DISCUSSION
In this multicentre population of young infants with severe AVB, the
failure rate of HFNC was 39%. Failure of this first step of respiratory
support occurred early, within 6 hours after HFNC initiation for half of
failing patients and within 24 hours for nearly 90% of them. Neither
ROX index, nor physiological variables usually collected in infants with
acute respiratory failure had early discriminatory capacity to predict
failure of management with HFNC.
In a previous study, Kannikeswaran et al. observed an association
between ROX index and HFNC failure, in other words the need for positive
pressure ventilation, in <2 years infant with bronchiolitis
[23]. This result was particularly relevant from the perspective of
directing these patients to the most suitable units downstream of the
emergency department. In this perspective, our study aimed to identify a
threshold for this index, associated with actual predictive capacities.
Our AUC results suggested a weak and non-significant relationship
between ROX and HFNC failure, which currently does not confirm the
interest of this tool in a clinical decision rule. The main difference
with studies suggesting that ROX index may be a good marker to predict
the risk of HFNC failure probably comes from different patient’s
characteristics [22, 23]. Our trial involved much younger patients
and, as our results indicate, age is a key risk factor for respiratory
failure in this population [12, 13]. In addition, the TRAMONTANE 2
study included patients probably affected by more severe forms of the
disease, as mWCAS > 3 was required to be eligible,
which signals unambiguous respiratory distress. In Kannikeswaranet al. study, the regression model to estimate the odds ratio of
PPV requirement was based on the highest ROX quartile, suggesting marked
heterogeneity in the severity of bronchiolitis [23].
The ROX index takes into account only two characteristics of a
respiratory distress, namely oxygenation and tachypnea. While these
parameters are critical in patients with acute hypoxemic respiratory
failure, they do not take into account all of the determinants of HFNC
failure in AVB. Indeed, AVB present different phenotypes: sometimes as a
restrictive parenchymal disease, but a majority of these infants
demonstrate a severe obstructive lung disease, with markedly increased
work of breathing and frequent apneas [4, 5, 25]. These two
elements, as well as comfort, are not integrated in the ROX index,
whereas they often intervene in the clinician’s decision to upgrade
respiratory support in a patient with AVB.
No variable, observed or calculated, was able to predict HFNC failure in
this work. However, half of failure occurred within 6 hours, which
suggests that such delay may be a relevant criterion for triage. Insofar
as the volume of patients in the emergency room allows it, it may be
consistent to maintain the infant with HFNC and adapted monitoring
during this timeframe before deciding admission to pediatric intensive
care unit.
This study has several strengths, including a large, multicentric and
homogeneous population, in terms of severity and age, of infants with
AVB. In addition, predefined HFNC failure criteria had been validated by
a panel of experts providing from the 16 participating centres in the
trial.
We acknowledge some limitations. Half of the children received a flow
rate of 2 L/min/kg, and the other half 3 L/min/kg, but we are uncertain
whether this affects the interpretation of our results because the
failure rate was exactly the same in the two groups.
The predictive value of the ROX index beyond the first hour of HFNC
delivery could have been evaluated, but it seemed more obvious that the
relevance of an urgent triage tool can hardly exceed 1 to 2 hours.