With TCD/TCCD, we can assess blood ow velocity non-invasively and compare its pulsa­tile waveform with the pulsatile waveform of ABP.CrCP can be then estimated as the intercept point of a regression line between arterial pres-

The intercept is the value of ABP at which the CBF (estimated through the blood ow velocity) is equal to zero [22, 24].

Another pressure threshold has been derived from CrCP and diastolic ABP (ABPd) which is the diastolic closing margin of the brain micro­vasculature (DCM=ABPd-CrCP) [27].

With TCD/TCCD, we can measure auto­regulation by measuring static autoregulation, studying the relationship between ‘steady-state’ changes in CPP or ABP and CBF, or by studying dynamic autoregulation through the assessment of continuous changes in CPP or ABP and CBF.

From MBP and FV, cerebrovascular resistance can be estimated as eCVR=MBP/FV.The static rate of autoregulation (SRoR) is the ratio of per­centage change in estimated CVR to the percentage change in CPP (or the percentage change in MBP if ICP is not measured). When MBP is used rather than CPP,results may be affected by an error asso­ciated with‘false autoregulation’, which occurs in the non-autoregulating brain when ICP rises con­sistently with MBP and leaves CPP constant.

Dynamic autoregulation can be assessed by rapidly varying ABP and CPP while measuring CBF.In order to measure dynamic autoregulation, the timing and magnitude of the reaction of CBF to the CPP/ABP challenge (induced or spontane­ous) are observed over time, which can only be possible when using monitoring modalities with high temporal resolution (i.e. TCD, laser Doppler owmetry, or near-infrared spectroscopy). One method that can be used is tight leg-cuff release in which the operator measures the recovery in FV after a rapid transient decrease in blood pres­sure that has been induced by the deation of large thigh cuffs [32]. The cuffs are placed and inated to 50mm Hg above systolic blood pres­sure for 3min. The deation of the cuff produces an approximately 20mm Hg drop in ABP. The ow velocity response to that drop can be ana­lysed through a validated algorithm to deter­mine the rate of dynamic cerebral autoregulation (dROR) or autoregulation index (ARI). ARI is a dimensionless index ranging from 0 to 9 that describes the response of CBF to a steep decrease in ABP. The threshold for good or disturbed autoregulation is an ARI value of around 5. The

Another method is the transient hyperaemic response test, which is performed by compressing the common carotid artery for 5–8s before observ­ing the change in ow velocity after the compres­sion has ceased [33]. The transient hyperaemic response ratio (THRR: the ratio of the hyperae­mic ow velocity recorded after carotid release and the precompression baseline ow velocity) is then calculated—see Fig. 7.4. Normal values in healthy volunteers range between 1.105 and 1.29. A threshold of 1.10 has been adopted as the lower limit of a normal response. However, this method could bring about some risk of releasing embo­lisms and extra variability of ABP.

Other methods include the squat-to-stand or sit-to-stand manoeuvres, controlled slow breath­ing, lower body negative pressure, Valsalva manoeuvreor analysis of spontaneous slow uc­tuations of ABP.

Mx [34] is the correlation coefcient index calculated between FVm and CPP, and Sx is the correlation coefcient index between FVs and CPP. When either Mx or Sx is positive, there is a positive association between FV and CPP, and thus disturbed autoregulation. A negative or 0 correlation coefcient signies intact autoregula­tion; see Fig.7.5. They both describe cerebrovas­cular reactivity and correlate with outcome after a head injury, and therefore either can be used to guide autoregulation-oriented therapy [35].

Mxa is the correlation coefcient between ABP and FV. A Mxa close to +1 denotes that slow uctuations of ABP produce synchronised slow changes in FV that describe deranged auto­regulation. According to previous literature in TBI patients, negative values or values <0.3 of Mxa indicate intact autoregulation, whereas values >0.3 indicate the failure of cerebral autoregulation [36].