細気管支炎治療における非侵襲的換気と神経調節換気補助（NAVA）の比較

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.