Assessment of cerebral autoregulation : the quandary of quantification

We assessed the convergent validity of commonly applied metrics of cerebral autoregulation (CA) to determine the extent to which the metrics can be used interchangeably. To examine between-subject relationships among low-frequency (LF; 0.07-0.2 Hz) and very-low-frequency (VLF; 0.02-0.07 Hz) transfer function coherence, phase, gain, and normalized gain, we performed retrospective transfer function analysis on spontaneous blood pressure and middle cerebral artery blood velocity recordings from 105 individuals. We characterized the relationships (n = 29) among spontaneous transfer function metrics and the rate of regulation index and autoregulatory index derived from bilateral thigh-cuff deflation tests. In addition, we analyzed data from subjects (n = 29) who underwent a repeated squat-to-stand protocol to determine the relationships between transfer function metrics during forced blood pressure fluctuations. Finally, data from subjects (n = 16) who underwent step changes in end-tidal PCO₂ (PetCO₂) were analyzed to determine whether transfer function metrics could reliably track the modulation of CA within individuals. CA metrics were generally unrelated or showed only weak to moderate correlations. Changes in PetCO₂ were positively related to coherence [LF: β = 0.0065 arbitrary units (AU)/mmHg and VLF: β = 0.011 AU/mmHg, both P < 0.01] and inversely related to phase (LF: β = -0.026 rad/mmHg and VLF: β = -0.018 rad/mmHg, both P < 0.01) and normalized gain (LF: β = -0.042%/mmHg² and VLF: β = -0.013%/mmHg², both P < 0.01). However, PetCO₂ was positively associated with gain (LF: β = 0.0070 cm⋅s⁻¹⋅mmHg⁻², P < 0.05; and VLF: β = 0.014 cm⋅s⁻¹⋅mmHg⁻², P < 0.01). Thus, during changes in PetCO₂, LF phase was inversely related to LF gain (β = -0.29 cm⋅s⁻¹⋅mmHg⁻¹⋅rad⁻¹, P < 0.01) but positively related to LF normalized gain (β = 1.3% mmHg⁻¹/rad, P < 0.01). These findings collectively suggest that only select CA metrics can be used interchangeably and that interpretation of these measures should be done cautiously