Apicoaortic Conduit
Encyclopedia
Introduction
Apicoaortic Conduit (AAC), also known as Aortic Valve Bypass (AVB), is a cardiothoracic surgical procedure that alleviates symptoms caused by blood flow obstruction from the left ventricleLeft ventricle
The left ventricle is one of four chambers in the human heart. It receives oxygenated blood from the left atrium via the mitral valve, and pumps it into the aorta via the aortic valve.-Shape:...
of the heart. Left ventricular outflow tract obstruction (LVOTO) is caused by narrowing of the aortic valve
Aortic valve
The aortic valve is one of the valves of the heart. It is normally tricuspid , although in 1% of the population it is found to be congenitally bicuspid . It lies between the left ventricle and the aorta....
(aortic stenosis) and other valve disorders. AAC, or AVB, relieves the obstruction to blood flow by adding a bioprosthetic valve to the circulatory system to decrease the load on the aortic valve. When an apicoaortic conduit is implanted, blood continues to flow from the heart through the aortic valve. In addition, blood flow bypasses the native valve and exits the heart through the implanted valved conduit. The procedure is effective at relieving excessive pressure gradient across the natural valve. High pressure gradient across the aortic valve can be congenital or acquired. A reduction in pressure gradient results in relief of symptoms.
Configuration
The figure depicts a typical apicoaortic configuration with a left ventricle connector sutured to the apex of the heartApex of the heart
The apex of the heart is the lowest superficial part of the heart.It is directed downward, forward, and to the left, and is overlapped by the left lung and pleura.-External anatomy:...
, and a conduit containing a bioprosthetic valve anastomosed to the descending thoracic aorta
Thoracic aorta
The thoracic aorta is contained in the posterior mediastinal cavity.It begins at the lower border of the fourth thoracic vertebra where it is continuous with the aortic arch, and ends in front of the lower border of the twelfth thoracic vertebra, at the aortic hiatus in the diaphragm where it...
. Blood exits the left ventricle
Left ventricle
The left ventricle is one of four chambers in the human heart. It receives oxygenated blood from the left atrium via the mitral valve, and pumps it into the aorta via the aortic valve.-Shape:...
either through the natural valve or the bypass conduit.
History
The concept of an apicoaortic conduit to bypass valvular aortic stenosis (AS) was conceived by Carrel in 1910, and performed experimentally by Sarnoff and colleagues on dogs in 1955. In 1962-63, Templeton implanted prostheses similar to those originally described by Sarnoff in five patients with severe aortic valve stenosis; one patient survived more than 10 years. In 1975, Bernhard and coworkers reported a reoperation in which a conduit was implanted between the left ventricle and the thoracic aorta. The procedure was also developed in the late 1970’s as a way to treat complex left ventricular outflow tract obstructions (LVOTO) in children and young adults. AAC is no longer performed on children as the Ross procedureRoss procedure
The Ross procedure is a cardiac surgery operation where a diseased aortic valve is replaced with the person's own pulmonary valve. A pulmonary allograft is then used to replace the patient's own pulmonary valve...
has superseded it.
Today, AAC is performed clinically on elderly aortic stenosis patients, and has gained in popularity in recent years. In the past 25 years, a number of case series on adults have been published. As of 2010, the estimated total number of AAC cases performed worldwide in the last 30 years is greater than 1500. The procedure has not been adopted widely because it is technically challenging and blood loss can be significant. The most difficult part of the procedure is the insertion of a left ventricle connector into the apex of the heart
Apex of the heart
The apex of the heart is the lowest superficial part of the heart.It is directed downward, forward, and to the left, and is overlapped by the left lung and pleura.-External anatomy:...
.
Devices
AAC requires specialized implants and installation tools. Hancock Laboratories, now part of Medtronic (Minneapolis, MN), developed and released a set of left ventricle connectors, valved conduits, and installation trocarTrocar
A trocar is a medical instrument with a sharply pointed end, often three-sided, that is used inside a hollow cylinder to introduce this into blood vessels or body cavities...
s in the 1970s. The Hancock left ventricle connectors, with inner diameters of 12 to 22 mm, have been used clinically for the vast majority of AAC procedures reported in the literature. For the valved conduit, surgeons have used the Medtronic Hancock Model 105 or 150 valved conduits which contain a complete porcine aortic valve. Other surgeons have constructed valved conduits on the back table using a variety of bioprosthetic valves such as the Medtronic Freestyle valve. The Hancock trocars for creating a hole in the left ventricle are rarely used clinically. Surgeons typically assemble their own tools to core and remove a plug of muscle from the left ventricular wall.
In 2011, Correx (Waltham, MA) released a complete kit for AVB with an installation tool that enables coring and insertion of a left ventricle connector on a beating heart while maintaining hemostasis
Hemostasis
Hemostasis or haemostasis is a process which causes bleeding to stop, meaning to keep blood within a damaged blood vessel . Most of the time this includes blood changing from a liquid to a solid state. Intact blood vessels are central to moderating blood's tendency to clot...
. Cardiopulmonary bypass (CPB) is not required. The kit is CE Marked and available in Europe. Several cases have been done in Canada under the Special Access Programme. This kit is not currently available for use or sale in the United States.
Procedure
The general procedure has evolved over the years as surgeons gained experience and improved techniques. A video of an AVB procedure performed by Dr. Jim Gammie of the University of Maryland Medical Center can be found here.Preparation
The patient is placed on the table in the supine position. Anesthesia is induced, and the patient is intubated with a double-lumen endotracheal tube, which facilitates one-lung ventilation and allows the surgeon to work within the left chest. The patient is positioned with his or her left side up (see figure). A left anterior-lateral thoracotomyThoracotomy
Thoracotomy is an incision into the pleural space of the chest. It is performed by a surgeon, and, rarely, by emergency physicians, to gain access to the thoracic organs, most commonly the heart, the lungs, the esophagus or thoracic aorta, or for access to the anterior spine such as is necessary...
is performed between the fifth and sixth ribs to expose the heart and provide access to the descending aorta
Descending aorta
The descending aorta is part of the aorta, the largest artery in the body. The descending aorta is the part of the aorta beginning at the aortic arch that runs down through the chest and abdomen. The descending aorta is divided into two portions, the thoracic and abdominal, in correspondence with...
. A portion of a rib may be removed to enable easier access and to minimize the chance of a rib fracture. The left lung is deflated and retracted.
Distal Anastomosis
With the descending aorta exposed, the surgeon applies a side clamp to the aorta, effectively pinching and isolating one side of the great vessel. Blood continues to flow to the body’s lower half through the unisolated remainder of the aorta. A lengthwise slit is made through the pinched face, and the open end of the valved conduit is carefully sutured to the slit. Once the suture line is checked for leaks, the aortic partial side clamp is removed and the valved conduit fills with blood up to the bioprosthetic valve. The valve acts as a check valve, thereby maintaining hemostasis.Left Ventricle Connector Installation
The surgeon next opens the pericardiumPericardium
The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...
and exposes the apex of the heart
Apex of the heart
The apex of the heart is the lowest superficial part of the heart.It is directed downward, forward, and to the left, and is overlapped by the left lung and pleura.-External anatomy:...
. The surgeon closely examines the surface of the heart and chooses and marks an insertion site near the apex. A ring of sutures reinforced with pledgets is installed around the insertion site. The free ends of the sutures are looped through a sewing ring on the left ventricle connector. At this point, CPB is typically initiated, although a few surgeons are able to complete the left ventricle installation off-pump (without CPB).
A circular core of myocardium is cut and removed using specialized coring tools. Care must be taken to avoid damaging structures inside the left ventricle such as the interventricular septum
Interventricular septum
Interventricular septum , abbreviated IVS, is the stout wall separating the lower chambers of the heart from one another....
, and the chordae tendineae
Chordae tendineae
The chordae tendineae, or heart strings, are cord-like tendons that connect the papillary muscles to the tricuspid valve and the mitral valve in the heart....
. The left ventricle connector is filled with sterile saline to displace any air and then inserted into the hole. The sutures are sequentially tightened and tied off to permanently secure the left ventricle connector to the apex. CPB is terminated.
With blood now flowing through the bypass conduit, the left ventricular outflow tract obstruction (LVOTO) is effectively relieved. 60% to 70% of the heart’s output flows through the bypass to the aorta. The load on the heart is markedly reduced, and symptoms such as dyspnea
Dyspnea
Dyspnea , shortness of breath , or air hunger, is the subjective symptom of breathlessness.It is a normal symptom of heavy exertion but becomes pathological if it occurs in unexpected situations...
, syncope
Syncope
In phonology, syncope is the loss of one or more sounds from the interior of a word; especially, the loss of an unstressed vowel. It is found bothin Synchronic analysis of languages and Diachronics .-Found synchronically:...
, and chest pain disappear.
Outcomes
Patients historically referred for apicoaortic conduit surgery have been considered high risk or ineligible for conventional aortic valve replacementAortic valve replacement
Aortic valve replacement is a cardiac surgery procedure in which a patient's failing aortic valve is replaced with an alternate healthy valve. The aortic valve can be affected by a range of diseases; the valve can either become leaky or partially blocked...
. The literature reports generally favorable outcomes for apicoaortic surgery given the age, frailty, and comorbidities of the patient population. Patients typically improve from New York Heart Association Functional Classification
New York Heart Association Functional Classification
The New York Heart Association Functional Classification provides a simple way of classifying the extent of heart failure. It places patients in one of four categories based on how much they are limited during physical activity; the limitations/symptoms are in regards to normal breathing and...
III or IV to Class I or II. AVB effectively relieves the LVOTO caused by aortic stenosis.
There is long-term stability of the left ventricular hemodynamics
Hemodynamics
Hemodynamics, meaning literally "blood movement" is the study of blood flow or the circulation.All animal cells require oxygen for the conversion of carbohydrates, fats and proteins into carbon dioxide , water and energy in a process known as aerobic respiration...
after AVB, with no further biologic progression of native aortic valve stenosis. Once the pressure gradient across the native valve is substantially reduced, the narrowing and calcification of the native valve halts. In one study of 47 patients whose average age was 82 years, the mean gradient across the native valve was reduced from 46 +/- 14 mm HG to 17 +/- 7 mm HG.
Low Incidence of Stroke
Throughout the published literature, perioperative strokeStroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
is uncommon. This is because the native valve and ascending aorta are not manipulated in any way. Calcified layers and other debris do not break free and migrate to the brain.
The literature also reports that postoperative stroke is rare. Whenever a prosthetic device such as a valve is introduced into the bloodstream, there is an ongoing risk of thrombus
Thrombus
A thrombus , or blood clot, is the final product of the blood coagulation step in hemostasis. It is achieved via the aggregation of platelets that form a platelet plug, and the activation of the humoral coagulation system...
formation which can lead to an embolism
Embolism
In medicine, an embolism is the event of lodging of an embolus into a narrow capillary vessel of an arterial bed which causes a blockage in a distant part of the body.Embolization is...
should the thrombus become mobile. Computational flow studies conclude that the presence of an aortic valve bypass conduit has no effect on cerebral blood flow. All blood flow to the brain continues to flow across the native valve. Should a thrombus form on the bioprosthetic valve in the bypass conduit, it will likely migrate to the lower half of the body and not to the brain. This fact is believed to reduce the long-term risk of cerebral thromboembolism (stroke) with AAC.
Need for Pacemakers is Uncommon
The published literature reports that the need for pacemaker after AAC is very rare as the (electrical) conduction center of the heart near the aortic valve is not disturbed in any way.Paravalvular Leak is Impossible
AAC avoids the risk of leakage around a replacement heart valve (aortic insufficiencyAortic insufficiency
Aortic insufficiency , also known as aortic regurgitation , is the leaking of the aortic valve of the heart that causes blood to flow in the reverse direction during ventricular diastole, from the aorta into the left ventricle....
), a serious complication of alternate treatments such as aortic valve replacement
Aortic valve replacement
Aortic valve replacement is a cardiac surgery procedure in which a patient's failing aortic valve is replaced with an alternate healthy valve. The aortic valve can be affected by a range of diseases; the valve can either become leaky or partially blocked...
(AVR) and percutaneous aortic valve replacement
Percutaneous aortic valve replacement
In Percutaneous aortic valve replacement or Transcatheter Aortic Valve Implantation , a replacement valve is passed through a hole in the groin by a puncture of the femoral artery and advanced up to the ascending aorta of the patient. It substitutes for a more invasive procedure in which the chest...
(PAVR or TAVI). With AAC, the native aortic valve is left completely undisturbed.
Patients receiving AAC
With aortic valve replacementAortic valve replacement
Aortic valve replacement is a cardiac surgery procedure in which a patient's failing aortic valve is replaced with an alternate healthy valve. The aortic valve can be affected by a range of diseases; the valve can either become leaky or partially blocked...
(AVR) as the accepted standard of care for critical AS, patients receiving apicoaortic conduits have often been refused AVR and as such are considered “no option” patients. AAC is also sometimes chosen by surgeons and cardiologists over other treatments, such as AVR and percutaneous aortic valve replacement
Percutaneous aortic valve replacement
In Percutaneous aortic valve replacement or Transcatheter Aortic Valve Implantation , a replacement valve is passed through a hole in the groin by a puncture of the femoral artery and advanced up to the ascending aorta of the patient. It substitutes for a more invasive procedure in which the chest...
(PAVR or TAVI), to treat critical AS. Patients who could benefit from AAC or AVB include those who:
- Have had coronary artery bypass surgeryCoronary artery bypass surgeryCoronary artery bypass surgery, also coronary artery bypass graft surgery, and colloquially heart bypass or bypass surgery is a surgical procedure performed to relieve angina and reduce the risk of death from coronary artery disease...
and have patent grafts - Have had mediastinal radiation or infection after a prior median sternotomy
- Have elevated risk of stroke, or history of stroke
- Have heavily calcified, or “porcelain” ascending aortas
- Have large or small aortic annular diameters
- Have low coronary ostia (these are the inlets to the left and right coronary arteries just above the aortic valve)
- Have heart (electrical) conduction disorders