Discussion
This study demonstrated that the proportion of backup ventilation decreased with increasing gestational age, especially after 26 weeks PMA. However, even the most immature infants triggered most of the ventilator breaths by their own respiration. An increase in the proportion of backup breaths anticipated clinical deteriorations, thereby showing the clinical value of observing the trends of backup ventilation.
Preterm infants have immature respiratory control leading to apnea.15,16 With increasing myelination of the brainstem, respiratory effort becomes stronger, and the number of apnea decreases.15–17 Postnatal maturation of the respiratory center explains our results: the decrease in backup ventilation and the increase in nRR with increasing gestational age. Our study showed that nRRs were already in the physiologic range of 40 – 60 breaths per minute, and the proportion of backup breaths was very low from 26 weeks of gestation. This justifies the implementation of NAVA from a very early PMA.
The development of neural respiratory control could be demonstrated because the study population was supported with NAVA rather than other controlled ventilatory modes. In previous studies, controlled mechanical ventilation led to diaphragmatic dysfunction and prolonged ventilation, whereas patient-triggered ventilation prevented diaphragm weakness and maintained spontaneous breathing activity.18–22 Since NAVA supports infant respiration synchronized with the Edi and provides backup ventilation only during neural apnea, it would be ideal for preventing diaphragm weakness and strengthening their respiratory effort.
This is the first study to provide information about the typical amount of backup ventilation in preterm infants at different gestational ages. However, the amount of backup ventilation is affected by a set apnea time and a backup rate during NAVA. Choosing an optimal apnea time and a backup rate is important to avoid hypo-/hypercarbia in premature infants.23 These parameters should be chosen and adjusted according to each infant’s age and clinical condition. In our study, the median apnea time was 3 and 5 seconds for invasive and noninvasive NAVA, respectively, and the median backup rate was 30 breaths per minute for both. The included infants remained stable with these settings except during their clinical deteriorations.
As the NAVA mode is based on the patient’s own respiratory drive, it is inevitably sensitive to detect an increase in apnea, which is the most common symptom of clinical deterioration in preterm infants. The current ventilators operating NAVA display trends in the proportion of backup ventilation and nRR for the last 24 or 72 hours. These trends can be useful to detect early signs of clinical deterioration in this vulnerable population.
This study has weaknesses inherent to observational studies. First, \souteach PMA group was heterogeneous in terms of the severity of respiratory distress and length of mechanical ventilation. However, our sample came from approximately 1 year of patient days (354) and included almost half of all ventilated preterm infants in a level III neonatal intensive care unit during the study period. Second, because we collected data for mechanically ventilated patients, the majority of study recordings was from infants having bronchopulmonary dysplasia, and they do not represent all preterm neonates. However, since they are the target population of ventilator care, it is meaningful to understand the developmental changes in their neural respiratory drive and the significance of an acute increase in backup ventilation.
In conclusion, we used the proportion of backup ventilation during NAVA to show how the control of breathing matured with increasing gestational age in preterm infants. It is important that even the most immature infants triggered most of their breaths by their own respiration. As an acute increase in the proportion of backup ventilation anticipated clinical deterioration, we identified the trends in backup ventilation as a useful tool to detect early warning signs of clinical deterioration in preterm infants.