IX. Maintenance
A. No one game plan can apply to all situations.
The following is only a sugestion.
B. You live or die by your twitch monitor.
Neuroanesthesia, limited as it is by the
constraints created by ICP constraints and
interactions with neurophysiologic monitoring,
has a high possibility of unwanted patient
movement.
1. Have the twitch monitor applied and ready
to go prior to the vecuronium taking effect.
Twitch monitors that automatically repeat
a train of four every 10 seconds are a
potential source of pain for an awake
patient if they are already cycling when
applied to the patient. Make sure that the
thumb has four full twitches before the
vecuronium hits (so you have a baseline
at any given electrode location).
If you change the location of the pads
after induction, and the response is gone,
maintain a high degree of suspicion. Despite
full paralysis with no twitchs, if the
electrodes are placed properly, you should
see at least a minimal response to 100 Hz
tetanic stimulus. If not, suspect electrode
placement, weak batteries or a bad stimulator.
2. Heavy individuals with thick skin may not
transmit the current. Clean the skin
throughly prior to application of the
electrodes.
3. Check the battery level. If your stimulator
displays output, it should show about 80-90
milli-volts if the batteries are new.
4. Insure solid application of the electrodes
at the wrist. Instances have occured where
only partial application of the electrodes
gave the impression that the blockade was
much deeper than actually existed.
Substantial patient movement occured.
5. If the twitch does not return after a
reasonble amount of time, assume that the
twitch monitor is not working before
assuming that the vecuronium is lasting
an inordinately long time.
6. If the electrodes are attached to a
paretic arm, the degree of blockade will
be underestimated - A profound block will
be present when twitches are still
present: extrajunctional receptor pro-
liferation will result in twitchs despite
adequate blockade in other parts of the body.
Thumb adduction is the best measure of ulnar
stimulation. Sometimes, however, this is not
the case. If the nlittle finger has four twitches
and the thumb has none, a confusing situation
exists. Assuming that the little finger is
moving as a result of direct muscle
stimulation can be dangerous. This
scenario has occured in the setting of
poor contact between the electrode and the
skin.
7. Taping the arm to the arm board can help
differentiate direct muscle stimulation
from true median nerve stimulation.
8. In the prone position with the arms
tucked, the most reliable muscle to use
for twitch monitoring is the masseter.
Place one electrode on the brow and the
other opposit the external auditory
meatus. Palpate the mentum during train of
four (TOF) stimulation. This technique is less
susceptible to direct muscle than placing
both electrodes on the forehead. Place the
electrodes before flipping the patient. If
the head is not in pins, however,
excessive movement of the head may cause
problems for the surgeon.
9. The goal of neuromuscular blockade is to
mantain the patient at 1/4 for the entire
case, right up to removal of the head
pins.
10. Patients on chronic phenytoin (dilantin) or
carbamazepine (tegretol) therapy usually have
activation of liver enzymes that greatly shorten
the duration of action of vecuronium.
11. If the median nerve is being repetitively
stimulated by the neurophysiologist, the
muscle may become fatigued. This may cause
an overestimation of the degree of
blockade present. It may be necessary to
ask that the median nerve stimulus be
stopped for a few minutes.
C. Is Nitrous a bad thing?
1. Absolute contraindication is the sitting
position and (?) the prior use of
pneumoencephalography
2. James Cottrel prefers no N20 at all
D. Dose ranges
1. Fentanyl
a. Induction dose: about 7mcg/kg
b. Dose prior to incision: total of
10mcg/kg
c. Constant infusion of 2mcg/kg/hr. If
the patient is paralyzed, this dose
may be gradually reduced later in the
case so that the patient will wake up at
the end of the procedure. As the infusion
rate is reduced, note the BP response. If
the patient is staying railroad tracks, it
may be possible to further reduce the
rate. Extreme caution should be used.
A lower limit of 1mcg/kg/hr is practical.
to assure ready awakening, try to turn
off the infusion about 45-60 minutes
before you plan to extubate the pt.
d. Keep track of the total dose
administered.
2. Forane is run at about 0.5% (expired
concentration). It is better to leave the
Forane as constant as possible and to vary
the level of anesthesia with the fentanyl.
3. Nitrous is kept at 50% if the patient can
tolerate it.
4. Pentothal infusions may be required as an
adjunct to ICP control for longer cases.
The dose is 2 mg/kg/hr.
E. A method for blood loss determination
X. Timing of Hyperventilation and Mannitol
A. When to start for different types of cases
B. How much Mannitol is needed and when should it
be augmented with Lasix
1. Excessive diuresis results in excessive IV
fluid administration.
2. Osmolality should rise by 10 mOsm with
Mannitol.
C. Mannitol can be given till the osmolality is
300. Keep in mind that aggresive diurectic use
will result in aggressive fluid replacement.
Fluids and Electrolytes