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In surgical procedures in which neurophysiological recordings
are requested, a careful titration of anesthetic agents is
necessary to maintain a patients' physiologic (e.g. blood
pressure, temperature) function, record neurophysiological
data and prevent awareness and movement. Halogenated
agents and nitrous oxide produce dose dependent depressions
in cortical activity and therefore affect the somatosensory
(SEP) and transcranial motor evoked potentials (tcMEP). In
addition, Electromyography and Transcranial stimulation require
the recording of muscle movements and thus prevent the use
of long lasting relaxants.
An awareness of the neurophysiological data required for
a particular procedure and the affects of anesthetic on the
physiology will assist in deciding the appropriate technique
for each patient.
Simple monitoring of spinal nerve roots or cranial nerves
(electromyography) rely on
the function of the neuromuscular junction because responses
are recorded from muscle. Therefore, a requirement is
no neuromuscular blockade.
For cases in which no baseline neurophysiological recordings
are required, a short acting paralytic may be appropriate
for intubation. If baseline recordings are necessary
then neuromuscular blockade should be avoided or reversed
as soon as possible.
Intraoperative neurophysiological monitoring techniques which
require the recording and/or stimulation of cortical neurons
are very sensitive to inhalational agents.
Transcranial electrical stimulation
and somatosensory evoked potentials
(usually recorded in conjunction with one another) require
synchronized cortical activity. Therefore, they are easily
affected by inhalational agents. In addition, the transcranial
stimulation technique relies on the recording of electromyographic
activity and therefore muscle relaxation must be avoided.
Total Intravenous Anesthesia (TIVA) is optimal
for tcMEP/SEPs monitoring:
Titrated profusion of analgesic and sedative agents produces
ideal recording conditions for somatosensory and transcranial
motor evoked potentials. In clinical studies, the use of TIVA
permits the generation and recording of highly reliable and
rapidly recorded responses. These reliable responses improve
the feedback to the surgeon regarding the functional status
of the patients' nervous system. This is essential during
surgical procedures in which the brain or spine are at risk.
In addition, TIVA permits the recording of electrical potentials
in neurologically compromised (e.g. mylopathic, neuropathic)
and difficult to assess patients (infants).
Several studies have shown that reliable responses can be
recorded with intravenous anesthesia and nitrous oxide up
to 50% (see Calancie et al.,1998). Greater than 50% depresses
cortical activity and thereby affects the stimulation (tcMEP)
and recording (SEP) of cortical neurons.
Relatively low level of halogenated inhalational agent (<0.5
MAC) with no nitrous oxide in most cases produces recordable
SSEP and motor responses to transcranial stimulation.
In 10-20 % of patients cortical recordings may be suppressed
and EMG response to transcranial stimulation obliterated with
this level of inhalational agent. The percentage of unreliable
responses may be even higher in severely neurologically compromised
or developing nervous systems.
High levels of halogenated agents (>0.5 MAC) significantly
depress cortical activity and therefore adversely affect cortically
recorded and cortically elicited responses. With high levels
of volatile agents the cortically recorded somatosensory responses
may have a loss of amplitude and increase latency and may
vary considerably from recording to recording. Transcranial
electrical stimulation may evoke only intermittent responses.
Changes in the patterns of stimulation may help elicit
a response (i.e. rapid presentation of pulse trains —
facilitation), however, the fluctuations in responses decrease
the validity of the intraoperative monitoring. Overall,
the use of inhalational agents at a minimum alveolar concentration
of 0.5% will result in decreased reliability for the evaluation
of the functional neurological status of the patient.
Higher levels of halogenated agents (1.5 MAC) typically results
in a loss of the SEP and tcMEPs altogether. With significant
cortical supression transcranial stimulation will likely not
activate enough motor strip neurons to elicit the muscle responses.
Additionally, even if the motor cortex does respond there
may be a lack of synchronization and thus, no activation at
the level of the anterior horn cell. Halogenated agents
can also directly affect the anterior horn cells and thereby
not permit the muscle contraction.
REFERENCES:
Calancie, B, et al., (1999) Distribution
and latency of muscle responses to transcranial magnetic stimulation
of motor cortex after spinal cord injury in humans.
J Neurotrama 1999 Jan; 16(1): 49-67.
Sloan, TB and Heyer, EJ (2002) Anesthesia
for intraoperative neurophysiological monitoring of the spinal
cord. J Clin Neurophysiology 2002 Oct; 19(5):
430-443.
Sloan, TB (2002) Anesthetics and
the brain. Anesthesiology Clinics of North America
2002 Jun; 20(2): 265-292.
Total Intravenous Anesthesia:
Kunisawa, T, et al. (2004) A comparison
of the absolute amplitude of motor evoked potentials among
groups of patients with various concentrations of nitrous
oxide. J Anesthesiology 2004; 18(3): 181-184.
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