Placental gas exchange during amniotic carbon dioxide insufflation in sheep

Abstract

Objective: Insufflation of the amniotic cavity with carbon dioxide (CO₂ ) is used clinically to improve visibility during complex fetoscopic surgery. Insufflation with heated, humidified CO₂ has recently been shown to reduce fetal hypercapnia and acidosis in sheep, compared with use of cold and dry CO₂ , but the underlying mechanisms are unclear. The aim of this study was to investigate whether differences in placental CO₂ and oxygen (O₂ ) exchange during insufflation with heated and humidified vs cold and dry CO₂ could explain these findings.

Methods: Thirteen fetal lambs at 105 days of gestation (term, 146 days) were exteriorized partially, via a midline laparotomy and hysterotomy, and instrumented with an umbilical artery catheter, an umbilical vein catheter and a common umbilical vein flow probe. Arterial and venous catheters and flow probes were also inserted into the maternal uterine circulation. Six ewes were insufflated with cold, dry CO₂ (22°C; 0-5% humidity) and seven with heated, humidified CO₂ (40°C; 95-100% humidity) at 15 mmHg for 180 min. Blood-flow recordings and paired arterial and venous blood gases were sampled from uterine and umbilical vessels. Rates of placental CO₂ and O₂ exchange were calculated.

Results: After 180 min of insufflation, fetal survival was 33% (2/6) using cold, dry CO₂ and 71% (5/7) using heated, humidified CO₂ . By 120 min, fetuses insufflated with heated, humidified CO₂ had lower arterial CO₂ levels and higher arterial pH compared to those insufflated with cold, dry gas. Insufflation decreased significantly placental gas exchange in both groups, as measured by rates of both (i) fetal CO₂ clearance and O₂ uptake and (ii) maternal O₂ delivery and CO₂ uptake from the fetal compartment.

Conclusions: Lower arterial CO₂ and higher pH levels in fetuses insufflated with heated and humidified, compared to cold and dry, CO₂ could not be explained by differences in placental gas exchange. Instead, heated and humidified insufflation appeared to reduce fetal CO₂ absorption from the uterus, supporting its use in preference to cold, dry CO₂ . © 2019 International Society of Ultrasound in Obstetrics and Gynecology.

Keywords: amniotic insufflation; carbon dioxide; fetal therapy; gas exchange; placenta.