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.