Glenn
Since the late 1950's the Glenn shunt, as the SVC-to-right pulmonary artery anastomosis is
called, has been performed on patients with diverse cyanotic congenital heart disease to
improve pulmonary blood flow.
The Glenn shunt does not create volume overload of the ventricle or increased work for the
ventricle, as is the case in systemic-pulmonary artery shunts. It provides venous flow to
the lung fields for oxygenation, rather than an arteriovenous mixture. The venous return is
under relatively low pressure, unlike systemic-pulmonary artery shunts, and the risk for
pulmonary artery distortion and late pulmonary vascular obstructive disease is
substantially less.
The Glenn shunt is palliative - not corrective. Depending on the diagnosis and the surgical
era, the Glenn shunt may be the only palliation for the cyanotic patient, one of several
palliative surgeries, or a step prior to corrective surgery or the Fontan form of total
right heart bypass.
COMPLETE
HISTORY
Experimental work (Carlon, Mondini and deMarchi, 1951, Glenn and Patino, 1954) explored the
feasibility of creating a connection between the superior vena cava (SVC) and the pulmonary
artery to increase pulmonary blood flow. William W.L. Glenn, a professor of surgery at Yale
University, first reported the clinical application of this concept in 1958.
INDICATIONS
Since the late 1950's the Glenn shunt, as the SVC-to-right pulmonary artery anastomosis is
called, has been performed on patients with diverse cyanotic congenital heart disease to
improve pulmonary blood flow. The Glenn shunt does not create volume overload of the
ventricle or increased work for the ventricle, as is the case in systemic-pulmonary artery
shunts. It provides venous flow to the lung fields for oxygenation, rather than an
arteriovenous mixture. The venous return is under relatively low pressure, unlike
systemic-pulmonary artery shunts, and the risk for pulmonary artery distortion and late
pulmonary vascular obstructive disease is substantially less. The Glenn shunt is
palliative, not corrective. Depending on the diagnosis and the surgical era, the Glenn
shunt may be the only palliation for the cyanotic patient, one of several palliative
surgeries, or a step prior to corrective surgery or the Fontan form of total right heart
bypass.
CLASSIC GLENN SHUNT
Dr. Glenn described an anastomosis between the transected distal end of the right pulmonary
artery and the side of the SVC, which is ligated distal to the anastomosis. The azygous
vein is ligated to prevent its decompressing flow from the SVC. Systemic venous return from
the head and upper extremities is to the right lung, driven by the pressure gradient from
the SVC to the left atrium. A patient having total right heart bypass for palliation of
cyanotic congenital heart disease with a Fontan procedure after a classic Glenn shunt
typically would have an anastomosis between the inferior vena cava (IVC) and the left lung,
using either a direct atrio-pulmonary connection or a conduit. Due to requisite extensive
dissection and technically difficult pulmonary artery reconstruction, patients after a
classic Glenn usually do not receive extra-cardiac IVC-pulmonary artery conduits as are now
commonly performed in the Fontan type procedure.
BI-DIRECTIONAL CAVO-PULMONARY SHUNT
The bi-directional cavo-pulmonary shunt was first performed in 1966. The transected end of
the SVC is anastomosed to the side of the undivided right pulmonary artery, allowing flow
to both lung fields. As with the classic Glenn shunt, the bi-directional cavo-pulmonary
shunt is far less likely to engender pulmonary vascular obstructive disease compared with
systemic-pulmonary shunts, and there is minimal distortion of the pulmonary artery
architecture. With newer surgical techniques the bi-directional Glenn (as it is sometimes
called) can be performed without cardiopulmonary bypass. If bilateral vena cavae exist,
both can be anastomosed end-to-side to the pulmonary artery. A bi-directional
cavo-pulmonary shunt may be done under six months of age, assuming the pulmonary vascular
resistance has declined.
The bi-directional cavo-pulmonary shunt currently most commonly is employed as the first
stage in a staged Fontan procedure. (Some surgeons feel that the staged Fontan improves
results. Others feel a fenestration in the Fontan circuit at the time of a single Fontan
operation with subsequent closure of the fenestration improves results‹by improving cardiac
output, minimizing systemic venous hypertension, and decreasing post-operative thoracostomy
drainage‹at the cost of relative desaturation until the fenestration is closed.) It may
serve as definitive palliation if the Fontan is thought too risky. In this setting and
depending on the specific cyanotic condition, the bi-directional cavo-pulmonary shunt can
provide a 1 _ ventricular repair; increased bilateral pulmonary blood flow is achieved yet
ventricular work and volume is not increased as is true with systemic-pulmonary shunts.
Aortopulmonary collaterals may occur after a bi-directional cavo-pulmonary anastomosis,
providing competitive pulsatile pulmonary blood flow. The prevalence may be as high as 36%
in patients after a bi-directional cavo-pulmonary anastomosis. Most aortopulmonary
collateral vessels originate from the internal mammary artery or thyrocervical trunk. The
significance these vessels and indications for closure are not yet clear.
OUTCOME
Operative mortality
In a series from Toronto the 30-day operative mortality rate after a Glenn shunt was 9.6%.
Death in the perioperative period is most commonly due to SVC syndrome, persistence
cyanosis and cerebral injury. Poorer results are seen in patients with increased pulmonary
vascular resistance, hypoplastic pulmonary arteries, or age younger than 18 months.
Long-term mortality
During follow-up in a series from Toronto the death rate was 20%. The majority of these
deaths were associated with subsequent repair procedures. Complications
Complications in the perioperative period include
- SVC syndrome, with excessive venous pressure from connection of systemic venous return
into the pulmonary circulation causing symptoms.
- cerebral edema, again due to excessive venous pressure. A patent azygous vein would allow
decompression of the elevated pressures in the SVC‹this would, however, decrease flow down
the Glenn shunt, promoting stasis and thrombosis.
- hemothoraces
- chylothoraces
- pulmonary artery thrombosis, possibly with infarction
- sinus node injury with subsequent rhythm disturbance.
- decreased flow down the Glenn shunt leading to thrombosis if bilateral SVCs are present
and the left SVC is not ligated at operation.
Later complications include protein-losing enteropathy (PLE). Higher pressures in the SVC
and thoracic duct can be transmitted to intestinal lymphatics, with gut loss of proteins
and subsequent abnormal fluid homeostasis. PLE is more common after a Fontan operation but
has been reported after a Glenn shunt.
The Glenn shunt can be taken down but with difficulty at subsequent surgery.
Mechanical ventilation in patients with a cavo-pulmonary anastomosis is challenging. Flow
from SVC to left atrium is non-pulsatile, down a pressure gradient. Even small amounts of
positive end-expiratory pressure (PEEP) can decrease substantially the cardiac output. It
is recommended that patients have continuous negative and extra thoracic pressure instead.
Long-term efficacy
Investigators at Yale University continue to define the long-term outcomes of the Glenn
shunt. The Glenn shunt is functional (that is, it provides some form of palliation with or
without further corrective or palliative procedures) at 10 years in 81% of patients, at 20
years in 50% of patients. Only a minority of patients, however, can expect to be free of
further operative procedures to augment pulmonary blood flow by 25 years after the Glenn
shunt.
Investigators at Toronto and elsewhere corroborate that the palliation achieved by the
Glenn shunt alone is adequate for 5-10 years in most patients. Thereafter further
intervention is usually necessary.
The palliation from the Glenn shunt can be augmented by the surgical creation of an
arteriovenous fistula, either axillary or brachial, bolstering the flow down the SVC. The
flow to the contralateral lung can be increased if feasible based on the specific anatomy
of the cyanotic heart condition.
Causes of worsening oxygenation late after a Glenn shunt include:
- decreased flow to the contralateral lung due to progression of the specific cyanotic
pathophysiology (e.g. increasing pulmonary stenosis‹whereas previously anterograde flow
through the pulmonary artery to the contralateral lung provided some pulmonary blood flow).
- intravascular pulmonary thrombosis.
- development of venous collateral flow from the SVC to the IVC resulting in decreased flow
down the Glenn shunt to the ipsilateral lung. If a bi
-directional cavo-pulmonary anastomosis is present, the collateral from the SVC into the
IVC drains into chambers leading to the systemic circulation, resulting in a right to left
shunt. These veno-venous collaterals may occur in up to 33% of patients.
- recanalization of the ligated connection between the SVC and the right atrium‹again
resulting in decreased flow down the Glenn shunt and, if a bi-directional cavo-pulmonary
anastomosis is present, a right to left shunt. This SVC to right atrium communication can
be occluded with an interventional catheterization procedure.
- thrombosis and stricture of the Glenn shunt. Thrombosis and stricture may contribute to
some cases of late cardiac death after the Glenn shunt.
- increased pulmonary vascular resistance--primarily or as a result of increasing blood
viscosity due to hypoxemia from the above causes
- pulmonary arteriovenous malformations (AVMs). Pulmonary AVMs may be the main reason of
late clinical deterioration in patients after the Glenn.
- pulmonary ventilation-perfusion abnormalities. The classic Glenn shunt results in
selective perfusion of the right lower lobe, with flow susceptible to gravitational
effects. The flow in the right upper and right middle lobe is minimal. After a
bi-directional cavo-pulmonary anastomosis blood flows preferentially to both lower lobes.
- SVC drainage as a percentage of systemic venous drainage is inversely proportional to the
age and size of a patient. Somatic growth with a relative decrease in venous return from
the head and upper extremities means the classic Glenn shunt or a bi-directional
cavo-pulmonary anastomosis placed in childhood may fail to provide adequate palliation in
the older patient.
- pulmonary arteriovenous malformations (AVMs)‹see below. AVMs result in a right to left
shunt.
Pulmonary AVMs
The prevalence of pulmonary AVMs increases with time. By 10 years it is 10% in patients
after a classic Glenn shunt. The prevalence as detected by pulmonary angiography rises to
20-25% over time, although the prevalence as detected by saline contrast injection into the
pulmonary arteries may reach 71%. The median time after the cavo-pulmonary anastomosis to
the development of pulmonary AVMs is 3.5 years.
The cause of pulmonary AVMs in patients with a cavo-pulmonary anastomosis is likely the
absence of perfusion of the pulmonary vasculature by hepatic venous return. Similar
pulmonary AVMs, with diffuse dilation of pre-capillary vessels, occur with cirrhotic liver
disease. Supporting the concept that normal hepatic venous return bathing the pulmonary
vasculature is protective against pulmonary AVMs is the resolution of pulmonary AVMs after
a cavo-pulmonary anastomosis if the anastomosis is taken down and the SVC is reconnected to
the right atrium, or if orthotopic heart transplantation is performed. (Pulmonary AVMs in
cirrhotic liver disease reverses after liver transplant.)
Surgical Perspectives:
Glenn Shunt (classic)
This venous shunt cannot be done in the neonatal period because of the relatively high
pulmonary vascular resistance (PVR)in the neonate. Since it is a venous, low pressure
shunt, it will only function adequately with an unobstructed pulmonary artery and low PVR.
The original Glenn shunt was performed through a right thoracotomy through the fourth
intercostal space. The pericardium is opened anterior to the phrenic nerve and the superior
vena cava, azygous vein, and entire right pulmonary artery are dissected. The azygous vein
is ligated and divided to prevent collateral run-off from the SVC. The right pulmonary
artery is transected at its origin and the proximal end oversewn. A side-biting clamp is
placed on the SVC to allow flow to continue into the right atrium from the SVC. The distal
end of the right pulmonary artery is then sewn end-to-side into the SVC. The clamp is
removed, and the final step consists of ligating the SVC at its junction with the RA, so
that now all SVC blood must flow into the pulmonary artery. The classic Glenn shunt is
rarely done today and has been supplanted by the bidirectional Glenn shunt (see below)
Bidirectional Glenn Shunt
This procedure is usually done as the second procedure in a patient with univentricular
heart, after having undergone a Blalock-Taussig shunt or pulmonary artery band as an
infant, usually between 5 to 8 months of age.
Operation is performed through a median sternotomy. The initial part of the procedure often
consists in taking down a previously constructed Blalock-Taussig shunt. A short period of
normothermic cardio-pulmonary bypass is necessary since there will be inadequate pulmonary
blood flow when the original shunt is removed. The SVC is cannulated superiorly, the IVS
and aorta are cannulated routinely. CPB is commenced and the previous shunt is removed. The
SCV is transected at its most proximal end and the cardiac end oversewn. An incision is
made on the superior-anterior aspect of the right pulmonary artery. An end to side
anastomosis is carried out between the distal end of the SVC and the side of the right PA.
Unlike the classic Glenn shunt, the bidirectional Glenn shunt allows for flow to both
lungs, and leaves the pulmonary artery tree intact. This facilitates the subsequent Fontan
operation which requires and intact, unobstructed pulmonary artery tree.
With the SVC as the only source of PBF in a child with normal PVR, oxygen saturations
usually settle in the 80 to 85% range. The volume load on the heart imposed by the
Blalock-Taussig shunt is eliminated allowing for myocardial size to decrease in preparation
for the Fontan operation.