Different CO 2 settings (6.0% vs 7.0%) do have an impact on extracellular pH of culture medium (pHe) and euploidy rates rather than on blastocyst development: a sibling oocyte stud

Abstract
Objective: To determine whether euploidy rates and blastocyst development differ in a continuous culture medium under different CO2 concentrations.

Design and method: A single-center retrospective study was performed from July 2018 to October 2019 including 44 fresh cycles with at least four fresh mature oocytes (MII) without severe male factor infertility. Sibling MII were injected and cultured in Global®Total®LP under 6.0% (pHe = 7.374 ± 0.014) or 7.0% (pHe = 7.300 ± 0.013) CO2, 5.0% O2, and 89.0% or 88.0% N2. Analyzed variables were normally fertilized oocytes (2PN), cleavage rate, blastulation rate on day 5/2PN, usable blastocyst (blastocysts biopsied/2PN), and euploidy rates. Blastocyst’s trophectoderm biopsy was performed on day 5, 6, or 7 for genetic testing and mitochondrial DNA (mtDNA) quantification by next-generation sequencing.

Results: Women’s mean age was 33.0 ± 6.6 years old. From a total of 604 MII, no differences were found in normal fertilization and cleavage rates on day 3 between 6.0 and 7.0% CO2 (72.3% vs 67.1%, p = 0.169 and 96.6% vs 96.3%, p = 0.897, respectively). Blastulation rate on day 5/2PN was comparable between 6.0 and 7.0% CO2 (68.1% vs 64.2%, p = 0.409). Although usable blastocyst rate was not different (54.3% vs 55.3%, p = 0.922), total euploidy rates differed significantly (58.7% vs 42.8%, p = 0.016) between 6.0% and 7.0% CO2, respectively. The mean blastocyst mtDNA content was significantly lower in 6.0% CO2 (30.4 ± 9.1 vs 32.9 ± 10.3, p = 0.037).

Conclusion: Blastocyst development is not affected when embryos are cultured in vitro at 6.0% or 7.0% CO2, while euploidy rates are significantly decreased at a higher CO2 concentration, therefore at a lower pHe.