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Intraoperative Use of Agonists in Neuroanesthesia
Alex Bekker M.D., Ph.D.
Director of Neuroanesthesia
New York University School of Medicine
Progress may have been all right once,
but it went on too long.
Ogden Nash
Activation of receptors leads to:
• Dose dependent sedation and anxiolysis
• Analgesia (supraspinal and spinal sites)
• Decrease plasma catecholamines
• Centrally mediated bradycardic and hypotensive effects
• Diuresis due to inhibition of ADH release and antagonism of ADH tubular effects
• Decongestant and antisialogogue effects
Qualifications for inclusion into the neuroanesthesia drug club:
• Controllability (e.g. rapid onset and offset of effect)
• Stability of intracranial homeostasis• Hemodynamic stability• Noninterference with neurophysiologic
monitoring• Neuroprotection• Antinonociception
Pharmacokinetics of IV agents
Dex Propofol Fentanyl Alfenta
Vdcc, l 16 16 30 10
Vdss, l 200 350 330 30
Cl, l/min 0.6 1.8 0.8 0.3
T1/2, min 6 4 6 4
T1/2hr 2 1.5 2.5 1
Context-sensitive Dexmedetomidine recovery times as a function of duration of infusion
Effect of Dexmedetomidine on Cerebral Blood Flow
• Animal models– Dex causes a reduction in CBF up to 45%
– Dex has no effect on the CMRO2
– Dex produces the concentration-dependent constriction of pial arteries and veins
– Dex limits hypercapnea- and hypoxia-induced cerebral vasodilation
Zornow MH et al, Anesth Analg; 1990Fale A et al, Anesth Analg; 1994Karlsson et al, Anesth Analg; 1991
Effect of Dexmedetomidine on Cerebral Blood Flow
• Human study (TCD)– Mean CBF velocity decreased with an increase
in plasma concentration of Dex– Pulsatility index increased at higher level of
Dex (indicates an increase in CVR)
Zornow MH et al, J Cereb Blood Flow Metab; 1993
Effect of Dexmedetomidine on ICP
• Animal model– ICP was unchanged despite an increase in systemic
blood pressure in rabbits– ICP was decreased in the presence of intracranial
hypertension Zornow MH et al, Anesth Analg 1992
• Human study– Dex has no effect on lumbar CSF pressure in patients
undergoing transphenoidal pituitary tumor resection
Talke P et al. Anesth Analg 1997
Dexmedetomidine effect on SSEPs and AEP
• There is a lack of effect on cortical AEP
• Dex does not affect cortical (P25-N35) response
• Dex depresses median nerve P15-N20 amplitudes
Thornton C et al. Br J Anaesth 1999
Median nerve SSEPs tracings after switching from propofol to
Dexmedetomidine infusion
Left
Right
Amplitudes of early and late SSEP waves at various stages of the surgery
EP Amplitudes
0
1
2
3
4
5
6
7
8
10:17 11:33 11:39 11:49 12:11 12:16 12:21 13:06 13:09 13:19
Time
uV
R P25-N35
L P25-N35
R N20-P25
L N20-P25
R P15-N20
L P15-N20
dd
Dexmedetomidine effect on the EEG
• Dex decreased MPF and 95% PF in cats
• Dex increased delta band power
• Halothane 2% produced similar EEG changes
• Animals on Dex responded to tail clamping purposefully
• BIS values after Dex infusion for 1 hour were:
65 at 0.2 g/kg/hr
60 at 0.6 g/kg/hr
• The volunteers were readily awakened from hypnosis by talking to them; BIS returned to awake level
Farber NE et al. Brain Research 1997 Hall JE et al. Anesth Analg 2000
BIS before and after subjects were asked to perform various tasks
Hall et al. Anesth Analg 2000
Neuroprotective effects of Dexmedetomidine
• Inhibition of ischemia induced NE release may be associated with neuroprotection
• Dex prevents delayed neuronal death after focal ischemia• Dex decreased total ischemic volume by 40% compared to
placebo
Jolkkonen J et al. Euro J Pharm 1999
Hoffman WE et al Anesthesiology 1991
• Dex enhances glutamine disposal by oxydative metabolism in astrocytes
Huang R et al. J Cereb Blood Metab 2000
Dexmedetomidine and Antinociception
2 – Agonists attenuate hemodynamic responses to laryngoscopy and intubation
Lawrence CJ et al Anaesthesia 1997
2 – Agonists decrease perioperative oxygen consumptionTaittonen MT Br J Anaesth 1997
• Dex reduces NE level during emergence from anesthesia (2 to 3 times lower than in placebo group)
Talke P et al. Anesth Analg 2000
Law of Conservation of Tsouris
The amount of aggravation in the universe
is a constant. If things are going well in one area,
they are going wrong in another.
Dexmedetomidine: Side Effects
• Hypotension • Transient hypertension• Bradycardia• Dry mouth• Limited amnestic effect• Animal studies show reduction in the
CBF/CMRO2 ratio
• Excessive sedation
Clinical Experience: Craniotomy
• In patient undergoing craniotomy, premedication with clonidine:
reduced anesthetic requirements
attenuated hemodynamic responses to intubation and pin fixation
Costello T et al Anesth Analg 1998
• Postoperative infusion of Dex in patients recovering from transphenoidal hypophysectomy reduced plasma catecholamines by 70%
Talke P et al Anesth Analg 1997
Clinical Experience: Spinal Fusion
• Perioperative administration of clonidine reduced postoperative morphine requirements by a factor of 3 in patients undergoing spinal fusion
Bernard et al Anesthesiology 1991
• Intraoperative switching from a propofol infusion to Dex in patients undergoing cervical fusion resulted in:– A neurological examination that was successfully performed in the
OR on an intubated patient
– Clinically insignificant hemodynamic changes during and after the switchover
Bloom M et al J Neurosurg Anesth 2001
2 – Agonists and Cognitive Function
• There is strong evidence that 2 – agonists improve prefrontal cortical function (PFC)
• PFC shares reciprocal projections with: – Parietal association cortex specialized for visuospatial processing
– Medial temporal lobe important to memory abilities
– Anterior cingulate cortex involved in organizing complex cognitive function
– Caudate nucleus that regulates motor behavior
• NE’s beneficial action in the PFC appear to result from stimulation of 2 (A) – receptors postjunctional to NE terminals
Arnstein et el. Arch Gen Psychiatry 1996
Clinical Experience: Carotid Endartrectomy
• A combination of superficial and deep cervical plexus blocks is the most common regional anesthetic technique in the NYU medical center
• Sedation with dexmedetomidine (0.2-0.4 mcg/kg/hr) offers a comfortable and cooperative patient during the operation
• Less agitation and respiratory depression than with a continuous infusion of propofol or repeated doses of fentanyl and/or midazolam
Clinical Experience: Functional Neurosurgery
• Dex infusion at 0.1 – 0.2 g/kg/hrallowed us to achieve a tranquil state sufficient to complete neuropsychiatric testing required for mapping of the cortical speech area, as well as to perform an awake tumor resection
• A lack of respiratory depression offers an advantage over other technique
Bekker A et al. Anesth Analg 2001
Is there a reason to add Dexmedetomidine to our practice?
• Dex properties include: – Reversible sedation without respiratory depression
– Analgesia
– Anesthetic sparing effect
– Cardiovascular stability
– Has minimal effect on ICP
– May offer neuroprotection
– A unique type of sedation in which a patient could be aroused
readily • Theoretical advantages have to be objectively justified in
clinical studies