Three types of neuromonitoring devices:
- Monitors of cerebral hemodynamics (TCD, stump pressure)
- Monitors of cerebral oxygen metabolism (NIRS, SjO2)
- Monitors of cerebral functional state (EEG, evoked potentials)
Awake (nonsedated) patient monitoring is the gold standard of neuromonitoring, but difficult to match during surgery.
Cerebral autoregulation (CAR): maintenance of constant CBF over a range of systemic BP (Figure 99-1)
- 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)
Figure 99-1. Autoregulatory Curve
Flow is generally shown as being stable between mean arterial pressures (or more strictly, perfusion pressure) of 50 and 150 mm Hg. However, although this expresses the concept of autoregulation well, it does not depict reality. Reproduced from Longnecker DE, Brown DL, Newman MF, Zapol WM: Anesthesiology. Figure 50-10A. Available at: www.accessanesthesiology.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.
- 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)
Figure 99-2. Sites to Obtain TCD Signal
Reproduced from Levitov A, Mayo PH, Slonim AD: Critical Care Ultrasonography. Figure 26-19. Available at: www.accessanesthesiology.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.
Figure 99-3. Circle of Willis
Depths of insonation from ipsilateral transtemporal window. Reproduced from Levitov A, Mayo PH, Slonim AD: Critical Care Ultrasonography. Figure 26-20. Available at: www.accessanesthesiology.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.
Problems encountered with TCD monitoring: no acoustic window (5%–15%), probe dislocation
- 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
Factors Affecting Velocity
|Factors associated with increased Vx signals||Factors associated with decreased Vx signals|
|Anesthetic inhalational agents||Anesthetic induction agents (except ketamine)|
|Vasopressors, hypertension with loss of CAR||Vasodilators, hypotension with loss of CAR|
|Increasing age, ...|
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