The effects of partial amniotic carbon dioxide insufflation in an ovine model

Objective: We aim to assess the effect of partial amniotic carbon dioxide insufflation (PACI) at increasing pressures on fetal acid-base, fetal-placental perfusion, and fetal membrane morphology in an ovine model. Method: Pregnant ewes and fetuses were instrumented under isoflurane anesthesia at 105 days gestation (term 145 days) to monitor utero-placental blood flow, fetal and maternal blood pressure, heart rate, and blood gas status. One group (n = 6) was exposed to PACI (unheated dry CO₂), involving 10 mm Hg stepwise increases in insufflation pressure (5 to 25 mm Hg), for 80 minutes followed by 20 minutes of desufflation. Un-insufflated controls (n = 5) were monitored for 100 minutes. At postmortem, fetal membranes were collected for histological analysis. Results: PACI at 25 mm Hg caused severe fetal hypercapnia (PaCO₂ = 143 ± 5 vs 54 ± 5 mm Hg, P < 0.001), acidosis (pH = 6.85 ± 0.02 vs 7.25 ± 0.02, P < 0.001), hypoxia (SaO₂ = 31 ± 4% vs 57 ± 4%, P = 0.01), and reduced uterine artery flow (50 ± 15 vs 196 ± 13 mL/min/kg, P = 0.005) compared with controls. These effects were greater at higher PACI pressures. PACI resulted in leukocyte infiltration in the amnion (1.77 × 10⁻⁵ ± 0.61 × 10⁻⁵ vs 0.38 × 10⁻⁵ ± 0.19 × 10⁻⁵ cells/μm², P = 0.04) and chorionic membranes (2.94 × 10⁻⁵ ± 0.67 × 10⁻⁵ vs 0.84 × 10⁻⁵ ± 0.42 × 10⁻⁵ cells/μm², P = 0.01). Conclusion: Higher PACI pressures results in larger disturbances in fetal acid-base, uterine blood flow, and fetal membrane inflammation in sheep. Differences between human and sheep utero-placental structure should be considered.