Effect of HFNC flow rate, cannula size, and nares diameter on generated airway pressures: an in vitro study

Increased use of non-invasive forms of respiratory support such as CPAP and HFNC in premature infants has generated a need for further investigation of the pulmonary effects of such therapies. In a series of in vitro tests, we measured delivered proximal airway pressures from a HFNC system while varying both the cannula flow and the ratio of nasal prong to simulated nares diameters. Neonatal and infant sized nasal prongs (3.0 and 3.7 mm O.D.) were inserted into seven sizes of simulated nares (range: 3-7 mm I.D. from anatomical measurements in 1-3 kg infants) for nasal prong-to-nares ratios ranging from 0.43 to 1.06. The nares were connected to an active test lung set at: TV 10 ml, 60 breaths/min, Ti 0.35 sec, compliance 1.6 ml/cm H₂O and airway resistance 70 cm H₂O/(L/sec), simulating a 1-3 kg infant with moderately affected lungs. A Fisher & Paykel Healthcare HFNC system with integrated pressure relief valve was set to flow rates of 1-6 L/min while cannula and airway pressures and cannula and mouth leak flows were measured during simulated mouth open, partially closed and fully closed conditions. Airway pressure progressively increased with both increasing HFNC flow rate and nasal prong-to-nares ratio. At 6 L/min HFNC flow with mouth open, airway pressures remained <1.7 cm H₂O for all ratios; and <10 cm H₂O with mouth closed for ratios <0.9. For ratios >0.9 and 50% mouth leak, airway pressures rapidly increased to 18 cm H₂O at 2 L/min HFNC flow followed by a pressure relief valve limited increase to 24 cm H₂O at 6 L/min. Safe and effective use of HFNC requires careful selection of an appropriate nasal prong-to-nares ratio even with an integrated pressure relief valve.