Carbon dioxide washout during high flow nasal cannula versus nasal CPAP support: An in vitro study

To compare CO₂ washout time at different levels of HFNC versus NCPAP in a premature infant lung model with simulated mouth-closed and mouth-open conditions using two sizes of nasal cannula and full- and half-prong HFNC insertion depths.

A piston-cylinder lung simulator, having a fixed volume of 30 ml and a 4.8 ml dead space, simulated spontaneous breathing (6.5 ml tidal volume, 50 br/min, Ti = 0.5 sec). Two Fisher & Paykel™ cannulas (Fisher & Paykel Healthcare Ltd., Auckland, New Zealand) (2.8 and 3.2 mm O.D.) and two Infant-Flow™ (CareFusion, Yorba Linda, CA) NCPAP cannulas (3.4 and 4.1 mm O.D.) were applied to simulated airways having either 3.5 or 4.5 mm I.D. nares. Simulated mouth opening was a 5 mm I.D. side tap below the nasal interface. The lung was primed with 5% CO₂ . Washout times were determined at HFNC settings of 3, 4, 5, 6, and 8 L/min and NCPAP at 3, 4, 5, 6, and 8 cm H₂ O with simulated open and closed-mouth conditions and full- and half-inserted HFNC prongs.

Overall combined mean washout times for NCPAP with mouth-closed were significantly longer than HFNC over all five pressure and flow device settings by 16.2% (P < 0.001). CO₂ washout times decreased as flow or pressure device settings were increased. There were negligible differences in washout times between NCPAP and HFNC with mouth-open. Mouth-open washout times were significantly less than mouth-closed for all conditions. Overall closed-mouth washout times for HFNC half-prong insertion were longer than for full-prong insertion by 5.3% (P < 0.022).

Significantly improved CO₂ elimination using HFNC versus NCPAP should be a particularly important consideration in premature infants having very high dead space-to-tidal volume ratio compared to larger infants. Pediatr Pulmonol. 2017;52:792-798. © 2017 Wiley Periodicals, Inc.