Neuroradiologic Procedures
Magnetic Resonance Imaging
The major constraints for this procedure are created by the powerful
magnetic field used. It creates three conditions
that have an impact on the anesthetic technique. The first is that any ferromagnetic
object that approaches the magnet has the potential to become a dangerous projectile.
The second is that a wide variety of electronic instruments will not function properly
in the vicinity of the magnet, including most notably those that contain oscilloscopes,
solenoids (e.g., some Dinamap-type blood pressure devices), or galvanometer-type
gauges. The third is that when moderately large metal objects, even non-ferromagnetic
objects, are brought into the vicinity of the magnet, they may degrade the image.
The equipment limitations have largely been circumvented. At present, MRI-compatible
ECGs, oximeters, capnographs, noninvasive blood pressure monitors, and gas machines
are available. It is only temperature monitoring that cannot be readily accomplished,
and with that exception, there is no longer any justification for incomplete monitoring
of patients undergoing MRI.
It is most frequently children, claustrophobic adults, and patients
with painful conditions who require anesthesia. Sedation with propofol and an unprotected
airway and general anesthesia with either an LMA or endotracheal tube have been used
successfully.
Interventional Neuroradiology (also
see Chapter 69
)
A wide variety of procedures are performed for the evaluation
and treatment of intracranial and extracranial disease. These procedures principally
include attempts to obliterate aneurysms or devascularize tumors and AVMs. Stenting
of extracranial carotid disease is occasionally performed. Vasospasm can be treated
by selective intraarterial instillation of papaverine or more commonly by balloon
dilatation. Most of these procedures can be accomplished without the involvement
of an anesthesiologist. The duration of a procedure, individual patient factors,
and occasionally, the necessity for precise physiologic control may result in requests
for monitored anesthesia care or general anesthesia. A chin-tucked position (to
remove the bones of the face from the line of the anteroposterior x-ray source) and
absolute immobility are commonly needed by the radiologist, and as a result, when
the anesthesiologist's assistance is requested, a general anesthetic will often be
necessary. In addition, anesthesiologists may become involved during the resuscitation
stage in the event of vascular rupture or migration of an intravascular device to
an incorrect location. When detachable devices (coils, balloons) are misplaced and
ischemia ensues, fluid loading and pressor administration may be requested to improve
collateral CBF while the device is retrieved.
Hyperventilation may be appropriate in an attempt to divert flow
away from normal brain and toward a lesion that is intended to receive the occlusive
device or material. Occasionally, the anesthesiologist will be asked to lower systemic
blood pressure and, it is hoped, cardiac output to facilitate the initial trapping
of glue or coils within a vascular lesion by briefly reducing the flow through it.
These requests have decreased in frequency as glue and balloons have been replaced
by platinum coils. Adenosine has proved effective when the occasion arises.[334]
The anesthesiologist may also be asked to participate in test
occlusion of various cerebral vessels that are candidates for sacrifice at a subsequent
procedure. In these circumstances, it is sometimes necessary to manage a patient
who is restless, confused, or abruptly unconscious.
Procedures that are likely to entail requests for blood pressure
manipulation are best done with an arterial catheter in place. The radiologist's
arterial line cannot be dedicated to blood pressure monitoring, and accordingly,
an independent arterial catheter should be established.
The relative roles of coiling and open clipping of intracranial
aneurysms and stenting and open endarterectomy for extracranial carotid disease are
yet to be defined. However, it seems inevitable that both coiling and stenting are
here to stay. A recently reported prospective multicenter (United Kingdom, Scandinavia,
northern Europe) comparison of clipping and coiling revealed significantly better
1-year disability-free survival with coiling, although some instances of rebleeding
occurred in the coiling patients.[335]
Proponents
of the open operation were critical because the surgery in that trial was not consistently
performed by neurosurgeons with practices dedicated to neurovascular surgery. The
discussion will go on, but it appears that coiling will inevitably have a role, potentially
an expanding one. The role of carotid angioplasty is similarly unclear at this time.
Although it has typically been reserved for patients deemed to have the highest
medical risk, morbidity and mortality similar to that associated with the open operation
have been reported recently.[336]
Note that the
results of older trials may not be applicable because in many instances, distal protection
devices (to protect the brain from embolized debris from the angioplasty site) were
not used. The hazards of that omission have been confirmed, and contemporary angioplasty
will use them. There are at least three ongoing prospective comparisons of carotid
endarterectomy versus angioplasty that should help define the optimal role of the
latter.[337]
That role may in large part be determined
by the durability of the stenting procedure.
For anesthesiologists asked to provide monitored anesthesia care
for these procedures, the significant issue is the bradycardia that occurs at the
time of balloon dilatation of the stenotic region. The anxiety about this physiologic
event was such that at one time, transvenous pacemakers were routinely placed by
some groups. This practice gave way to the availability of transthoracic pacers
and finally to the realization that prophylactic glycopyrrolate and occasionally
reactive administration of atropine will suffice. As the issue of profound bradycardia
has become less threatening, the practice of asking for anesthesia assistance has
all but ceased in many institutions (including ours).
