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Objective: Insufflation of the amniotic cavity with carbon dioxide (CO 2 ) is used clinically to improve visibility during complex fetoscopic surgery. Insufflation with heated, humidified CO 2 has recently been shown to reduce fetal hypercapnia and acidosis in sheep, compared with use of cold and dry CO 2 , but the underlying mechanisms are unclear. The aim of this study was to investigate whether differences in placental CO 2 and oxygen (O 2 ) exchange during insufflation with heated and humidified vs cold and dry CO 2 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 CO 2 (22°C; 0-5% humidity) and seven with heated, humidified CO 2 (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 CO 2 and O 2 exchange were calculated., Results: After 180 min of insufflation, fetal survival was 33% (2/6) using cold, dry CO 2 and 71% (5/7) using heated, humidified CO 2 . By 120 min, fetuses insufflated with heated, humidified CO 2 had lower arterial CO 2 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 CO 2 clearance and O 2 uptake and (ii) maternal O 2 delivery and CO 2 uptake from the fetal compartment., Conclusions: Lower arterial CO 2 and higher pH levels in fetuses insufflated with heated and humidified, compared to cold and dry, CO 2 could not be explained by differences in placental gas exchange. Instead, heated and humidified insufflation appeared to reduce fetal CO 2 absorption from the uterus, supporting its use in preference to cold, dry CO 2 . © 2019 International Society of Ultrasound in Obstetrics and Gynecology., (© 2019 International Society of Ultrasound in Obstetrics and Gynecology.)

Objective: Partial amniotic carbon dioxide (CO2 ) insufflation (PACI) is used to improve visualization and facilitate complex fetoscopic surgery. However, there are concerns about fetal hypercapnic acidosis and postoperative fetal membrane inflammation. We assessed whether using heated and humidified, rather than cold and dry, CO2 might reduce the impact of PACI on the fetus and fetal membranes in sheep.Methods: Twelve fetal lambs of 105 days' gestational age (term = 145 days) were exteriorized partially, via a midline laparotomy and hysterotomy, and arterial catheters and flow probes were inserted surgically. The 10 surviving fetuses were returned to the uterus, which was then closed and insufflated with cold, dry (22 °C at 0-5% humidity, n = 5) or heated, humidified (40 °C at 100% humidity, n = 5) CO2 at 15 mmHg for 180 min. Fetal membranes were collected immediately after insufflation for histological analysis. Physiological data and membrane leukocyte counts, suggestive of membrane inflammation, were compared between the two groups.Results: After 180 min of insufflation, fetal survival was 0% in the group which underwent PACI with cold, dry CO2 , and 60% (n = 3) in the group which received heated, humidified gas. While all insufflated fetuses became progressively hypercapnic (PaCO2  > 68 mmHg), this was considerably less pronounced in those in which heated, humidified gas was used: after 120 min of insufflation, compared with those receiving cold, dry gas (n = 3), fetuses undergoing heated, humidified PACI (n = 5) had lower arterial partial pressure of CO2 (mean ± standard error of the mean, 82.7 ± 9.1 mmHg for heated, humidified CO2 vs 170.5 ± 28.5 for cold, dry CO2 during PACI, P < 0.01), lower lactate levels (1.4 ± 0.4 vs 8.5 ± 0.9 mmol/L, P < 0.01) and higher pH (pH, 7.10 ± 0.04 vs 6.75 ± 0.04, P < 0.01). There was also a non-significant trend for fetal carotid artery pressure to be higher following PACI with heated, humidified compared with cold, dry CO2 (30.5 ± 1.3 vs 8.7 ± 5.5 mmHg, P = 0.22). Additionally, the median (interquartile range) number of leukocytes in the chorion was significantly lower in the group undergoing PACI with heated, humidified CO2 compared with the group receiving cold, dry CO2 (0.7 × 10-5 (0.5 × 10-5 ) vs 3.2 × 10-5 (1.8 × 10-5 ) cells per square micron, P = 0.02).Conclusions: PACI with cold, dry CO2 causes hypercapnia, acidosis, hypotension and fetal membrane inflammation in fetal sheep, raising potential concerns for its use in humans. It seems that using heated, humidified CO2 for insufflation partially mitigates these effects and this may be a suitable alternative for reducing the risk of fetal acid-base disturbances during, and fetal membrane inflammation following, complex fetoscopic surgery. © 2018 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of the International Society of Ultrasound in Obstetrics and Gynecology. [ABSTRACT FROM AUTHOR]