Changing the start temperature and cooling rate in a slow-freezing protocol increases human blastocyst viability

Objective: To determine the effect of start temperature and cooling rate of a slow freezing protocol on human blastocyst viability. Design: Controlled-rate freezing of human blastocysts using different start temperatures and cooling rates. Setting: Private assisted reproductive technology unit. Patient(s): Patients donated with consent cryopreserved pronucleate embryos. Intervention(s): Culture of thawed pronucleate embryos in G III series media, containing hyaluronan, followed by cryopreservation of 36 blastocysts with subsequent noninvasive analysis of embryo metabolism. Main Outcome Measure(s): Pyruvate and glucose consumption and blastocyst reexpansion and quality. Result(s): Glucose consumption and blastocyst reexpansion after thaw were significantly higher when a start temperature of -6degreesC and a cooling rate of 0.5degreesC/min to -32degreesC were used compared with a start temperature of 20degreesC and a cooling rate of 2degreesC to -6degreesC, followed by cooling at 0.3degreesC to -35degreesC. Pyruvate uptake after thaw was not affected by the freezing procedure. Clinical use of the lower start temperature and quicker cooling rate, combined with culture in hyaluronan-based media, has led to the establishment of a 30% implantation rate. Conclusion(s): Human embryos cultured to the blastocyst stage in hyaluronan-based sequential media are readily cryopreserved and maintain their viability after thaw. (C) 2003 by American Society for Reproductive Medicine.