Authors: Tzeng YC, Ainslie PN, Cooke WH, Peebles KC, Willie CK, Macrae BA, Smirl JD, Horsman HM, Rickards CA.
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 between low (LF, 0.07- 0.2 Hz) and very low (VLF, 0.02-0.07 Hz) frequency 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) between spontaneous transfer function metrics and the rate of regulation index and auto-regulatory 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 partial pressure of end-tidal CO(2) (P(ET)CO(2)) 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 P(ET)CO(2) were positively related to coherence (LF, β=0.0065 AU mm Hg(-1); VLF, β=0.011 AU mm Hg(-1); both P<0.01) and inversely related to phase (LF, β=-0.026 rad mm Hg(-1); VLF, β=-0.018 rad mm Hg(-1); both P<0.01) and normalized gain (LF, β=-0.042 % mm Hg(-2); VLF, β=-0.013 % mm Hg(-2); both P<0.01). However, P(ET)CO(2) were positively associated with gain (LF, β=0.0070 cm s(-1) mm Hg(-2), P<0.05; VLF, β=0.014 cm s(-1) mm Hg(-2), P<0.01). Thus, during changes in P(ET)CO(2), LF phase was inversely related to LF gain (β=-0.29 cm s(-1) mm Hg(-1) rad(-1), P<0.01) but positively related to LF normalized gain (β=1.3 % mm Hg(-1) rad(-1), 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.