The Physiologic Effect of the Flow Generated by High Flow Nasal Cannula in Mild Respiratory Failure

Oxygen (O2) therapy delivered by means of High Flow Nasal Cannula (HFNC) is being increasingly used. This device, capable of delivering high flow, oxygen enriched gas, has been shown to reduce intubation rates in patients with hypoxemic respiratory failure. The primary components of HFNC are "nasal prongs" (or cannulae) that are inserted into the nostrils, a device which provides a warmed and humidified air / oxygen mixture and regulators that control oxygen concentration, temperature and total gas flow. Commercially available systems can deliver up to 40 - 60 liters per minute (LPM) of oxygen flow.

This method of non-invasive oxygen delivery has several presumed important working principals. Firstly, the high flow rate may meet the patient's own inspiratory flow rate. This enables accurate delivery of a high fractional inspired concentration of oxygen (FiO2). Second, the high gas flow might also produce a certain degree of Continuous Positive Airway Pressure (CPAP). In addition, the heated and humidified gas might also wash out/reduce the physiological airway dead space of the patient. Furthermore, HFNC is more comfortable and might be better tolerated than other Non-Invasive Ventilation (NIV) devices, such as Bi-level positive airway pressure (BiPAP). This, in turn, can prolong the time the patient is comfortably connected to the HFNC device.

Information regarding the basic physiologic influence of HFNC on oxygenation, ventilation and vital signs is missing in the literature. Patients with mild respiratory failure provide an opportunity to examine this topic. On one hand, these patients suffer from respiratory failure manifested by hypoxemia, on the other hand the respiratory failure is mild to the extent that removal of the conventional oxygen therapy (COT) will not lead to immediate respiratory collapse. This prospective study evaluates the physiologic effects of HFNC in 28 patients with mild respiratory failure.

Inclusion criteria

• Minimal age 18
• The patient had invasive blood pressure monitoring via an arterial line in situ
• The patient had a mild respiratory failure manifested by the need for oxygen support via nasal prongs in order to maintain oxygen saturation (SpO2) above 90% for at least one hour prior to the start of the intervention

Exclusion criteria

• Shortness of breath
• Patients regularly using O2 at home
• No patient consent for participation