## 4 Discussion
contrasts with the recent single-center, Phase II, uncontrolled trial assessing the safety and efficacy of nebulized surfactant, in which HFNC was one of the permitted NRS modalities for aerosol delivery to premature infants [27]. In this trial, [PERSON] et al. randomized 149 premature infants (24-37 weeks' gestational age) to either receive 100 or 200 mg/kg of surfactant aerosols (_Beractant_, Survanta, Abbot Laboratories) generated with jet or vibrating-membrane nebulizers during NRS. Compared to retrospective controls, the intervention with nebulized surfactant significantly reduced the need for intubation within 72 h. Notably, the authors reported that all infants receiving surfactant aerosol during HFNC (n = 37) avoided intratracheal intubation in the first 72 h. The authors acknowledged that at their institution, HFNC is used in the early stages of RDS with escalation to nasal CPAP followed by NIPPV with worsening respiratory distress. Moreover, 24 out of 37 infants (64%) managed with HFNC belonged to the 33-37 weeks' gestational age stratum, who are at a lower risk of developing RDS. Surfactant lung deposition was not reported in the trial, and no benchmark studies of lung dose estimation were referenced. Therefore, the positive outcome of the infants treated with nebulized surfactant delivered with HFNC in the [PERSON] et al. study may be partly explained by the inclusion of premature infants with uncomplicated respiratory distress who would do well just with NRS. Several preclinical studies have investigated the aerosol delivery efficiency during HFNC in vitro and in vivo. [PERSON] et al. [33] compared the lung dose of albuterol delivered with a vibrating mesh nebulizer (Aerogen Solo), either placed before the humidifier of the OptiflowTM system or proximal to the airway cast of a premature infant (DiBlasi model, based on a 26 weeks' gestation premature infant [34]). In this study, HFNC ventilation was delivered using infant intermediate NCs (Fisher & Paykel) at a flow of 3 L/min and the breathing pattern of an infant (VT 9 mL, RR 50 bpm, and Ti 0.5 s). Placing the nebulizer proximal to the airway cast, an albuterl lung dose of 0.90 \(\pm\) 0.26% was achieved, which was slightly increased to 1.30 \(\pm\) 0.17% by placing the nebulizer before the humidifier. [PERSON] et al. observed a flow-dependent variation of the aerosol lung deposition after nebulization during HFNC with a vibrating-membrane nebulizer (Aerogen Solo) in both benchmark and in vivo scintigraphy studies conducted in spontaneously breathing macaques (3.2-3.6 kg) [35]. The nebulizer was placed right before the humidifier of the OptiflowTM system. Their benchmark study used a neonatal-size NC placed at the Sophia anatomical infant nose-throat model (SAINT model, based on a 9-month-old infant [36]), which was further connected to a pump that simulated a tidal volume of 25 mL. The amount of radioactive tracer collected in the lung dose filter was significantly reduced by increasing the flow; at a flow rate of 2 L/min, the lung dose was 4.15 \(\pm\) 1.75% and was, respectively, reduced to 3.29 \(\pm\) 1.70% and 0.52 \(\pm\) 0.23% at flows of 4 and 8 L/min. The lung deposition further decreased in spontaneously breathing macaques to, respectively, 0.85 \(\pm\) 0.57%, 0.49 \(\pm\) 0.44%, and 0.09 \(\pm\) 0.04% at 2, 4 and 8 L/min. The lung doses achieved in our benchmark study are comparable with the outcomes of the [PERSON] et al. study, which also used a flow of 8 L/min. During our benchmark studies, surfactant bubbling could be observed at the outlet of the NCs, regardless of the NC type, and most of the surfactant leaked out of the NCs as a liquid film, depositing in the face of the PrINT model. This observation implies a high impaction of surfactant aerosol particles within the narrow tubing of the NCs, which dramatically reduced the lung dose.