Bronchiolitis is a common diagnosis in pediatric hospitals and critical care units. Viral
infection in younger patients often results in increased work of breathing, hypoxemia,
impaired ventilation, and increased secretion burden. In some cases, treatment of severe
respiratory failure includes intubation and mechanical ventilation. Current practice for
patients with bronchiolitis who require hospital admission is to initially provide
non-invasive ventilation to improve the patient's respiratory mechanics. This
non-invasive respiratory support can range from simple nasal cannula, to high-flow nasal
cannula, to non-invasive positive pressure ventilation.
The high-flow nasal cannula (HFNC) provides warm, humidified, oxygen-enriched air.
Therapy commonly is prescribed with a prescribed fraction of inhaled oxygen (FiO2)
delivered at 1-2 L/kg/min. This helps to improve oxygenation as the high rate of flow can
"wash-out" carbon dioxide in the upper airways and thus reduce the volume of dead space
ventilation.
Non-invasive ventilation (NIV) essentially provides a similar method of support as
invasive ventilation without the use of endotracheal tube. Prescribed airway support is
instead delivered non-invasively through a specialized nasal cannula or for larger
children an occlusive facemask of appropriate size. The ventilator provides positive-end
expiratory pressure (PEEP) with a prescribed delivery rate of a set inspiratory pressure
(positive inspiratory pressure, or pressure control). This ventilator support enables the
delivery of a set FiO2, helps maintain open airways to reduce atelectasis and allow for
improved oxygenation with better V/Q matching, and improves work of breathing. The
ventilator analyzes the flow generated by the patient's inspiratory effort and attempts
to provide the prescribed positive inspiratory pressure at the time of the patient's own
effort.
One of the major drawbacks of non-invasive ventilation for young pediatric patients with
bronchiolitis is the difficulty in achieving synchrony between patient effort and
ventilator-delivered positive inspiratory pressure. This is secondary to the large air
leak given the non-invasive apparatus and the low inspiratory flows generated by this
patient population. Thus, the ventilator and patient are often dyssynchronous which may
actually increase work of breathing and agitation while impeding on the ventilatory
support provided.
Neurally-Adjusted Ventilatory Assistance (NAVA) attempts to mitigate the harms of
ventilator/patient dyssynchrony. This modality utilizes a specialized catheter placed
into the esophagus, often via a nasogastric route, which has the capability of monitoring
the electrical activity of the patient's diaphragm. This catheter can also be utilized to
deliver feeds similarly to a basic nasogastric tube. The NAVA catheter monitors both the
activation of the patient's diaphragm (indicating patient respiratory effort) and the
strength of this activation in, referred to as the electrical activity of the diaphragm
(Edi) and measured in millivolts (µV). Both human and animal studies have positively
correlated the peak Edi values with work-of-breathing and demonstrated higher Edi values
when respiratory pathology is present . Based on the Edi tracing, the ventilator can then
deliver positive inspiratory pressure that is synchronous with both the patient's
respiratory effort and proportional to the strength of this effort through a multiplier
referred to as the NAVA level on the ventilator. This modality has been shown to improve
patient agitation levels, reduce the need for sedating medications, and enhance synchrony
in non-invasive ventilation modes.
The current practice model of the investigators entails that patients with bronchiolitis
who require more than 1.5 L/kg of HFNC or require non-invasive ventilation, whether via a
conventional or NAVA modality, are managed in the Pediatric Intensive Care Unit (PICU).
Both modalities for non-invasive ventilation (conventional and NAVA) are used routinely.
Patient respiratory status is aggregated into a value known as the Respiratory Severity
Score (RSS) which accounts for respiratory rate, dyspnea, retractions, and auscultatory
findings adjusted for the age of the patient. The RSS value is a validated assessment
tool with good interobserver reliability between Medical Doctors (MDs), Registered Nurses
(RNs), and Respiratory Therapists (RTs). It is calculated on a 4-hour basis for all
patients with bronchiolitis in the investigator's PICU and helps determine clinical
improvement or deterioration and better guide decisions to increase or decrease support.
While multiple physiologic studies demonstrate a reduced work of breathing with invasive
NAVA ventilation, the majority of pediatric studies focused on non-invasive NAVA
ventilation were designed to determine improvements in patient/ventilator
synchronization. The investigators' project aims are two-fold. The study team
hypothesizes that Edi levels and RSS scores will positively correlate for patients with
bronchiolitis, allowing for another metric to gauge clinical status. The investigators
also hypothesizes that the improved synchronization on NAVA-NIV may improve respiratory
status as measured by RSS scores and Edi levels, reduce further escalations in
respiratory support, shorten the length of non-invasive ventilation required, and reduce
intubation rates. This improvement will be more substantial when transitioning from HFNC
to NAVA-NIV compared to transitioning to conventional-NIV.
