P-262 Should ultra-low oxygen tension (2%) be established as the widespread reference for culture to blastocyst stage?

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Abstract Study question Does embryo culture using biphasic O2 concentration strategy from low (5%) to ultra-low (2%) at day 3 improve blastocyst development when performed for all patients? Summary answer Our results suggest equivalent outcomes using a biphasic embryo culture with 2% O2 compared to monophasic culture at 5% O2 in sibling embryos. What is known already For decades, 20% atmospheric O2 concentration ([O2]) was used in embryo culture. However many studies confirmed better outcomes when lowering [O2] to 5%. In humans [O2] is around 5% in the fallopian tube (fertilization to cleaved embryo) and decreases to 2% in the uterus (morula to blastocyst). The oxygen gradient decreases at the same time as the metabolic shift from oxidative phosphorylation to glycolysis. A biphasic [O2] culture from low (5%) to ultra-low (2%) seems thus logical to mimic the in-vivo environment. Certain studies have shown an improvement in blastocyst rates with biphasic [O2], nevertheless these results are still controversial. Study design, size, duration Prospective study from February 2024 to January 2025 in a single private hospital. Sibling embryos from 146 patients were cultured in time-lapse incubators until day 3 (D3) in 5% [O2].At D3 they were randomized into the control group with 5% [O2] (group A) or the study group with 2% [O2] (group B) for extended culture. Participants/materials, setting, methods 2456 eggs were obtained, 1994 were mature (81.2%). Only the first attempts with ≥6 developing embryos on D3 were studied (n = 125 cIVF, n = 21 ICSI). 1441 embryos were assigned into the study groups. Fertilisation rate (FR), abnormal cleavage rate, total blastulation rate (BR), top blastulation rate (TBR) and morphokinetic features were compelled. Embryos with chaotic or direct-cleavage 1 were discarded. Blastocysts were graded using Gardner’s classification. Data were analysed with Chi-square and Fisher’s tests using RStudio2023.09.1. Main results and the role of chance Patients were aged 34.9±4.4. FR was 71.3% for cIVF and 70.6% for ICSI. Number of embryos with abnormal cleavage was similar (21.9% group A vs 22.3% group B). No significant differences were seen in the BR between the two groups (77.7% vs 79,7% p = 0,369) nor in the TBR. However, TBR seems higher in group B without statistical significance (54,9% vs. 59,8% p = 0.069). No total blastulation failure was noticed simultaneously in both groups and very few patients showed blastulation failure in only one of the groups (0.1% vs. 0.6%, p = 0.215). Nevertheless, we observed a tendency regarding top blastulation failure (TBF) in group A (1.7% vs. 0.6% in the B group, p = 0.083). Only 3 patients (0.2%) had TBF in both groups. When observing the rate of vitrified blastocysts in the two groups, there is a slight increase tendency towards group B, although no significant differences were seen (54.9% vs 59.8%). No significant differences were observed in blastocysts morphokinetics. Embryos obtained in both groups shared similar times of start of blastulation (tSB) and reached the full blastocyst stage (tB) at similar times, suggesting that they develop at similar speeds (tSB= 98±9.3 vs 97.4±14.9 p = 0.964, tB = 107.9±10.8 vs 106.1±18.6 p = 0.87). Limitations, reasons for caution Since the study was conducted in 2024, more time is needed to observe pregnancy outcomes and assess any potential impact of the blastocysts obtained using biphasic ultra-low [O2]. Wider implications of the findings Overall results were similar between group A and group B, suggesting there is no need to lower [O2] to 2% systematically for all patients. Further studies are needed to determine whether ultra-low [O2] conditions could benefit to specific cases where embryos undergo certain difficulties in their development. Trial registration number No

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.