DISCUSSION
In our study, HFNC was a proper respiratory management for patients admitted to PICU because of asthma. We observed that there was no difference in PS at admission between children which only received HNFC or those with NIMV support. Children with only HNFC received less pharmacological therapies and have fewer days of hospitalization. Finally, they did not require escalating therapeutic measures the days of PICU admission were lower. We observed that lower SatO2/ FiO2 and higher PCO2 at admission were related to the use of NIMV.
The HFNC delivers a flow of warm and humidified air, with a variable oxygen fraction (between 0,21-1) and a flow between 2 and 60 litres. From a theoretical point of view, the HFNC reduces the oropharyngeal dead space, reduces CO2rebreathing26, improves mucociliary clearance27and generate an end positive pressure in the airway up to 6 cmH2O during expiration17,28,29. The growing interest in the use of HFNC as respiratory support derives from these properties added to a higher comfort for the patient24,29. It also does not need synchronisation with patient breathing and requires less nursing care (versus a NIMV device). The HFNC would help the respiratory muscles to overcome the auto-peep characteristic of obstructive pathologies. Also, this constant flow may facility the renewal of nasopharyngeal air that would contribute to improving CO2 washing and oxygenation17.
As previously said, asthma is one of the leading causes of PICU admission. The evidence about the utility of HFNC as optimal respiratory support is scarce. Recently, Ramnarayan et al. carried out a pilot study to assess it through a multicenter and randomised clinical trial. They found that it was common to switch treatment from HFNC to CPAP. Also, HFNC patients experienced fewer ventilator-free days at day 2821. Added to this, an observational study in 42 asthmatic children by Pilar et al.concluded that initial support with HFNC was no optimal and delay support with NIMV20. In our prospective observational study, the HFNC was the most frequent respiratory assistance applied. In 74% of patients, the HFNC was their first and only support. None of these children needs to escalate to NIMV. In the rest cases, the HFNC was always used as de-escalation of NIV.
About this absence of necessity to switch from HFNC to NIMV, we must do some commentaries. As can be seen in our series children with higher PCO2 and lower SatO2/ FiO2 received NIMV as first support. Furthermore, there is a tendency to use HFNC in older patients (median age 25 versus 14 months in the case of NIV, p> 0.05). There were also differences among groups regarding chest radiography, initiation of empirical antibiotic therapy, administration of magnesium sulfate, and continuous nebulisation of salbutamol. Although we did not find differences in PS we thing that children with NIMV support were more severe at PICU admission. As explained previously the management was not protocolised. The attending experience influenced the respiratory assistance chosen and act as a selection bias that for sure influences our results.
About the days of PICU or hospital admission, they were no longer in HNFC group that in others (Table 3). These findings are contrary to what has been described in other studies21. This observation may be related to this selection bias in which HNFC will be effective.
This study has several limitations. It is a single-centre study. Patients under the age of two are included. Although those with acute bronchiolitis diagnosis were excluded, this fact should be taken into account. It is also observed that this was the age group in which the need for non-invasive was higher. The choice between different therapeutic options was influenced by the know-how of the physician in charge of each patient. This non-protocolised approach influenced the results through a selection bias about the utility of HFNC. It also informs about the importance of clinical experience to ensure the success of this respiratory approach.
In summary, in our study, the HFNC was a safe approach that did not require escalation to NIV. It did not increase the length of hospital admission. Initial blood gases and the absence of high oxygen requirements on admission could be helpful to select children that will be good responders to HNFC. The external validation of our results is complex. Randomised and multicenter clinical trials are needed to verify this data.