Page 72 - Physiological based CPAP for preterm infants at birth Tessa Martherus

Chapter 3
Delivery of preterm kittens and experimental preparations (Figure 1b)
Kittens were delivered via caesarean section. The umbilical cord remained intact and was covered with diclofenac diethylammonium (Voltaren Emugel, GlaxoSmithKline Consumer Health B.V, UK). A custom-made oesophageal tube was inserted, with the tip located in the mid thoracic region. This tube exited through a custom-made, soft rubber face mask that was placed over the kitten’s head and sealed with tissue glue to prevent leak if necessary (small kittens). The umbilical cord was then clamped, and the kitten weighed before it received naloxone (0.2 mg/kg), anexate (10 μg/kg) and caffeine (20 mg/kg) injected intraperitoneally to reverse maternally administered sedatives/analgesics and to stimulate spontaneous breathing. Kittens were then transferred to the imaging hutch, placed on their right side on a heated platform and connected to a temperature probe, oximeter and ECG leads. The face mask was connected to a custom-build ventilator in CPAP mode (33), whereas the oesophageal tube was connected to a pressure transducer to detect breathing (BP Transducer/Cable Kit, ADInstruments, New South Wales Australia). All devices were connected to a PowerLab and LabChart data acquisition system (ADInstruments, NSW, Australia) for continuous physiological recording.
All kittens were included, except if they were stillborn or were too small for instrumentation (<15 g).
Respiratory support and CPAP intervention protocol (Figure 1c)
The experiment began when the kittens were attached to the recording equipment and imaging commenced as soon as the kittens were judged to have a stable breathing pattern. Respiratory support was initiated with CPAP and 100% oxygen, while tactile stimulation was used to encourage breathing. Kittens continued to receive 100% oxygen throughout the study period. On the indication of apnoea (absence of breathing for 4-5 s), IPPV was applied (PEEP 5, PIP 35 cmH2O, 1:1 inflation: deflation time of 500 ms each). If kittens did not re-establish stable breathing following IPPV, imaging was interrupted, the imaging room entered and kittens received tactile stimulation.
Kittens were randomly divided into five groups although we aimed not to include more than one kitten from each mother to anyone group. Depending on the group, respiratory support was commenced with an initial CPAP level of 0, 5, 8, 12 or 15 cmH2O CPAP. Kittens that initially received 15 cmH2O were further sub-divided into three groups to either (i) maintain CPAP continuously throughout the experiment or to decrease CPAP to (ii) 5 or (iii) 8 cmH2O. In the latter two groups (called the dynamic CPAP groups), once the lungs were visibly aerated (as indicated by imaging) and the kittens had achieved a stable breathing pattern, CPAP was decreased in a stepwise fashion at a rate of ~2 cmH2O per 30 s. These pressures (5-8 cmH2O) reflect the lower- and upper limit of the recommended CPAP range in the Australian and New Zealand neonatal resuscitation guidelines.34 The administered CPAP levels were adjusted by measuring the delivered pressure at the face mask outlet.
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