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Three types of neuromonitoring devices:
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- Monitors of cerebral hemodynamics (TCD, stump pressure)
- Monitors of cerebral oxygen metabolism (NIRS, SjO2)
- Monitors of cerebral functional state (EEG, evoked potentials)
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Awake (nonsedated) patient monitoring is the gold standard of neuromonitoring, but difficult to match during surgery.
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Cerebral autoregulation (CAR): maintenance of constant CBF over a range of systemic BP (Figure 99-1)
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- Lower limit of autoregulation (LLA): BP under which CBF decreases with BP (here about 50 mm Hg)
- Upper limit of autoregulation (ULA): BP above which CBF increases with BP (here about 150 mm Hg)
- CAR shift: modification of LLA and/or ULA set points, associated with altered relationship between systemic BP and CBF (LLA right shift in chronic HTN and anemia)
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- Measures cerebral blood velocity (Vx), which correlates with cerebral blood flow (CBF)
- Relative changes over time are more accurate than absolute values
- Three main sites to obtain Doppler signal (Figures 99-2 and 99-3): temporal (most common, used essentially to measure Vx in MCA), suboccipital (posterior cerebral circulation), orbital (anterior cerebral circulation)
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Problems encountered with TCD monitoring: no acoustic window (5%–15%), probe dislocation
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- Peak systolic velocity (PsVx) and End-diastolic Velocity (EdVx): measured directly
- Mean velocity (MVx): derived from PsVx and EdVx
- Pulsatility index (PI): maximal variation of Vx from systole to diastole, weighted by MVx
- Used to study changes in distal cerebral vascular resistance if HR and systemic BP pulsatility are maintained constant
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