At 126 ± 2 days gestation (term ∼ 148 days), Border-Leicester ewes were anaesthetised with an intravenous bolus of 5% sodium thiopentone (Pentothal; 1g in 20 ml) and, following intubation, maintained with inhalation of 1.5–3% halothane in air. The fetal head and neck were exposed via caesarean section and an ultrasonic flow probe (3 mm: Transonic Systems, Ithaca, NY, USA) was placed around a carotid artery. Heparinised saline-filled polyvinyl catheters were inserted into the other carotid artery and into a jugular vein. The fetal trachea was intubated with a 4.0 mm cuffed endotracheal tube and lung liquid was drained passively for ∼ 10 seconds or until liquid ceased exiting the airways. A transcutaneous arterial oxygen saturation (SpO2) probe (Masimo, Radical 4, CA, USA) was placed around the right forelimb and the output recorded continuously. A Near Infrared Spectroscopy optode (Casmed Foresight, CAS Medical Systems Inc, Branford, CT, USA) was placed over the left frontal cortex and used to continuously measure cerebral tissue oxygen saturation (SctO2). After completion of instrumentation, the ewe was rotated onto its side and the fetus was completely exteriorised from the uterus, still attached to the umbilical cord, dried, and placed on a delivery table immediately next to the ewe. Physiological parameters were allowed to stabilise prior to the birth procedures being initiated (see below)—the delay between exteriorisation and intervention was a mean (SD) of 181 ± 31 seconds for all lambs.
Each fetus was randomised to either umbilical cord clamping prior to initiation of ventilation (Clamp 1st; n = 10) or the initiation of ventilation prior to umbilical cord clamping (Vent 1st; n = 7) groups. In Clamp 1st lambs, the umbilical cord was immediately clamped and cut, the lamb transferred to an infant warmer (CosyCot, Fisher and Paykel, Auckland, New Zealand) and ventilation commenced as soon as possible. In Vent 1st lambs, ventilation commenced while the umbilical cord remained patent. Umbilical cord clamping was delayed until after PBF had increased, indicating cardiopulmonary transition, whereupon the cord was clamped and cut. In both groups ventilation was initiated using positive pressure ventilation in volume guarantee mode with a tidal volume of 7 mL/kg and a positive end-expiratory pressure of 5 cmH2O using warmed and humidified inspired gases, with an initial fraction of inspired oxygen (FiO2) of 21% (Babylog 8000+, Dräger, Lübeck, Germany). Peak inspiratory pressure was limited to 40 cmH2O to avoid pneumothoraces. FiO2 was adjusted to target SpO2 at 70–90% by 3 min, 80–90% by 5 min and 90–96% by 10 min.
Regular blood gas analysis (ABL30, Radiometer, Copenhagen, Denmark) and real-time SpO2 measurements were used to monitor the lambs. All lambs received sedation (Alfaxane i.v. 5–15 mg/kg/h; Jurox, East Tamaki, Auckland, New Zealand) in 5% dextrose via the jugular vein catheter to minimize spontaneous breathing during the experiment. The ewes were humanely euthanized using sodium pentobarbitone (100 mg/kg i.v) after caesarean section and the lambs were euthanized after completion of the ventilation study, at 30 min—2 h later, depending on the study.
Each fetus was randomised to either umbilical cord clamping prior to initiation of ventilation (Clamp 1st; n = 10) or the initiation of ventilation prior to umbilical cord clamping (Vent 1st; n = 7) groups. In Clamp 1st lambs, the umbilical cord was immediately clamped and cut, the lamb transferred to an infant warmer (CosyCot, Fisher and Paykel, Auckland, New Zealand) and ventilation commenced as soon as possible. In Vent 1st lambs, ventilation commenced while the umbilical cord remained patent. Umbilical cord clamping was delayed until after PBF had increased, indicating cardiopulmonary transition, whereupon the cord was clamped and cut. In both groups ventilation was initiated using positive pressure ventilation in volume guarantee mode with a tidal volume of 7 mL/kg and a positive end-expiratory pressure of 5 cmH2O using warmed and humidified inspired gases, with an initial fraction of inspired oxygen (FiO2) of 21% (Babylog 8000+, Dräger, Lübeck, Germany). Peak inspiratory pressure was limited to 40 cmH2O to avoid pneumothoraces. FiO2 was adjusted to target SpO2 at 70–90% by 3 min, 80–90% by 5 min and 90–96% by 10 min.
Regular blood gas analysis (ABL30, Radiometer, Copenhagen, Denmark) and real-time SpO2 measurements were used to monitor the lambs. All lambs received sedation (Alfaxane i.v. 5–15 mg/kg/h; Jurox, East Tamaki, Auckland, New Zealand) in 5% dextrose via the jugular vein catheter to minimize spontaneous breathing during the experiment. The ewes were humanely euthanized using sodium pentobarbitone (100 mg/kg i.v) after caesarean section and the lambs were euthanized after completion of the ventilation study, at 30 min—2 h later, depending on the study.
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