Au K, et al. - Researchers employed biased-coin sequential allocation in order to determine the effective time interval in 90% of healthy parturients to attain a target endpoint end-tidal oxygen of ≥ 90% using standard flow rate facemask and high-flow nasal oxygen. They randomly assigned 80 Eighty parturients to standard facemask (n = 40) or high-flow nasal oxygen (n = 40) groups; half of the parturients in the high-flow nasal oxygen group also used a simple no-flow facemask to minimize air entrainment. For standard facemask, the effective time interval for 90% of parturients to attain the target endpoint was 3.6 min (95%CI 3.3–6.7 min), but it could not be determined for the high-flow nasal oxygen groups with or without an additional simple facemask, as 8 minutes was inadequate to achieve the target endpoint for 55% and 92% of parturients, respectively. Furthermore, 71% in the standard facemask group vs 0% in the high-flow nasal oxygen groups reached the target endpoint after 3 minutes. After 4 minutes, 100% in the standard facemask, 80% in the high-flow nasal oxygen with simple facemask and 67% in the high-flow nasal oxygen groups attained the target endpoint. Beyond four minutes, no improvement in preoxygenation success was noted employing high-flow nasal oxygen. In conclusion, this study suggests that the effective time interval for 90% of parturients to obtain an end-tidal oxygen ≥ 90% for standard flow rate facemask was estimated to be 3.6 min. However, it could not be estimated for high-flow nasal oxygen groups even after eight minutes.