Table 2 Summary of autoregulation indices
From: Regulation of the cerebral circulation: bedside assessment and clinical implications
Autoregulation metric | Input signals | Calculation | Interpretation | Comment |
|---|---|---|---|---|
Autoregulation index (ARI) | ABP, Fv | Compares the CBF response to changes in ABP with those predicted from a parametric model with 10 different ‘strengths’ of autoregulation [110] | ARI = 0 absent autoregulation, ARI = 9 perfect autoregulation | Moderately complex signal processing required |
Flow index (Mx, Sx, Dx) | ABP (CPP), Fv | Pearson correlation between CPP and mean Fv (300-s window of 10-s averages). Sx and Dx calculated with systolic and diastolic flow velocity, respectively | Impaired autoregulation = higher Mx, Dx, and Sx | Simplistic yet prognostically relevant |
Transfer function (phase, gain, coherence) | ABP, Fv | Derived from the transfer function of fast Fourier transform of ABP and Fv signals. Phase is the shift required to align Fv and ABP signals, gain the transmission from ABP to Fv, and coherence the statistical association between ABP and Fv | Impaired autoregulation = low phase, high gain, high coherence | Moderately complex signal processing. Some prognostic relevance |
TOx, COx, THx, HVx | ABP (CPP), NIRS oxygenation | Pearson correlation between 30 consecutive 10-s means of ABP and tissue oxygenation (or total haemoglobin for THx and HVx) | Impaired autoregulation = higher TOx, COx, THx, HVx | Correlated with TCD methods but allows for longer term monitoring |
TOIHRx | HR, NIRS oxygenation | Correlation between 30 consecutive 10-s means of HR and NIRS oxygenation | ?Higher TOIHRx = impaired autoregulation | Used in preterm infants. Further comparisons with standard autoregulation indices required |
Transfer function (phase, gain, coherence) | ABP, NIRS oxygenation | Derived from the transfer function of fast Fourier transform of ABP and oxygenation signals. Phase is the shift required to align oxygenation and ABP signals, gain the transmission from ABP to NIRS oxygenation, and coherence the statistical association between ABP and NIRS oxygenation | Impaired autoregulation = low phase, high gain, high coherence | Moderately complex signal processing |
PRx | ABP, ICP | Correlation between 30 consecutive 10-s means of ABP and ICP | Higher PRx = impaired autoregulation | Robust measure for long monitoring periods. Simplistic and prognostically relevant |
PAx | ABP, amplitude of ICP | Correlation between 30 consecutive 10-s means of ABP and ICP | Higher PAx = impaired autoregulation | Similar to PRx, may allow better estimate of pressure reactivity when the “pressure–volume” compensatory curve is flat, i.e. at low ICP |
ORx | CPP (ABP), PBTO2 | Correlation between 30 consecutive 10-s means of ABP and PBTO2 | High ORx = impaired autoregulation | Further validation required |