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804 PART 6: Neurologic Disorders

few seconds to several minutes to identify slow vasocycling responses in Another approach is measuring cerebral pressure reactivity (PRx),
perfusion and arterial pressures. An advantage of such an approach lies which is the ability of the cerebral vascular smooth muscle to respond
in its relative noninvasiveness. However, it is likely that these correla- to transmural pressure changes such as ICP variations. As an analogy to
tions are vulnerable to variation based on other important predictors the calculations employed to derive Mx, slow waves of MAP and ICU
changes, other medications, are compared to obtain a PRx. For example, good reactivity would imply
of cerebral hemodynamics such as Pa CO 2
intrinsic reflexes such as Cushing responses, etc. that a change of MAP will lead to an inverse adjustment in cerebral arte-
Transcranial Doppler (TCD), an ultrasound-based evaluation of the rial tone and CBV, which will lead to either a smaller or larger change
cerebral arteries, can delineate the pulsatile component of the cardiac in ICP depending on the position of the slope of the ICP-volume curve.
cycle within the cerebral vasculature and links the linear dynamic A negative MAP and ICP correlation identifies intact PRx and hence,
systems of cerebral blood flow velocity (CBFV) to the arterial blood intact autoregulation and vice versa; a positive correlation indicates a
pressure (ABP). As a result, TCD can be used to monitor autoregulatory disturbed PRx. The PRx has shown good correlations with TCD-based
mechanisms as well as directly assess intracranial blood flow. This pro- indices (ie, Mx) and PET-CBF measurements. 51,52 Values for mean PRx
vides a mechanism for evaluating the brain’s compensation for elevated in head injury patients have been plotted against mortality rate and a
ICP by increasing CBF, detecting such compensation failure through cut-off value of 0.3 indicated a mortality increase from 20% to 70%. 53
decreased blood flow, and indicating the need for treatment to lower ICP The Mx shows abnormalities in SAH patients developing vaso-
and increase CBF, avoiding potential ischemia. In normal brain, autoreg- spasm, and in patients with ischemic stroke and subsequent poor
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ulation allows the cerebral vasculature to react to alterations in the ABP outcome. Both Mx and PRx are abnormal in hydrocephalus and TBI
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and respiratory pattern within seconds via constant adjustments in the as well as during rewarming after hypothermia. It has been shown that
diameter of cerebral arterioles, maintaining a constant CPP (Fig. 86-7). the CPP where cerebral autoregulation measured by PRx is strongest
This relationship, however, is unpredictable or absent in the injured identifies “optimal CPP” in an individual patient, and mortality in head
brain as mechanisms of autoregulation are disrupted. As a result, ICP trauma may be improved when deviation between average and optimal
is altered in proportion to changes in systemic pressure and respiration. CPP is minimized. 12
tion, but slower respiratory and slow-ICP waves can be used to analyze ■ ADJUNCT BRAIN MONITORING MODALITIES
Generally, cyclic cardiac time scales are too fast to assess for autoregula-
autoregulatory responses. Derived terms include the gain and phase shift Newer technologies in neuromonitoring have opened a very excit-
of the wave relationships as a measure for autoregulatory intactness. ing, multimodal strategy for use in parallel with ICP monitoring to
Other markers of intact or disrupted autoregulation are available. provide real-time information regarding multiple variables in cerebral
A coherence function, which is derived from transformation of post- pathophysiology at various stages of acute brain injury (Table 86-9).
Fourier correlation between each frequency component within the So far, however, ICP-control and CPP-stabilizing strategies provide the
frequency range of interest, is used to indicate linearity (and hence, clinician with the most practical information in dealing with patients
intactness) of the autoregulatory response system. The autoregulation suffering from intracranial hypertension. Nevertheless, additional intra-
index (ARI) is obtained from mathematical modeling of the mean CBFV cranial monitoring modalities have come to guide our understanding
reaction to spontaneous ABP fluctuations in order to find the best fit of brain injury by addressing secondary injury mechanisms not directly
of the resulting impulse response with one of 10 hypothetically created related to ICP elevations such as hypothermia, cerebral oxygenation,
response models. An ARI of zero indicates total absence of autoregu- blood flow abnormalities, and abnormalities in brain metabolism.
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latory response, whereas 9 delineates complete intactness. The mean Most of these measures, however, provide monitoring results that are
velocity index (Mx) is a correlation coefficient between the mean CBFV highly dependent on the location of the catheter tip (which is ideally
and CPP (or MAP, in cases where ICP is normal or not measured). To positioned within injured or susceptible brain tissue, that is, penum-
obtain the Mx, a series of consecutive time-averaged samples are cor- bra or pericontusional regions). These parenchymal monitor readings
related and, in head injury, a positive coefficient identifies a positive reflect abnormalities only within a few millimeters of the probe, so cau-
association between CBFV and CPP and hence, abnormal autoregula- tion must be taken in generalizing the findings to tissue regions beyond
tion and vice versa. Cross-validation of the Mx against other measures the brain sampling area.
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of autoregulation (ie, CO reactivity, leg-cuff test) resulted in good cor- The most commonly implemented examples of parenchymal tissue
2
relations and both Mx and ARI correlated in head injury patients. 47,48 monitors that are used in conjunction with ICP monitors are cerebral
Repeated Mx calculations over time allow continuous assessment of blood flow and tissue oxygenation probes. Continuous quantita-
autoregulation displayed along a time graph. Finally, to better adjust for tive monitoring of regional CBF is accomplished by the insertion
nonlinear and nonstationary relationships of the measured values, an of parenchymal cerebral blood flow monitors that utilize thermal
advanced computational method, the multimodal pressure-flow analy- diffusion within a very small region around the catheter tip. These
sis or MMPF, was created. An intervention, such as Valsalva maneuver, probes use thermal diffusion based on the tissue’s ability to dissipate
induces a characteristic phase lag between ABP and CBFV and the heat. The probe tip is inserted into the brain white matter to obtain
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identified oscillations are applied to MMPF and used as an index of normal CBF values at a range of 18 to 25 mL/100 g/min. Values lower
intactness of cerebral autoregulation. 48 than 15 mL/100 g/min can indicate tissue ischemia around the cath-
As TCD is a time-consuming procedure with limitations due to eter tip, while readings of less than 10 mL/100 g/min may indicate
significant user-dependent variability, alternate methods of assessing tissue infarction. Brain tissue oxygen pressure (PBt O 2 ) monitoring
autoregulatory mechanisms and CBFV have been attempted. For example, provides an estimate of the balance between regional oxygen supply
near-infrared spectroscopy (NIRS) and cerebral oximetry are methods of and oxygen use. Some PBt O 2 probes also allow monitoring of local
sampling the ratio of local brain oxygen delivery and utilization. This tissue pressure of carbon dioxide (PBt CO 2 ) and pH. A cutoff point
ratio depends largely on CBF, which can be used to replace CBFV in the indicating cerebral ischemia with PBt O 2 seems to be in the range of
calculation of autoregulatory indices. Studies assessing autoregulation 8 to 25 mm Hg and targeted therapy is therefore implemented at levels
have shown a significant correlation between TCD- and NIRS-derived less than 25. Small clinical studies identify good correlation between
Mx indices. NIRS obtained measures are not only much easier to obtain CBF and PBt O 2 .
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but multichannel NIRS would provide an elegant opportunity to assess A final adjunct monitoring method for intracranial pathology is
and monitor autoregulatory responses across various brain regions. cerebral microdialysis. This procedure allows for hourly monitoring of
) various extracellular brain substances that can contribute to elevations
Similar to NIRS, but as an invasive method, tissue oxygenation (PBt O 2
readings directly obtained via parenchymal probes allow for focal moni- in ICP or that are associated with tissue ischemia, indicating a risk for
toring of autoregulation. 50 elevated ICP (eg, glucose, lactate, pyruvate). The lactate/pyruvate ratio,

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