The frequency response of cerebral autoregulation

Abstract

The frequency-response of pressure autoregulation is not well delineated; therefore, the optimal frequency of arterial blood pressure (ABP) modulation for measuring autoregulation is unknown. We hypothesized that cerebrovascular autoregulation is band-limited and delineated by a cutoff frequency for which ABP variations induce cerebrovascular reactivity. Neonatal swine (n = 8) were anesthetized using constant minute ventilation while positive end-expiratory pressure (PEEP) was modulated between 6 and 0.75 cycles/min (min(-1)). The animals were hemorrhaged until ABP was below the lower limit of autoregulation (LLA), and PEEP modulations were repeated. Vascular reactivity was quantified at each frequency according to the phase lag between ABP and intracranial pressure (ICP) above and below the LLA. Phase differences between ABP and ICP were small for frequencies of >2 min(-1), with no ability to differentiate cerebrovascular reactivity between ABPs above or below the LLA. For frequencies of <2 min(-1), ABP and intracranial pressure (ICP) showed phase shift when measured above LLA and no phase shift when measured below LLA [above vs. below LLA at 1 min(-1): 156° (139-174°) vs. 30° (22-50°); P < 0.001 by two-way ANOVA for both frequency and state of autoregulation]. Data taken above LLA fit a Butterworth high-pass filter model with a cutoff frequency at 1.8 min(-1) (95% confidence interval: 1.5-2.2). Cerebrovascular reactivity occurs for sustained ABP changes lasting 30 s or longer. The ability to distinguish intact and impaired autoregulation was maximized by a 60-s wave (1 min(-1)), which was 100% sensitive and 100% specific in this model.

Coherent hemodynamics spectroscopy in a single step

“…(4) or to the fact that we are measuring local autoregulation at the microcirculation level as opposed to organ-level autoregulation is a subject for future studies. However, we observe that one recent study also reported a low autoreguation cutoff frequency of ~0.03 Hz [28], consistent with our results.…”

Cerebral Autoregulation in the Microvasculature Measured with Near-Infrared Spectroscopy

“…Even though this transfer function analysis description of cerebral autoregulation is common, 3,11,[20][21][22] we are aware of only a few studies that are explicitly based on a high-pass filter analysis involving the concept of autoregulation cutoff frequency. 11,17,35 In this work, we focus on the assessment of autoregulation cutoff frequencies as a measure of the effectiveness of cerebral autoregulation: namely, the higher the cutoff frequency, the better the suppression of CBF oscillations at any frequency, and the better the autoregulation.…”

Pressure reactivity index: journey through the past 20 years

“…When we arrived home, we programmed our computers, running ICM (intensive care monitor) software [7], to calculate a moving correlation coefficient from 30 consecutive 10-s averages of ICP and ABP waveforms. We called this the PRx index (pressure reactivity index) [6].The rationale for averaging ICP and ABP waveforms over 10 s was that only slow waves, of frequencies lower than 0.05 Hz, can carry information about autoregulation [11]. Simple averaging is a sufficient method of filtration of all faster components (mainly respiratory and pulsatile), which do not contain or contain only a little of any autoregulationrelated signatures.…”

“…The rationale for averaging ICP and ABP waveforms over 10 s was that only slow waves, of frequencies lower than 0.05 Hz, can carry information about autoregulation [11]. Simple averaging is a sufficient method of filtration of all faster components (mainly respiratory and pulsatile), which do not contain or contain only a little of any autoregulationrelated signatures.…”