Materials and Methods
A prospective observational study took place from March 2020 to December
2020 in the level III neonatal intensive care unit of Turku University
Hospital, Finland. We included preterm infants born before 36 weeks of
gestation who received at least 24 hours of ventilatory support with
invasive or noninvasive NAVA using Servo-i or Servo-n ventilators
(Getinge, Solna, Sweden).
As NAVA was the primary ventilation mode in this unit, for intubated
preterm infants, we began NAVA whenever the infants had spontaneous
breathing. As soon as possible we extubated them and applied noninvasive
NAVA. Before the start of NAVA mode, a standard feeding tube was
replaced with a dedicated electrode-equipped catheter to detect Edi (Edi
catheter, Getinge, Solna, Sweden). The trigger Edi level was set as 0.5
μV above the minimum Edi. Attending clinicians decided and adjusted the
NAVA level, positive end-expiratory pressure (PEEP), fraction of
inspired oxygen (FiO2) and backup ventilation (pressure
control mode) settings according to the infant’s condition. The apnea
time was set as 2 to 10 seconds. After the set apnea time without the
Edi signal, the backup ventilation started with a preset peak pressure
and frequency until the Edi trigger resumed. Our usual backup rate was
set as 20 to 40 breaths/min and the inspiratory time was set as 0.4 to
0.5 seconds. The safety limit for peak inspiratory pressure (PIP) was
set as 25 to 35 cmH2O. If the infant lost his/her
cardiorespiratory stability during NAVA, the ventilator mode was
changed, and this episode was defined as ‘clinical deterioration.’
For the infant participating in the study, ventilator settings and
respiratory variables were recorded every day and exported to a specific
computer using Servo Record Viewer version 1.0 (Maquet Critical Care AB,
Getinge, Solna, Sweden). The collected data provided the values for PIP,
mean airway pressure, PEEP, expiratory tidal volume, peak Edi, minimum
Edi, measured respiratory rate (RR), neural respiratory rate (nRR) and
percentage time spent in backup ventilation for each minute. All the
ventilator data were inspected and compared with the event logs recorded
automatically from the ventilator, which included all alarm
notifications, mode and setting changes, cable connections and
disconnections. Data during disconnection of the Edi cable, malfunction
or dislocation of the Edi catheter were excluded from the analysis. The
mean values during each day were computed for each ventilatory variable.
If there was a change in the ventilatory setting parameters, we chose
the parameter that was applied for a longer duration in the 24-hour time
period. For clinical deterioration, ventilator data 30 hours prior to
the episode were reviewed at 6-hour intervals. The zero time point was
defined as the moment when the ventilator mode was switched from NAVA to
others because of clinical deterioration.
The following data were collected from medical records: gestational age
at birth, birth weight, sex, postmenstrual age (PMA) and body weight at
the study date. For clinical deterioration, information about its cause
and interventions were collected. The study was approved by the Ethics
Committee of the Hospital District of Southwest Finland as well as by
the scientific research committee of the Pediatric Department of Turku
University Hospital, Finland. Written informed consent was obtained from
both parents according to the guidelines of the Research and Ethics
Board before patient enrollment in this study.
Correlations between respiratory parameters and PMA were analyzed using
the Spearman test. Changes in the proportion of backup ventilation
before clinical deterioration were analyzed using the Friedman test. Ap value < 0.05 was considered to be statistically
significant. Statistical analyses were performed using SPSS v27.0 (IBM,
Armonk, NY, USA).