BACKGROUNDClassically, cerebral autoregulation (CA) entails cerebral blood flow (CBF) remaining constant by cerebrovascular tone adapting to fluctuations in mean arterial pressure (MAP) between ∼60 and ∼150 mmHg. However, this is not an on-off mechanism; previous work has suggested that vasomotor tone is proportionally related to CA function. During propofol-based anaesthesia, there is cerebrovascular vasoconstriction, and static CA remains intact. Sevoflurane-based anaesthesia induces cerebral vasodilation and attenuates CA dose-dependently. It is unclear how this translates to dynamic CA across a range of blood pressures in the autoregulatory range.OBJECTIVEThe aim of this study was to quantify the effect of step-wise increases in MAP between 60 and 100 mmHg, using phenylephrine, on dynamic CA during propofol-and sevoflurane-based anaesthesia.DESIGNA nonrandomised interventional trial.SETTINGSingle centre enrolment started on 11 January 2019 and ended on 23 September 2019.PATIENTSWe studied American Society of Anesthesiologists (ASA) I/II patients undergoing noncardiothoracic, nonneurosurgical and nonlaparoscopic surgery under general anaesthesia.INTERVENTIONIn this study, cerebrovascular tone was manipulated in the autoregulatory range by increasing MAP step-wise using phenylephrine in patients receiving either propofol-or sevoflurane-based anaesthesia. MAP and mean middle cerebral artery blood velocity (MCAVmean) were measured in ASA I and II patients, anaesthetised with either propofol (n = 26) or sevoflurane (n = 28), during 10 mmHg step-wise increments of MAP between 60 and 100 mmHg. Static CA was determined by plotting 2-min averaged MCAVmeanversus MAP. Dynamic CA was determined using transfer function analysis and expressed as the phase lead (°) between MAP and MCAVmeanoscillations, created with positive pressure ventilation with a frequency of 6 min-1.MAIN OUTCOMESThe primary outcome of this study was the response of dynamic CA during step-wise increases in MAP during propofol-and sevoflurane-based anaesthesia.RESULTSMAP levels achieved per step-wise increments were comparable between anaesthesia regiment (63 ± 3, 72 ± 2, 80 ± 2, 90 ± 2, 100 ± 3 mmHg, and 61 ± 4, 71 ± 2, 80 ± 2, 89 ± 2, 98 ± 4 mmHg for propofol and sevoflurane, respectively). MCAVmeanincreased more during step-wise MAP increments for sevoflurane compared to propofol (P≤0.001). Dynamic CA improved during propofol (0.73° mmHg-1, 95% CI 0.51 to 0.95; P ≤ 0.001)) and less pronounced during sevoflurane-based anaesthesia (0.21° mmHg-1(95% CI 0.01 to 0.42, P = 0.04).CONCLUSIONSDuring general anaesthesia, dynamic CA is dependent on MAP, also within the autoregulatory range. This phenomenon was more pronounced during propofol anaesthesia than during sevoflurane.TRIAL REGISTRATIONNCT03816072 (https://clinicaltrials.gov/ct2/show/NCT03816072).