## 4 Discussion  
The aim of the present study was to explore the delivery efficiency of surfactant aerosols during HFNC to estimate the potential of this treatment for spontaneously breathing premature infants at high risk of developing RDS. The benchmark study revealed the surfactant lung dose to be relatively low independent of types of NCs (<2%). We further compared, in vivo, the effect of nebulized surfactant delivered during HFNC with the clinical surfactant administration method via intratracheal instillation. The modest lung doses achieved in the benchmark study anticipated the lack of effect seen in the in vivo nebulized surfactant study, where arterial oxygenation and lung mechanics did not improve and were significantly worse than the intratracheal instillation of surfactant. The aerosol delivery of exogenous surfactant during NRS has been a long-pursued goal of neonatal research. Two recently published randomized, controlled trials have reported the positive effects of nebulized surfactant during NRS. [PERSON] et al. reported a significant reduction in the need for mechanical ventilation in premature infants (GA 29-34 weeks) treated with a nominal dose of 200 mg/kg of nebulized _Poractant alfa_ delivered with the eFlow Neos nebulizer [25]. [PERSON] et al. reported a significant reduction in the intubation rate for surfactant instillation in infants (gestational age 23-41 weeks) receiving 210 mg/kg of nebulized _Calfactant_ using a Solarys nebulizer modified with an oral interface, namely a \"pacifier adapter\", while NRS was simultaneously delivered with a separate nasal interface [26]. The beneficial effects of nebulized surfactant in both studies were restricted to the more mature premature infants (>31 weeks' gestation), who have a higher likelihood of less severe RDS. It is worth noting that CPAP was exclusively used in the [PERSON] study during surfactant nebulization and was the predominant NRS type (70% of enrolled infants) in the [PERSON] et al. study. Over the last years, HFNC has gained momentum as a first-line treatment for mild and moderate RDS [3; 5]. Therefore, effective aerosol delivery strategies compatible with this NRS modality would be highly desired. However, to the best of our knowledge, aerosol lung deposition in premature infants managed with HFNC remains unknown. [PERSON] et al. investigated the aerosol deposition of a radioactive tracer in 18-term infants with congenital cardiac disease (median age 26 days) supported with NCs [32]. The radioactive tracer was delivered using a vibrating-membrane nebulizer (Aerogen Solo)  
Figure 4: Box plots showing (**A**) dynamic compliance (C\({}_{\text{dyn}}\)) in adult rabbits at baseline, after inducing respiratory distress (after BALs) and 180 min after receiving different treatments, and (**B**) pressure registered after applying 30 mL of air (P\({}_{\text{V30 mL}}\)) post mortem into the lungs of animals treated with a humidified high-flow nasal cannula (HFNC) only (control, grey boxes), with 200 mg/kg of intratracheal surfactant (SF instillation, black boxes), or with 400 mg/kg of nebulized surfactant in combination with HFNC (white boxes). The boxes display the median (horizontal line) and the first and third quartiles. The \(x\) within the boxes indicates the mean of each group, and the whiskers display the maximum and minimum values within the dataset. The dots beyond the whiskers represent outlier values. * Between SF instillation and HFNC-only groups, \(p<0.01\); * Between SF instillation and HFNC + nebulized surfactant (400 mg/kg) groups, \(p<0.01\). placed right before a corrugated tube connected to the NCs. With a flow of 2 L/min, the estimated lung deposition was 0.46% of the nominal dose, which could be increased to 1.10% by lowering the flow to 0.2 L/min. The authors also reported that pilot studies with a flow of 3 L/min were associated with high levels of nasal deposition. The relatively low lung deposition observed by [PERSON] et al.