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NeuroRadiology (Cerebral Coiling)  Endovascular Carotid Stents
Evoked Potentials &
Anesthetic Techniques		  Neuro ICP & Sat Monitoring Devices (pdf) sUPDATE
Posterior Spinal Fusion (Scoliosis)  Indocyanine Green (IC Green) 
for Angiography NEW
"Awake" Craniotomy  

Evoked Potentials & Anesthetic Techniques

(from Jameson & Sloan, Anes. Clin., 2006, 24:777-791)

Somatosensory Evoked Potentials (SSEPs or SEPs)

Sensory (visual, auditory, tactile, or position) stimuli activate afferent nerves that synapse in the spinal cord or brainstem and ultimately arrive in the contralateral area of the designated cerebral cortex. 
Sensory evoked potential monitoring provides a standard stimulus with recording of the electrical neuronal response over the appropriate area of sensory cortex.
The most commonly used sensory tests are somatosensory evoked potentials (SSEPs or SEPs) electrical stimulation of a mixed motor sensory nerve, and brainstem auditory evoked potentials (BAEPs), standard sound stimulation of the cochlear nerve.
Visual evoked potentials are not commonly performed due to difficulties in obtaining reliable responses. 

Motor Evoked potentials (MEPs)

Motor evoked potentials (MEPs) do not involve areas of sensory cortex but are produced by the electrical stimulation of the motor cortex and measurement of the electromyoqraphic response (EMG) in skeletal muscle, usually abductor pollicis brevis (APB), tibialis anterior (TA), medial gastrocnemius (MG), and interphalangeal foot muscles, 
It is common practice to record EMG responses in arms, APB, and two or more locations in the lower extremities, TA or MG and foot. 


Stimulus and Analysis

For SEP and BAEP the stimulus/response cycle is repeated and averaged between 150 and 1,000 times to allow identification of the response. 
The MEP response is a single skeletal muscle response produced by four to six stimuli. 
All waveforms are analyzed for latency (time from stimulus to peak response), amplitude (absolute difference between negative and positive wave deflection), and configuration or shape. 
Waveform changes that are considered to indicate possible functional change are a 50% decrease in amplitude or a 30% increase in latency.

Pathologic changes in shape, amplitude, and latency are caused by tissue ischemia in the nerve, spinal cord, brainstem, or subcortical or cortical neural structures. 
Sources of tissue ischemia include:
• hypoxia
• hypoperfusion from systemic or local hypotension
• blood vessel damage
• local compression of tissue and vessels with retraction
• stretching of nerve, spinal cord, or brain stem
• decreased blood flow
• inadequate oxygen-carrying capacity (anemia)
• local or systemic hypothermia (vasoconstriction)


Effects of Anesthetic Agents:

Anesthetic agents produce varying degrees of change in the evoked potential:
• Increasing concentrations of volatile anesthetics produce a dose-dependent decrease in amplitude and increase in latency. 
• Nitrous oxide effect can range from none to substantial decreases in amplitude and increases in latency. 
• Opioids have little effect on evoked potentials. 
• Hypnotic intravenous drugs such as propofol and dexmedetomidine also have little effect. 
• Ketamine can cause increases in the amplitude but has little effect on the latency. 


For intraoperative evoked potential monitoring to be effective, the anesthetic protocols must be designed to minimize the drug effect on the evoked potential response. Low stable concentrations of volatile anesthetics with intravenous hypnotic agents or just intravenous hypnotic agents alone are usually advocated. Supplementation with low-dose ketamine infusion will often increase the waveform amplitude making it easier to identify changes due to surgical manipulation, particularly in patients with significant underlying abnormalities. Since minimal concentrations of volatile anesthetics and nitrous oxide can make it impossible to obtain an MEP, intravenous techniques without muscle relaxants are used. 

Recommended intraoperative monitoring tests and anesthetic management by surgical procedure are listed in Table 1.

A couple of reprints of good journal articles are available in the Library under "E" for Evoked Potentials.
IndoCyanine GREEN 
Video-angiography in Cerebrovascular Surgery NEW


December 2010

IC Green (Indocyanine Green ) video-angiography is a new technique of blood-flow measurement that is being used in neurosurgery. The dye is administered intravenously and used in combination with a special microscope to obtain high-resolution, high-contrast video images during micro-neurovascular surgery.
Used in the OR at MMC by neurosurgery

* The OR will order on a patient specific basis. To view order form for printing click here.
* Send one vial and one amp of diluent to the OR as requested. It is stored in the Carousel. Each box contains 6 vials of IC green and 6 amps of diluent. 

  • -Only the aqueous solvent provided should be used for reconstitution. It is specially prepared sterile water for injection. There have been reports of incompatibility with some commercially available sterile water for injection products. 
    •
  • -The vial of IC Green should be reconstituted with the entire contents of the supplied diluent. 
    Solution must be used within 6 hrs of reconstitution. 
  • -Normal dose is one vial of reconstituted IC Green, 25mg, administered via IV push
  • -Flush lines with NS after administration
  • -Anaphylactic reactions have occurred. Use in caution in patients with an allergy to iodine. 
  • -Unknown effects in pregnancy. Use only when clearly indicated. 

 

Indocyanine green will transiently lower SpO2 readings on a pulse oximeter (Reference). If persistent or otherwise unexplained, confirm with Arterial Blood Gas determination.

J Clin Monit. 1987 Oct;3(4):249-56.
Methylene blue and indocyanine green artifactually lower pulse oximetry readings of oxygen saturation. Studies in dogs.

Sidi A, Paulus DA, Rush W, Gravenstein N, Davis RF.

Department of Anesthesiology, College of Medicine, University of Florida, Gainesville 32610-0254.

The effects of fluorescein, methylene blue, and indocyanine green on hemodynamic variables and on pulse oximetry and co-oximetry measurements of arterial hemoglobin oxygen saturation (SaO2) and oxyhemoglobin percentage (% HbO2) were evaluated in 16 anesthetized dogs in vitro by co-oximetry (% HbO2) and in vivo by pulse oximetry (SaO2). The light absorbance (optical density) in plasma (range 500 to 800 nm) was measured by a spectrophotometer. Fluorescein did not affect

oximetry measurements, plasma light absorbance in the range measured, or hemodynamic variables. Methylene blue caused dose-dependent decreases in measurements made with both forms of oximetry for up to 30 minutes, the decrease being greater and longer lasting with pulse oximetry (P less than 0.05).

Hemodynamic measurements in 5 dogs showed that methylene blue (1 to 5 mg/kg) increased arterial pressure transiently, after which cardiac output, stroke index, and left ventricular stroke work index decreased and left ventricular end-diastolic pressure and systemic and pulmonary vascular resistances increased (P less than 0.05 with 5 mg/kg). Methemoglobin concentration measured by co-oximetry increased significantly (to 19.9 +/- 1.4%, P less than 0.05) 1 minute after 5 mg/kg of methylene blue was injected. Methylene blue had a dose- and time-dependent effect on plasma light absorbance, and this effect peaked in the 660- to 670-nm range. The data do not distinguish the relative contributions of

physiology (hemodynamic change), chemistry (methemoglobin production), and physics (optical properties) to the decrease in pulse oximetry and co-oximetry measurements that follows injection of methylene blue. Indocyanine green affected neither hemodynamic variables nor co-oximetry readings but decreased pulse oximetry readings for up to 10 minutes dose dependently.