Examining the upper frequency limit of dynamic cerebral autoregulation: Considerations across the cardiac cycle during eucapnia.

There are differences within the literature regarding the upper frequency cut‐off point of the dynamic cerebral autoregulation (CA) high‐pass filter. The projection pursuit regression approach has demonstrated that the upper frequency limit is ∼0.07 Hz, whereas another approach [transfer function analysis (TFA) phase approaching zero] indicated a theoretical upper frequency limit for the high‐pass filter of 0.24 Hz. We investigated how these limits accurately represent the CA upper frequency limit, in addition to extending earlier findings with respect to biological sexes and across the cardiac cycle. Sixteen participants (nine females and seven males) performed repeated squat–stand manoeuvres at frequencies of 0.05, 0.10, 0.15, 0.20 and 0.25 Hz, with insonation of the middle and posterior cerebral arteries. Linear regression modelling with adjustment for sex and order of squat completion was used to compared TFA gain and phase with 0.25 Hz (above the theoretical limit of CA). The upper frequency limit of CA with TFA gain was within the range of 0.05–0.10 Hz, whereas TFA phase was within the range of 0.20–0.25 Hz, and consistent between vessels, between sexes and across the cardiac cycle. Females displayed greater middle cerebral artery gain compared with males (all P < 0.047), and no phase differences were present (all P > 0.072). Although sex‐specific differences were present for specific TFA metrics at a given frequency, the upper frequency limit of autoregulation was similar between cerebral conduit vessels, cardiac cycle phase and biological sex. Future work is warranted to determine whether an upper frequency limit exists with respect to hysteresis analyses. What is the central question of this study?There is conflict in the current literature regarding the upper frequency limit of dynamic cerebral autoregulation, with proposed limits ranging from 0.07 to 0.24 Hz. The vast majority of previous work on the upper cerebral autoregulation frequency limit has been performed in males and with mean blood pressure and cerebral blood velocity.What is the main finding and its importance?The upper frequency limit for gain was 0.05–0.10 Hz and for phase it was 0.20–0.25 Hz. This occurred irrespective of cardiac cycle phase or biological sex. [ABSTRACT FROM AUTHOR]