Is humidified high flow nasal prong oxygen a physiologically different therapy in adults with the mouth open and closed: evidence from patient and volunteer studies

High flow humidified nasal prong (HFNP) therapy involves the delivery of warmed humidified gas, often oxygen enriched at or above peak inspiratory flow rates via prongs in the nostrils. A version of this therapy was first described by Lomholt involving delivery of flows of up to 30 litres per minute of humidified gas flow into one nostril in 1968. Since this initial description HFNP has been further developed, becoming a cornerstone therapy in both critical care and ward settings. HFNP is indicated in hypoxic respiratory failure, to prevent the need for intubation or re-intubation in high-risk patients and may be noninferior for a population of patients with COPD. The therapeutic effects of HFNP are thought to be mediated by well described physiologic effects: (i) High gas flows at the nares meet with the expiratory stream to increase positive end expiratory pressure (PEEP) in the airways. (ii) Gas flows delivered to the nares equal to or in excess of peak inspiratory flow rate enhance the delivery of a desired fraction of inspired oxygen (FiO2). (iii) Improved CO2 clearance is mediated by washout of anatomic dead space. Each of these physiologic effects is potentially different with open versus closed mouth breathing. (i) During nasal breathing the upper airways account for half of the resistance to airflow. It would be expected that provided normal patency of the pharyngeal airway opening of the mouth would decrease the resistance to airflow during expiration and decrease airway pressure for patients using HFNP. (ii) Provided the rate of flow delivered at the nares is greater than peak inspiratory flow rate it has been intuited that the FiO2 delivered at the nostril would equal that transiting the larynx. Entrainment of air in the oropharynx from a venturi effect with the mouth open and high resistance to nasal airflow due to variant anatomy and/or turbulent flow create theoretical potential for the FiO2 inhaled through the larynx to be less than that delivered via HFNP. (iii) An open versus closed mouth may facilitate increased anatomic dead space washout from the mouth and impact CO2 clearance. We aimed to review available literature to answer the question: in adult patients or volunteers, is HFNP different when the mouth is closed versus when the mouth is open for the physiologic outcomes of PEEP, measured FiO2 in or close to the trachea, or CO2 clearance?