Revisiting human cerebral blood flow responses to augmented blood pressure oscillations

Abstract

Key points Cerebral autoregulation is most effective in buffering against pressure fluctuations slower than 0.03 Hz (∼30 s). This suggests that frequency bands for characterizing cerebral autoregulation should be redefined Low cross‐spectral coherence below 0.03 Hz highlights the limitations of transfer function approaches Haemodynamic changes induced by lower body pressure could not fully explain the differences in autoregulation estimated from spontaneous vs. augmented fluctuations, and thus, observations of spontaneous fluctuations should not be relied on whenever possible. Abstract There is currently little empirical basis for time scales that are considered to be most significant in cerebrovascular counter‐regulation of changes in arterial pressure. Although it is well established that cerebral autoregulation behaves as a ‘high‐pass’ filter, recommended frequency bands have been largely arbitrarily determined. To test effectiveness of cerebral autoregulation, we refined oscillatory lower body pressure (LBP) to augment resting pressure fluctuations below 0.1 Hz by a factor of two in 13 young male volunteers, and thoroughly characterized the time and frequency responses of cerebral autoregulation. We observed that despite a threefold increase in arterial pressure power <0.03 Hz with oscillatory LBP, there was no change in cerebral blood flow power, indicating near perfect counter‐regulation. By contrast, in the range 0.03–0.10 Hz, both cerebral blood flow and arterial pressure power more than doubled. Our data demonstrate that cerebral autoregulation is most effective in buffering against pressure fluctuations slower than 0.03 Hz (∼30 s). This suggests that frequency bands of interest should be redefined and recording length should be increased considerably to account for this. Furthermore, low cross‐spectral coherence below 0.03 Hz, even when pressure fluctuations were augmented, highlights the uncertainty in transfer function approaches and the need to either report precision or use non‐linear approaches. Finally, haemodynamic changes induced by LBP could not fully explain the differences in autoregulation estimated from spontaneous vs. augmented fluctuations, and thus, observations of spontaneous fluctuations should not be relied on whenever possible.

Dynamic Cerebral Autoregulation Post Endovascular Thrombectomy in Acute Ischemic Stroke

“…We have previously shown that the coherence at the frequency bands used in this study is a relatively reliable measure of the integrity of autoregulation, although the gain and phase shift may be less reliable [29]. By definition, effective autoregulation should be a low coherence state, which in turn, creates uncertainties around the estimated gain and phase relation [31,32].…”

“…Third, we used conventional frequency ranges to assess cerebral autoregulation. However, as noted before [ 27 , 28 ], these were chosen to standardize the assessment of autoregulation across studies, and in reality, autoregulation is most effective at frequencies at or lower than 0.03 Hz [ 29 ]. Fourth, cerebral autoregulation may be affected by multiple variables that were not adequately assessed in our study, such as medications [ 41 ], pre-morbid vascular risk factors, and critical illness, including sepsis [ 42 ].…”

“…The transfer coherence, gain, and phase shift were calculated for each subject within the standard frequency ranges (very low frequency (VLF): <0.03 Hz, and low frequency (LF): 0.03-0.07 Hz; the lower limit of the VLF range was 0.02 due to the relatively short duration of the recordings) [27]. These frequency bands were chosen based on convention [28] and on recent data suggesting that autoregulation is most effective at frequencies close to and lower than 0.03 Hz [29]. The coherence indicates the extent of the linear relationship between the arterial pressure and the cerebral blood flow fluctuations (a lower coherence indicates more effective autoregulation).…”

Heat transfer, physiological responses, and subjective perceptions during short contact time with wood or other materials

“…Generally, neurogenic (sympathetic and parasympathetic) components cause vasoconstriction in counteracting a rise in arterial pressure. In addition, cerebral autoregulation has been reported to be effective for a slower change in blood pressure (> 30 s), whereas it is less so during faster changes [16,17]. Based on the previous findings, the changes in cerebral blood flow in this study may be explained as follows: the fastest parasympathetic and the subsequent sympathetic reflexes counteracted the fast increase in blood pressure, thus resulting in vasoconstriction that was reflected in the decreased oxy-and deoxy-hemoglobin signals of the NIRS.…”

Cerebrovascular Neuroprotection after Acute Concussion in Adolescents

“…This was to elicit fluctuations in bloodthus, perfusionpressure sufficiently large enough to engage cerebral autoregulatory mechanisms. 19,29,30 Autoregulation was determined by deriving the relationship between mean arterial pressure (independent variable; input variable) and MCAv (dependent variable; output variable) fluctuations using a validated nonlinear and nonparametric approach based on projection pursuit regression (PPR). 31 This approach has been validated and shown to have excellent reproducibility (Lin's concordance coefficient >0.95 across repeat-assessments).…”