Background: During cardiac surgery, maintaining a mean arterial pressure (MAP) within the range of cerebral autoregulation (CA) may prevent postoperative morbidity. The lower limit of cerebral autoregulation (LLA) can be determined using the mean velocity index (Mx). The standard Mx is averaged over a ten second period (Mx{}_{\text{10s}}) while using a two second averaging period (Mx{}_{\text{2s}}) is faster and may record more rapid variations in LLA. The objective of this study is to compare a quick determination of LLA (qLLA) using Mx{}_{\text{2s}} with the reference LLA (rLLA) using Mx{}_{\text{10s}}. Methods: Single center, retrospective, observational study. Patients undergoing cardiac surgery with cardiopulmonary bypass. From January 2020 to April 2021, perioperative transcranial doppler measuring cerebral artery velocity was placed on cardiac surgery patients in order to correlate with continuous MAP values. Calculation of each patient’s Mx was manually determined after the surgery and qLLA and rLLA were then calculated using a threshold value of Mx >0.4. Results: 55 patients were included. qLLA was found in 78% of the cases versus 47% for rLLA. Despite a –3 mmHg mean bias, limits of agreement were large [–19 mmHg (95% CI: –13; –25), and +13 mmHg (95% CI: +6; +19)]. There was an important interobserver variability (kappa rLLA = 0.46; 95% CI: 0.24–0.66; and Kappa qLLA = 0.36; 95% CI: 0.20–0.52). Conclusions: Calculation of qLLA is feasible. However, the large limits of agreement and significant interobserver variability prevent any clinical utility or interchangeability with rLLA.