ANESTHETIC MANAGEMENT
Surgery on the ascending aorta routinely
uses median sternotomy and CPB. The conduct of anesthesia is similar to that
for cardiac operations involving CPB, but the intraoperative course may be
complicated by long aortic cross-clamp times and large intraoperative blood
losses. TEE is espe-cially useful. Blood loss can be reduced by adminis-tration
of ε-aminocaproic acid or tranexamic acid. Concomitant
aortic valve replacement and coro-nary reimplantation are often necessary
(Bentall procedure). The radial artery cannulation site should be guided by the
possible need for clamp-ing of either the subclavian or innominate arteries
during the procedure. Nicardipine or nitroprusside may be used for precise
blood pressure control. β-Adrenergic blockade should also be
employed inthe presence of an aortic dissection. On the other hand, bradycardia
worsens aortic regurgitation and should be avoided. The arterial inflow cannula
for CPB is placed in a femoral artery for patients with dissections. In the
event that sternotomy may rup-ture an aneurysm, prior establishment of partial
CPB (using the femoral artery and femoral vein) should be considered.
These procedures are usually performed
through a median sternotomy with deep hypothermic circula-tory arrest
(following institution of CPB). Additional considerations focus on achieving
optimal cerebralprotection with systemic and topical hypother-mia (above).
Hypothermia to 15°C, barbiturate or propofol infusion to
maintain a flat EEG, methyl-prednisolone or dexamethasone, mannitol, and
phe-nytoin are also commonly administered (but there is vanishingly small
evidence for efficacy of these drug treatments). The necessarily long rewarming
peri-ods probably contribute to the larger intraoperative blood losses commonly
observed after CPB.
Surgery limited to the descending
thoracic aorta may be performed through a left thoracotomy without CPB, with or
without (so-called “clamp-and–run” technique) a heparin-impregnated left
ventricular apex to femoral artery shunt; or using partial right atrium to
femoral artery bypass. A thoracoabdomi-nal incision is necessary for lesions
that also involve the abdominal aorta. One-lung anesthesia greatly facilitates
surgical exposure. Correct positioning of the endobronchial tube (even with fiberoptic
bron-choscopy) may be difficult because of distortion of the anatomy. A
right-sided double-lumen tube or a regular endotracheal tube with a bronchial
blocker may be necessary.
The aorta must be cross-clamped above
and below the lesion. Acute hypertension develops above the clamp, with
hypotension below when there is no shunt or partial bypass. Arterial blood
pressure should be monitored from the right radial artery, as clamping of the
left subclavian artery may be necessary. The sudden increase in left
ventricular afterload after application of the aortic crossclamp during aortic
surgery may precipitate acute left ventricular failure and myocardial ischemia,
particularly in patients with underlying ventricular dysfunction or coronary
disease; it can also exacer-bate preexisting aortic regurgitation. Cardiac
output falls and left ventricular end-diastolic pressure and volume rise. The
magnitude of these changes is inversely related to ventricular function. These
effects can be ameliorated by the use of shunting or partial bypass. Moreover,
the adverse effects of aor-tic clamping become less pronounced the more dis-tal
on the aorta that the clamp is applied. A vasodilator infusion is often needed
to prevent excessive increases in blood pressure. In patients with good
ventricular function, increasing anes-thetic depth just prior to cross-clamping
may also be helpful.
Excessive intraoperative bleeding may
occur during these procedures. Prophylaxis with anti-fibrinolytic agents may be
helpful. A blood scav-enging device (cell saver) for autotransfusion is
routinely used. Adequate venous access and intraop-erative monitoring are
critical. Multiple large-bore (14-gauge) intravenous catheters (preferably with
blood warmers) are useful. Pulmonary artery cath-eterization and intraoperative
TEE are often used. The period of greatest hemodynamic instability fol-lows the
release of the aortic cross-clamp; the abrupt decrease in afterload together
with bleeding and the release of vasodilating acid metabolites from the ischemic
lower body can precipitate severe systemic hypotension and less commonly
hyperkalemia. Decreasing anesthetic depth, volume loading, and partial or slow
release of the cross-clamp are helpful in avoiding severe hypotension. A bolus
dose of a vasopressor may be necessary. Sodium bicarbonate is often used,
particularly for persistent severe meta-bolic acidosis (pH < 7.20) in association with hypo-tension. Calcium
chloride may be necessary when symptomatic hypocalcemia follows massive
transfu-sion of citrated blood products.
Spinal cord ischemia can complicate
thoracic aortic cross-clamping. The incidence of transient post-operative
deficits and postoperative paraplegia are 11% and 6%, respectively. Increased
rates are asso-ciated with cross-clamping periods longer than 30 min, extensive
surgical dissections, and emer-gency procedures. The classic deficit is an
anterior spinal artery syndrome with loss of motor function and pinprick
sensation but preservation of vibration and proprioception. Anatomic variations
in spinal cord blood supply are responsible for the unpredict-able occurrence
and variable nature of deficits. The spinal cord receives its blood supply from
the ver-tebral arteries and from the thoracic and abdomi-nal aorta. One anterior
and two posterior arteries descend along the cord. Intercostal arteries feed
the anterior and posterior arteries in the upper thoracicaorta. Textbook
descriptions suggest that in the lower thoracic and lumbar cord, the anterior
spinal artery is supplied by the thoracolumbar artery of Adamkiewicz. The truth
is that a single large feed-ing artery usually cannot be identified. When
pres-ent, this artery has a variable origin from the aorta, arising between T5
and T8 in 15%, between T9 and T12 in 60%, and between L1 and L2 in 25% of
indi-viduals; it nearly always arises on the left side. It may be damaged
during surgical dissection or occluded by the aortic cross-clamping. Monitoring
motor and somatosensory evoked potentials may be useful in preventing paraplegia,
but clearly surgical technique and speed are most important.
As noted earlier, use of a temporary
heparin-coated shunt or partial CPB with hypothermia maintains distal perfusion
and decreases the inci-dence of paraplegia, hypertension, and ventricular
failure. Partial CPB has the disadvantage of requir-ing heparinization, which
increases blood loss. Using a heparin-coated shunt precludes the need for
heparinization. It is usually positioned proximally in the left ventricular
apex and distally in a femoral artery. Other therapeutic measures that may be
pro-tective of the spinal cord include methylpredniso-lone, mild hypothermia,
mannitol, and drainage of cerebrospinal fluid (CSF) to reduce the CSF pressure
in the spine; magnesium is also protective in some animal models. The efficacy
of mannitol appears to be related to its ability to lower CSF pressure by
decreasing its production. Spinal cord perfusion pressure is mean arterial
blood pressure minus CSF pressure; the rise in CSF pressure following experi-mental
cross-clamping of the aorta may explain how a mannitol-induced decrease in CSF
pressure could improve spinal cord perfusion pressure dur-ing cross-clamping.
Any protective effect of drain-age of CSF via a lumbar catheter may have a
similar mechanism.
The excessive use of vasodilators to
control the hypertensive response to cross-clamping may be a contributing
factor in spinal cord ischemia, as drug actions also occur distal to the
cross-clamp. Excessive reduction in blood pressure above the cross-clamp should
therefore be avoided to prevent inadequate blood flow and excessive hypotension
below it.
An increased incidence of kidney failure
following aortic surgery is reported after emergency proce-dures, prolonged
cross-clamping periods, and pro-longed hypotension, particularly in patients
with preexisting kidney disease. A variety of “cocktails” have been employed in
the hope of reducing the risk of kidney failure, including infusion of
manni-tol (0.5 g/kg) prior to cross-clamping, furosemide, fenoldopam (or
low-dose dopamine), etc. Low (renal)-dose dopamine or fenoldopam will increase
renal blood flow and may be used as an adjunct for a persistently low urinary
output after the cross-clamp is released, however there is no convincing
evidence that these treatments alter renal outcomes.
Stents are most often placed via
catheters inserted in a femoral artery. When an open technique is chosen,
either an anterior transperitoneal or an anterolat-eral retroperitoneal
approach can be used to access the abdominal aorta. Depending on the location
of the lesion, the cross-clamp can be applied to the supraceliac, suprarenal,
or infrarenal aorta. Heparin is usually administered prior to aortic clamping.
Intraarterial blood pressure can be monitored from either upper extremity. In
general, the more distally the clamp is applied to the aorta, the less the
effect on left ventricular afterload. In fact, occlusion of the infrarenal
aorta frequently results in minimal hemo-dynamic changes. In contrast, release
of the clamp usually produces hypotension; the same techniques that were
described earlier (see above) may be used. The large incision and extensive
retroperitoneal sur-gical dissection increase fluid requirements beyond
intraoperative blood loss. We recommend colloid to maintain intravascular
volume and crystalloid for maintenance fluids. Fluid replacement may be guided
by monitoring central venous pressure, noninvasive monitors of stroke volume,
or TEE. Pulmonary artery catheters are rarely used.
Renal prophylaxis with mannitol should
be considered, particularly in patients with preexist-ing renal impairment.
Clamping of the infrare-nal aorta has been shown to decrease renal blood flow,
which may contribute to postoperative kid-ney failure. The decrease in renal
blood flow is not prevented by epidural anesthesia or blockade of the
renin–angiotensin system.
Some centers use continuous epidural
anesthe-sia combined with general anesthesia for abdominal aortic surgery. This
combined technique decreases the general anesthetic requirement and appears to
suppress the release of stress hormones. It also pro-vides an excellent route
for administering postop-erative epidural analgesia. Systemic heparinization
during surgery introduces concern regarding the risk of paraplegia secondary to
an epidural hematoma; however, all credible studies suggest that there when the
catheter is placed well in advance of hepariniza-tion and removed after
reversal of anticoagulation there is no increased risk of neuraxial hematomas
as a consequence of epidural catheter placement.
Those undergoing stenting may not
require intu-bation either during or after the procedure. Most patients
undergoing open surgery on the ascend-ing aorta, the arch, or the thoracic
aorta will remain intubated and ventilated for 1–24 h postoperatively. As with
cardiac surgery, the initial emphasis in their postoperative care should be on
hemodynamic sta-bility and monitoring for postoperative bleeding. Patients
undergoing open abdominal aortic surgery may be extubated at the end of the
procedure. These patients typically continue to require a marked increase in
maintenance fluids for several hours postoperatively.
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