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Pulmonary thromboendarterectomy is the definitive treatment
for chronic pulmonary hypertension as the result of thromboembolic
disease. Although pulmonary embolism (PE) is one of the
more common cardiovascular diseases affecting Americans,
pulmonary thromboendarterectomy remains an uncommon procedure,
mainly because this form of chronic pul-monary hypertension
remains an underdiagnosed condition. These patients may
present with a variety of debilitating cardiopulmonary
symptoms. However, once diagnosed there is no curative
role for medical management, and surgery remains the only
option.
The exact incidence of PE remains unknown, but there
are some valid estimates. Acute PE is the third most common
cause of death (after heart disease and cancer). Approximately
75% of autopsy-proven PE is not detected clinically.1
Dalen and Alpert 2 calculated that PE results in 630,000
symptomatic episodes in the United States yearly, making
it about half as common as acute myocardial infarction,
and three times as common as cerebral vascular accidents.
This is, however, a low estimate, since in 70% to 80%
of the patients where the pri-mary cause of death was
PE, premortem diagnosis was unsus-pected. 3,4 The disease
is particularly common in hospitalized elderly patients.
Of hospitalized patients who develop PE, 12% to 21% die
in the hospital, and another 24% to 39% die within 12
months.5-7 Thus approximately 36% to 60% of patients who
survive the initial episode live beyond 12 months, and
may present later in life with a wide variety of symptoms.
More than 90% of clinically detected pulmonary emboli
are associated with lower extremity deep vein thrombosis
(DVT), but in two-thirds of patients with DVT and PE,
the DVT is asymptomatic.8,9 Greenfield 10 estimates that
approximately 2.5 million Ameri-cans develop DVT each
year.
The prognosis for patients with pulmonary hypertension
is poor, and it is worse for those who do not have intracardiac
shunts. Thus, patients with primary pulmonary hypertension
and those with pulmonary hypertension due to pulmonary
emboli fall into a higher risk category than those with
Eisenmenger’s syndrome and encounter a higher mortality
rate. In fact, once the mean pulmonary pressure in patients
with thromboembolic disease reaches 50 mmHg or more, the
3-year mortality approaches 90%.11 Surgical options are
dependent on both the primary disease process and the
reversibility of the pulmonary hypertension. With the
exception of thromboembol-ic pulmonary hypertension, lung
transplantation is the only effective therapy for patients
with pulmonary hypertension, when the disease reaches
end stage. Pulmonary transplantation is also still used
in some centers as the treatment of choice for those with
thromboembolic disease. However, a true assess-ment of
the effectiveness of any therapy should take into account
the total mortality once the patient has been accepted
and put on the waiting list. Thus, the mortality for transplanta-tion
(and especially double-lung or heart-lung transplantation)
as a therapeutic strategy is much higher than is generally
appreciated because of the significant loss of patients
awaiting donors. Considering the long-term use of antirejection
medica-tions with their associated side effects, the higher
operative morbidity and mortality, the long waiting time,
and inferior prognosis even after transplantation, transplantation
is clearly an inferior option to pulmonary thromboendarterectomy.
We consider it to be inappropriate therapy for this disease.
Pulmonary Thromboendarterectomy:
Indications
Although there were previous attempts, Allison et al 12
did the first successful pulmonary “thromboendarterectomy”
through a sternotomy using surface hypothermia, but only
fresh clots were removed. The operation was done 12 days
after a thigh injury that led to PE, and there was no
endarterectomy. Since then, there have been many occasional
surgical reports of the surgi-cal treatment of chronic
pulmonary thromboembolism,13, 14 but most of the surgical
experience in pulmonary endarterectomy has been reported
from the UCSD Medical Center. Braunwald commenced the
UCSD experience with this operation in 1970, which now
totals more than 1500 cases. The operation des-cribed
below 15 , using deep hypothermia and circulatory arrest,
is now the standard procedure.
When the diagnosis of thromboembolic pulmonary hyper-tension
has been firmly established, the decision for operation
is made based on the severity of symptoms and the general
condition of the patient. Early in the pulmonary endarterectomy
experience, Moser and colleagues 16 pointed out that there
were three major reasons for considering thromboendarterectomy:
hemodynamic, alveolo-respiratory, and prophylactic. The
hemodynamic goal is to prevent or ameliorate right ventricular
com-promise caused by pulmonary hypertension. The respiratory
objective is to improve respiratory function by the removal
of a large ventilated but unperfused physiologic dead
space. The prophylactic goal is to prevent progressive
right ventricular dys-function or retrograde extension
of the obstruction, which might result in further cardiorespiratory
deterioration or death.16 Our subsequent experience has
added another prophylactic goal: the prevention of secondary
arteriopathic changes in the remaining patent vessels.
Most patients who undergo operation are within New York
Heart Association (NYHA) class III or class IV. The ages
of the patients in our series have ranged from 8 to 85
years. A typical patient will have a severely elevated
pulmonary vascular resistance (PVR) level at rest, the
absence of significant comorbid disease unrelated to right
heart failure, and the appearance of chronic thrombi on
angiography that appear to be in balance with the measured
PVR level. Exceptions to this general rule, of course,
occur.
Although most patients have a PVR level in the range
of 800 dynes/sec/cm-5 and pulmonary artery pressures less
than sys-temic, the hypertrophy of the right ventricle
that occurs over time makes pulmonary hypertension to
suprasystemic levels possible. Therefore, many patients
(perhaps 20% in our prac-tice) have a level of PVR in
excess of 1000 dynes/sec/cm-5 and suprasystemic pulmonary
artery pressures. There is no upper limit of PVR level,
pulmonary artery pressure, or degree of right ventricular
dysfunction that excludes patients from operation. We
have become increasingly aware of the changes that can
occur in the remaining patent (unaffected by clot) pulmonary
vascular bed subjected to the higher pressures and flow
that result from obstruction in other areas. Therefore,
with the increasing experience and safety of the operation,
we are tend-ing to offer surgery to symptomatic patients
whenever the angiogram demonstrates thromboembolic disease.
A rare patient might have a PVR level that is normal at
rest, although elevated with minimal exercise. This is
usually a young patient with total unilateral pulmonary
artery occlusion and unaccept-able exertional dyspnea
because of an elevation in dead space ventilation. Operation
in this circumstance is performed to reperfuse lung tissue,
to reestablish a more normal ventilation, perfusion relationship
(thereby reducing minute ventilatory requirements during
rest and exercise), and to preserve the integrity of the
contralateral circulation. If not previously implanted,
an inferior vena caval filter is routinely placed sev-eral
days in advance of the operation.
Guiding Principles of the
Operation
There are several guiding principles for the operation.
It must be bilateral because, for pulmonary hypertension
to be a major factor, both pulmonary arteries must be
substantially involved. The only reasonable approach to
both pulmonary arteries is through a median sternotomy
incision. Historically, there were many reports of unilateral
operation, and occasionally this is still performed, in
inexperienced centers, through a thoracotomy. However,
the unilateral approach ignores the disease on the contralateral
side, subjects the patient to hemodynamic jeop-ardy during
the clamping of the pulmonary artery, and does not allow
good visibility because of the continued presence of bronchial
blood flow. In addition, collateral channels develop in
chronic thrombotic hypertension not only through the bronchial
arteries but also from diaphragmatic, intercostal, and
pleural vessels. The dissection of the lung in the pleural
space via a thoracotomy incision can therefore be extremely
bloody. The median sternotomy incision, apart from providing
bilateral access, avoids entry into the pleural cavities
and allows the ready institution of cardiopulmonary bypass.
Cardiopulmonary bypass is essential to ensure cardiovascu-lar
stability when the operation is performed and to cool
the patient to allow circulatory arrest. Very good visibility
is required, in a bloodless field, to define an adequate
endarterec-tomy plane and to then follow the pulmonary
endarterectomy specimen deep into the subsegmental vessels.
Because of the copious bronchial blood flow usually present
in these cases, periods of circulatory arrest are necessary
to ensure perfect vis-ibility. Again, there have been
sporadic reports of the perform-ance of this operation
without circulatory arrest. However, it should be emphasized
that although endarterectomy is possible without circulatory
arrest, a complete endarterectomy is not. We always initiate
the procedure without circulatory arrest, and a variable
amount of dissection is possible before the circulation
is stopped, but never complete dissection. The circulatory
arrest periods are limited to 20 minutes, with restoration
of flow between each arrest. With experience, the endarterectomy
usu-ally can be performed with a single period of circulatory
arrest on each side.
A true endarterectomy in the plane of the media must
be accomplished. It is essential to appreciate that the
removal of visible thrombus is largely incidental to this
operation. Indeed, in most patients, no free thrombus
is present; and on initial direct examination, the pulmonary
vascular bed may appear normal. The early literature on
this procedure indicates that thrombectomy was often performed
without endarterectomy, and in these cases the pulmonary
artery pressures did not improve, often resulting in death.
Surgical Technique
After a median sternotomy incision is made, the pericardium
is incised longitudinally and attached to the wound edges.
Typically the right heart is enlarged, with a tense right
atrium and a variable degree of tricuspid regurgitation.
There is usual-ly severe right ventricular hypertrophy,
and with critical degrees of obstruction, the patient’s
condition may become unstable with the manipulation of
the heart. Anticoagulation is achieved with the use of
beef-lung heparin sodium (400 units/kg, intra-venously)
administered to prolong the activated clotting time beyond
400 seconds. Full cardiopulmonary bypass is instituted
with high ascending aortic cannulation and two caval cannulae.
These cannulae must be inserted into the superior and
inferior vena cavae sufficiently to enable subsequent
opening of the right atrium. The heart is emptied on bypass,
and a temporary pulmonary artery vent is placed in the
midline of the main pul-monary artery 1 cm distal to the
pulmonary valve. This will mark the beginning of the left
pulmonary arteriotomy.
When cardiopulmonary bypass is initiated, surface cooling
with both the head jacket and the cooling blanket is begun.
The blood is cooled with the pump-oxygenator. During cooling
a 10°C gradient between arterial blood and bladder or
rectal tem-perature is maintained.17 Cooling generally
takes 45 minutes to an hour. When ventricular fibrillation
occurs, an additional vent is placed in the left atrium
through the right superior pulmonary vein. This prevents
atrial and ventricular distension from the large amount
of bronchial arterial blood flow that is common with these
patients. It is most convenient for the primary sur-geon
to stand initially on the patient’s left side. During
the cool-ing period, some preliminary dissection can be
performed, with full mobilization of the right pulmonary
artery from the ascend-ing aorta. All dissection of the
pulmonary arteries takes place intrapericardially, and
neither pleural cavity should be entered. An incision
is then made in the right pulmonary artery from beneath
the ascending aorta out under the superior vena cava and
entering the lower lobe branch of the pulmonary artery
just after the take-off of the middle lobe artery.
Any loose thrombus, if present, is now removed. It is
most important to recognize, however, that first, an embolectomy
without subsequent endarterectomy is quite ineffective
and, second, that in most patients with chronic thromboembolic
hypertension, direct examination of the pulmonary vascular
bed at operation generally shows no obvious embolic material.
Therefore, to the inexperienced or cursory glance, the
pulmonary vascular bed may well appear normal even in
patients with severe chronic embolic pulmonary hypertension.
If the bronchial circulation is not excessive, the endarterectomy
plane can be found during this early dissection. However,
although a small amount of dissection can be performed
before the initia-tion of circulatory arrest, it is unwise
to proceed unless perfect visibility is obtained because
the development of a correct plane is essential.
When the patient’s temperature reaches 20°C, the aorta
is crossclamped and a single dose of cold cardioplegic
solution (1L) is administered. Additional myocardial protection
is obtained by the use of a cooling jacket. The entire
procedure is now performed with a single aortic crossclamp
period with no further administration of cardioplegic
solution. A modified cere-bellar retractor is placed between
the aorta and superior vena cava. When blood obscures
direct vision of the pulmonary vascular bed, thiopental
is administered (500 mg to 1 g) until the electroencephalogram
becomes isoelectric. Circulatory arrest is then initiated,
and the patient undergoes exsanguination. It is rare that
one 20-minute period for each side is exceeded. Although
retrograde cerebral perfusion has been advocated for total
circulatory arrest in other procedures, it is not helpful
in this operation because it does not allow a completely
bloodless field, and with the short arrest times that
can be achieved with experience, it is not necessary.
Removing Thromboembolic
Material
Any residual loose thrombotic debris encountered is removed.
Then, a microtome knife is used to develop the endarterectomy
plane posteriorly. Dissection in the correct plane is
critical because if the plane is too deep the pulmonary
artery may per-forate, with fatal results, and if the
dissection plane is not deep enough, inadequate amounts
of the chronically thromboembol-ic material will be removed.
Once the plane is correctly devel-oped, a full-thickness
layer is left in the region of the incision to ease subsequent
repair. The endarterectomy is then performed with an eversion
technique. Because the vessel is everted and subsegmental
branches are being worked on, a perforation here will
become completely inaccessible and invisible later. This
is why absolute visualization in a completely blood-less
field provided by circulatory arrest is essential. It
is impor-tant that each subsegmental branch is followed
and freed individually until it ends in a “tail,” beyond
which there is no further obstruction. Residual material
should never be cut free; the entire specimen should “tail
off’ and come free spontaneously. Once the right-side
endarterectomy is completed, circulation is restarted,
and the arteriotomy is repaired with a con-tinuous 6-0
polypropylene suture. The hemostatic nature of this closure
is aided by the nature of the initial dissection, with
the full thickness of the pulmonary artery being preserved
immedi-ately adjacent to the incision.
After the completion of the repair of the right arteriotomy,
the surgeon moves to the patient’s right side. The pulmonary
vent catheter is withdrawn, and an arteriotomy is made
from the site of the pulmonary vent hole laterally to
the pericardial reflection, avoiding entry into the left
pleural space. Additional lateral dissection does not
enhance intraluminal visibility, may endanger the left
phrenic nerve, and makes subsequent repair of the left
pulmonary artery more difficult. The left-sided dis-section
is virtually analogous in all respects to that accomplished
on the right. The duration of circulatory arrest intervals
during the performance of the left-side dissection is
subject to the same restriction as the right. After the
completion of the endarterectomy, cardiopulmonary bypass
is reinstituted and warming is commenced. Methylprednisolone
(500 mg, intra-venously) and mannitol (12.5 g, intravenously)
are adminis-tered, and during warming a l0°C temperature
gradient is main-tained between the perfusate and body
temperature. If the systemic vascular resistance level
is high, nitroprusside is admin-istered to promote vasodilatation
and warming. The rewarming period generally takes approximately
90 minutes but varies according to the body mass of the
patient.
When the left pulmonary arteriotomy has been repaired,
the pulmonary artery vent is replaced at the top of the
incision. The right atrium is then opened and examined,
unless prior to car-diopulmonary bypass, a negative “bubble”
test was confirmed on transesophageal echocardiography.
Otherwise, any intraatri-al communication (present in
about 20% of patients) is closed at this point. Although
tricuspid valve regurgitation is invariable in these patients
and is often severe, tricuspid valve repair is not performed.
Right ventricular remodeling occurs within a few days,
with the return of tricuspid competence. If other cardiac
procedures are required, such as coronary artery or mitral
or aortic valve surgery, these are conveniently performed
during the systemic rewarming period. Myocardial cooling
is discon-tinued once all cardiac procedures have been
concluded. The left atrial vent is removed, and the vent
site is repaired. All air is removed from the heart, and
the aortic crossclamp is removed.
When the patient has rewarmed, cardiopulmonary bypass
is discontinued. Dopamine hydrochloride is routinely administered
at renal doses, and other inotropic agents and vasodila-tors
are titrated as necessary to sustain acceptable hemody-namics.
The cardiac output is generally high, with a low sys-temic
vascular resistance. Temporary atrial and ventricular
epi-cardial pacing wires are placed. Despite the duration
of extra-corporeal circulation, hemostasis is readily
achieved, and the
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