2. This device uses the passover humidity system to humidify the delivered gas which also acts as a way to not transport contaminated viruses that can be carried with water droplets. The manufacturer prides itself on the better use of patient compliance with their device as flows are less forceful through their large bore tubing. This device also notes patient compliance since it is quieter and easier to tolerate. Also, the device can be used in the home setting since it can be connected to an oxygen concentrator.

Other benefits of this device is the ability to achieve secretion mobilization and mucociliary transport at 100% relative humidity delivered via nasal cannula, oxygen mask with adapter, or trach interface.  Nebulized treatments can be given in line with the use of the Aerogen Solo which attaches to the water chamber. There is a transport mode, however, during transport, the heater plate will switch off causing humidity output to decrease until the unit is plugged back in.  

Extubate to HFNC

Some studies note, patients with comorbidities and/or patients with an increased length of ventilator days may benefit from extubation directly to HFNC. Patients must be extubated directly to high flow and flow rate settings must start high.  As discussed already, high flow rates reduce the work of breathing.  Another study states starting HFNC immediately after extubation reduces respiratory fatigue. Next time you extubate a patient, think about pulling that HFNC out of your closet.