Section 20: Conduit for Coronary Bypass Grafting

Selection, Evaluation, and Harvesting

Kerry Bryant, PA-C

 

The goal of coronary artery bypass grafting is to provide long-term patency in the native coronary arterial system.  Proper conduit selection is critical to the success of a coronary graft and is associated with an uneventful post-operative course and better long-term survival.

    

Indications for surgical coronary revascularization are most often unstable angina despite medical therapy and prolongation of life in high-risk patients (left main disease, triple and double vessel coronary artery disease, or complex LAD stenosis not amenable to percutaneous coronary angioplasty).  The conduit to be selected in coronary bypass grafting depends in part on the site of native vessel obstruction, the clinical condition of the patient, as well as the quality (and quantity) of available autologous conduit.

    

There is recently an overwhelming enthusiasm for the use of catheter-based coronary revascularization procedures (PTCA, coronary atherectomy, laser coronary angioplasty, and coronary stents).  These procedures have shown to be effective in highly selected group of patients.  However, the high incidence of restenosis in coronary arteries after percutaneous intervention continues to be a major obstacle in the routine use of these modalities.  Surgical revascularization has been more effective in relieving angina and providing better longevity.1 The success of the coronary artery bypass grafting procedures is dependent upon the proper selection of the conduit and it’s quality.

    

Moreover, the steady increase in coronary bypass grafting reoperations continues to challenge cardiac surgeons and physician assistants to search for new and alternative conduit material.2  Previous myocardial revascularization, peripheral arterial reconstruction (i.e.: femoral-popliteal bypass), varicose vein ligation procedures are a few circumstances which necessitate the use of alternative conduit due to absent or unsuitable autologous conduit.1  

    

The goal of this unit will be to provide the physician assistant resident in cardiac surgery an overall summary of conduit options used in surgical coronary revascularization including: indications and contraindications for conduit use, patency rates, pre and intra-operative assessment for conduit quality, harvesting techniques based on anatomic landmarks, and complications associated with the harvesting of various conduits.

    

Historically, venous conduits, specifically the greater saphenous vein (GSV), are the most frequently used coronary conduits since the beginning of coronary bypass surgery.

 

Venous conduits include:

     Autologous:          Greater Saphenous Vein

                                   Lesser (Short) Saphenous Vein

                                   Upper Extremity Vein (cephalic or basilic)

     Non-Autologous:  Greater Saphenous Vein (cadaveric, homografts)

                                    Umbilical Vein

 

Due to the high patency rates associated with internal mammary artery use, the efficacy of other arterial conduits in coronary artery bypass surgery has been examined.

 

Arterial conduits include:

 

          Autologous:          Internal Thoracic (Mammary) Artery

                                        Right Gastroepiploic Artery

                                        Inferior Epigastric Artery

                                        Radial Artery

                                        Splenic Artery

                                        Gastroduodenal Artery

                                        Left Gastric Artery

                                        Intercostal Artery

 

          Non-Autologous:  Bovine Internal Thoracic (Mammary) Artery

 

 

 

Greater Saphenous Vein

The Greater Saphenous Vein is the most frequently used venous conduit.  It can be removed from the lower leg and/or thigh and can be tailored to varying lengths as a graft to fit anywhere across the epicardium.1   It is especially useful in difficult situations: distal LAD, massive ventricular hypertrophy, and inadequate internal mammary artery.3 

   

Contraindications to the use of the Greater Saphenous vein use include: very large varicosities and severe coronary-graft size mismatch.

    

The one-year patency rates for aortocoronary GSV grafts are between 80-90%.  With the advantage of long-term follow-up of patients who have undergone CABG with GSV grafts, an important limitation to recognize is their late degeneration.  Diffuse intimal thickening (fibroplasias or hyperplasia) is found in most grafts.  However, late occlusion is usually related to graft atherosclerosis.  Studies have shown that approximately 50% vein grafts closed 10 years post-operatively and of those patent, 50% of them demonstrated atherosclerotic wall changes.1

     

A retrospective study of 723 patients by Lytle and associates 2 showed that later vein graft stenosis carries a greater risk than native coronary artery stenosis, especially when the LAD artery is involved.

    

Studies have also shown that, ”in response to injury, normal endothelium promotes platelet aggregation and coagulation.  Vasospasm, occlusive intimal hyperplasia, and accelerated atherosclerosis can result from endothelial injury during graft harvest”.2  This has led to the growing trend toward minimally invasive or modified minimally invasive techniques for GSV harvesting.

    

The physician assistant usually performs greater saphenous vein harvesting while the surgeon is completing the sternotomy and harvesting the internal mammary artery.   Pre-operative assessment should include an examination of the patient’s legs for gross, deep varicosities, vein ligation procedure scars, and palpable flow.  If a question of vein integrity is an issue, ultrasonic vein mapping can be performed.  Optimal vessel diameter measurements should be uniform without varicosities and between 2-4mm in diameter.  The legs are positioned in the frog-legged position in order to externally rotate the legs and expose the landmarks for saphenous vein harvest.

 

The distal landmark is the superior edge of the medial malleolus and follows the medial edge of the tibia.  At the knee, it is approx 1 fingerbreadth medial to the tibia below the knee crease and approximately 2 fingerbreadths below the medial femoral head.  The saphenous vein can be found proximally approximately 2 fingerbreadths medial to the femoral pulse and 2-3 finger-width distal to that point.  An incision is made along these landmarks, and dissection is performed with scissors to divide connective tissue and fat to expose the vein.

 

Refer to the Human Anatomy book in LaWaun and Diane’s office:

     p. 292 Figure A (femoral triangle)

     p. 301 Figure D. (GSV at the knee)

     p. 318 Figure A. (GSV at ankle)

     p. 316 Figure B  (GSV at ankle)

 

 

 

The vein is prepared for bypass by cannulating the distal end and distending it with saline.

 

Branches are carefully tied with 4-0 silk ties.  The vein is assessed for consistency,

 

uniform diameter, wall thickness, varicosities, and flow.

 

 

 

 

 

 

 

One (modified) minimally invasive technique for GSV harvest is a skin bridging technique.  The patient is positioned in the supine position with standard prepping and draping of the (frog-legged) lower extremities.  A small (2-4cm) incision is made either in the thigh or in the calf.  The vein is then found under direct visualization.  A retractor (i.e.: Army-Navy, Richardson, vein retractor, deever) is then inserted anterior to the vein and used to create a tunnel.  Branches are then clipped and ligated under direct vision.  This process is repeated at the other end (distal or proximal) to the incision.  A counter-incision is then made a few centimeters beyond the tip of the retractor and the same steps are repeated with the retractor used to connect the two tunnels, thereby completing the dissection.  The vein can be ligated proximally and distally.  Two to three of these incisions can be made above and below the knee in order to harvest the GSV from just distal to the saphenofemoral junction to just above the medial malleolus.  This method of vein procurement provides limited skin incisions, improved cosmesis, and fewer wound complications without additional cost to the patient.5 

 

 

 

Endoscopic Vein Harvesting:  There exists numerous devices and equipment which have evolved over time in order to improve the efficacy of endoscopic vein harvesting.  Harvesting vein endoscopically has become standard in some institutions. The initial incision is made either above or below the knee crease carefully to avoid crossing the knee crease.  The incision is only 2-3 cm in length, with just enough space to pass the instruments through, but not too large to allow carbon dioxide to escape.  The vein is identified and dissected with the help of retractors both proximally and distally from the incision.  When a tunnel is created the scope is passed along the plane of the vein.  This is usually done proximally in the thigh, but may also be passed distally in the lower leg, depending on how much length of vein is required for adequate revascularization.   Passing the dissector around the vein while viewing the monitor performs dissection of the surrounding tissue.  A balloon is inflated to ensure an adequate seal in the tunnel that is created by the dissection.  Carbon dioxide is then used to insufflate the tunnel, thereby maximizing exposure and ease of harvest.  When the dissection is complete, another instrument is then passed through the port, either a bisector or a scissor to be used together with a “C-arm” to divide the vein branches.  Then, a counter (stab) incision is made at the proximal (or distal) end of the tunnel and a clamp is used to grab the vein and pull it though the incision.  The vein is ligated and pulled through the initial 2 cm incision and prepared for use as coronary conduit.     

     

Complication rates from GSV harvest vary, but can be a source of morbidity, increased length of stay, and thus, increased cost.  The potential benefits from the minimally invasive harvesting techniques include: decreased wound complication rates, decreased lower extremity pain and discomfort, earlier ambulation, improved cosmesis, and overall better patient satisfaction.

 

Lesser Saphenous Vein

Lesser saphenous vein grafts are a good alternative to greater saphenous vein grafts.  When GSV has been previously removed secondary to prior coronary artery bypass, revascularization, or vein ligation procedures, the lesser (short) saphenous vein can prove to be sufficient conduit material.  Because of its short length, it is often necessary to harvest the lesser saphenous vein bilaterally.  Circumferential prepping and exposure to the posterior calf can make the harvest of the lesser saphenous vein difficult.  The physician assistant must be careful to avoid injury to the sural and posterior cutaneous nerves during dissection.1  The patency rate of LSV grafts is 60% at 3 years.10

 

Please refer to the Human Anatomy book in LaWaun and Diane’s office:

     p. 311 Figure C (proximal LSV)

     p. 312 Figure B (distal LSV)    

 

    

 

Upper Extremity Veins

Upper extremity veins are used less often than greater saphenous or lesser saphenous veins.  The cephalic and basilic veins are of adequate length for bypass, however, they are generally thin-walled and more prone to kinking, twisting, and aneurysmal dilatation.  Harvest of the arm veins can be more tedious.  It is easier to traumatize the thin-walled vein when handling it during dissection.  Jarvin et al4 have shown upper extremity vein graft patency of 87% at a mean of 15 months follow-up in 15 patients.  Prieto et al5 demonstrated a 90% patency in 13 patients who had angiography less than 9 months post-operatively and a patency rate of 63% in those seen after one year (mean=42 months).  Stoney et al6 showed a 57% patency in 28 patients at two years post coronary artery bypass.  Siefert et al7 demonstrated a patency rate of 66% in 17 patients at a mean follow-up of 8 months.  Overall, upper extremity vein grafts have a lower patency rate when compared with greater saphenous vein grafts or internal mammary grafts.  Patency rates have been reported as low as 57% at two years and 47% at 4.6 years.10

 

Please refer to the Human Anatomy book in LaWaun and Diane’s office:

     p. 133 Figure E

 

 

Internal Mammary Artery

As early as 1972, preliminary studies reported that internal mammary artery (IMA) coronary anastomoses were superior to those of saphenous vein grafts.  Later studies confirmed this in 1983.  IMA grafts have since become the backbone of coronary artery bypass surgery. Although better than synthetic grafts, early and late failure of vein grafts did occur with increasing frequency as time progressed.  Szylagyi4 and colleagues believed the most important factor in success of the coronary revascularization to be the quality of the vein itself.  Experiments demonstrated that vein grafts were more vulnerable to atherosclerosis than pedicled IMA grafts.  It was thought that early failure of saphenous vein grafts were caused by subintimal hyperplasia.  As efforts were made toward more suitable matching of the size of the vein graft to the size of the artery, the incidence of hyperplasia in vein grafts diminished.  There was a continued trend toward the recurrence of angina one year post-operatively, which was shown angiographically to be due to vein grafts failure.  Failure of vein grafts increased in frequency beyond five years post-operatively secondary to accelerated atherosclerosis.

    

Impairment of vasa vasorum in vein grafts creates atherosclerotic lesions.  The disruption of the vasa vasorum leaves the walls of the veins without capillary flow for 72 hours.  Approximately six weeks are needed for maximal return of arteriolar flow.  This flow, regardless of placement does not come from it’s own lumen; it comes from neighboring arteries.6  Deficiencies of vasa vasorum preclude the development of atherosclerosis.  Grondin9 reported at 10 years, only 21% of SVG’s showed no adverse changes, whereas 95% of IMA pedicled grafts showed no adverse changes. 

    

The IMA is somewhat resistant to atherosclerotic changes because of the thinness of the wall (also, ironically, a drawback to it’s surgical use), and because the pedicled graft carries it’s “environment “ with it. When taken as a pedicle, neural innervation, venous and lymphatic drainage remains intact.1 The IMA graft will enlarge in response to flow demand as long as the anastomosis is not restrictive.6

    

In the past two decades, several studies confirmed that the patency rate for IMA grafts results in an improvement in survival of 10-30% and a lower incidence of cardiac events when compared with coronary artery bypass with SVG’s only.  There is evidence that for those patients with diffuse coronary artery disease, the single most important survival predictor is an IMA bypassed to the left anterior descending artery.  The use of an IMA graft was consistently associated with better survival rates, regardless of age, sex, percent stenosis, left main disease, or left ventricular function. 

    

The New England Journal of Medicine published a study looking at the effect of IMA grafts on ten-year survival.  In patients who have coronary artery bypass, the operative technique and the pathology of the grafted coronary artery are the limited determinants of graft patency.  However, after the fifth year post-operatively, the major determinant of long-term patency is the type of conduit.  Because the saphenous vein is prone to intimal proliferation, angiographic studies were performed to look at patency.  A 2% per year vein graft attrition rate from years 1-7 post-operatively increases to 5% per year from years 7-12.  At the end of the tenth year only 38-45% vein grafts are open.  Similar studies showed a patency rate of 85-95% at 7-10 years post-operatively with IMA grafts.  Patients with SVG’s were found to have 1.61 times greater risk of death, 1.4 times greater risk of late myocardial infarction, two times the risk for re-operations, and 1.27 times risk of later cardiac events than those with IMA grafts.11 

    

IMA grafts today are essentially used on almost all patients.  Exceptions include:  radiation-induced atherosclerosis  (breast cancer), atherosclerosis involving innominate, carotid, or subclavian arteries, or re-operation with patent, large diameter SVG (a smaller IMA may result in hypotension), and a dissected internal mammary artery.  Hypoperfusion may be a result of: (1) graft-recipient artery mismatch (2) high dose vasoconstrictors (3) subclavian steal (proximal occlusion of subclavian causes retrograde flow from coronary circulation through the IMA, “stealing” blood from the left coronary system.7

    

Studies have shown similar, excellent results when looking at bilateral, free, sequential, and “T” or “Y” grafts with other arterial grafts.7  Windler et al8 looked at patency and flow rates of free right IMA – left IMA “T” grafts and radial artery – left IMA “T” grafts with a mean of 4 and 4.3 anastomoses respectively.  One week, six months post-operative flow was measured in the proximal left IMA with a Doppler guide wire.  There was no significant difference between baseline flow, maximum flow and coronary reserve flow.  Flow reserve for the left IMA is adequate for multiple (sequential) anastomoses regardless of the choice of the second arterial graft.8  Free IMA grafts are typically reserved for short pedicled IMA’s (i.e.: distal LAD) or as a result of accidental division during harvesting.1

Another study looked at the benefit of using bilateral IMA grafts.  Fiore et al 9 compared a series of 100 patient operated on between 1972 and 1975 who had bilateral IMA grafts as well as SVG’s with a similar group of 100 patients who had left IMA grafts as well as SVG’s.  The two groups were similar with respect to age, sex, risk factors, left ventricular function, number of grafts, and extent of coronary disease.  Single IMA graft mortality was 2% and double IMA graft mortality was 9%.  Mean follow-up was 14.4  (+/- 2.7 years) with all but 7 patients having follow-up at 10 years.  At 13 years, patency of right IMA was 85% and left IMA was 82%.  The study suggested a survival benefit with the use of double IMA grafts (74% vs. 59%; p=0.05).  Furthermore, patients with double IMA grafts had fewer occurrences of myocardial infarction (75% vs. 59%; p<0.025), recurrent angina (36% vs. 27% p<0.025), and total ischemic events (37% vs. 18%; p<0.001), as well as less need for percutaneous intervention and reoperation.9  Indications for the use of bilateral IMA grafts include patients of younger age (<60 years old), unsuitable saphenous vein, and severe ascending aortic atherosclerosis in order to avoid a proximal anastomosis. 1 Contraindication to bilateral IMA grafts is generally reserved for obese or brittle diabetic patients in whom sternal infections may become an issue in healing.

   

The internal mammary (or internal thoracic) artery originates from the proximal subclavian artery and gives off a pericardiophrenic artery, mediastinal branches, anterior intercostals arteries, and perforating branches.  It divides and terminates at the musculophrenic artery and superior epigastric artery.  Harvesting technique involves taking it as a muscular pedicle.

 

 

 

 

 

 

 

 

 

 

 

Overall graft patency of a left IMA – LAD anastomosis has been consistently over 90% at ten years follow-up.1

   

Pre-operative assessment of the IMA includes a detailed physical exam and chart review.  Has the patient had a documented history of subclavian stenosis or chest wall radiation?  Did you listen for carotid and subclavian bruits?  Formal arterial studies with color flow ultrasound will help determine if the IMA is useable and if there is any question of the flow to the artery.  Intra-operatively, a sterile Doppler may be used to assess flow through the artery.

Radial Artery

As a result of the high patency rates associated with the use of IMA grafts, the efficacy of other arterial conduits has been examined. In 1973, Carpentier actually demonstrated very good results with patency rates of 90% at 1 and 10 months post-operatively.  The success of the radial artery use was attributed to the caliber of the radial artery, the absence of atherosclerotic disease, and the character of the vessel wall, which is designed for arterial flow1.  Despite these initial promising results, subsequent reports of radial artery use showed very poor results.  The failure of these grafts was attributed to the number of fenestrations in the internal elastic membrane and a thick media, the possibility of injury to the graft during harvest, and intimal injury were all entertained as possible causes for early graft failure within the first few months.  Acar and associates followed a patient with a successful radial artery graft for 15 years.  They began using radial artery grafts again in 1989 and followed 122 grafts in 104 patients.  All of these patients received diltiazem as an antispasmodic intra and post-operatively.  These patients also received aspirin following surgery.  Fifty patients showed patent grafts two weeks post-op by angiography.  Late studies performed at a mean of 9.2 months after surgery demonstrated a patency rate of 93.5%.  They attributed the poor results initially to the extreme sensitivity to mechanical handling and vasoconstriction by using blood and paparverine to prepare it for use as a bypass graft. 2

    

Advantages of radial artery use as a bypass conduit include: (1) it can be dissected simultaneously with other conduit harvesting (2) its long length can allow for grafting of all vessels (3) its diameter matches the diameter of most coronary vessels, and (4) its thicker wall makes it relatively easy to handle14. 

    

Disadvantages of radial artery use include its tendency to spasm due to its thick, muscular microscopic structure and its high sensitivity to mechanical stimuli.  In addition, there exists a potential complication with regards to the vascular supply of the forearm and hand. 

    

The forearm and the hand are primarily supplied by the ulnar artery and its collateral branches.  Pre-operative assessment of collateral circulation in the hand involves performing an Allen test.  A 5 second or greater delay in refill with radial artery compression suggests inadequate collateral flow. (Ideal delay in refill is less than 3 seconds).  This test, which is very subjective and yields too many false positive and false negative results can be modified.  A modified Allen test is performed in the same manner except with a pulse oximeter monitor tested on the thumb and then on the index finger.  This method of plethysmography is used to evaluate blood flow through the superficial palmar arch.  With radial artery compression, the waveform should return within a few seconds (<5seconds).  (Of note: Some of the computer monitors will automatically recalibrate a low amplitude after a few seconds).  Another noninvasive study of the radial artery to prevent ischemic complications of the hand is Doppler flow measurements.  Blood flow velocities are measured at the subclavian, axillary, brachial, radial, and ulnar arteries as well as digital pressures.  Measurements are made with and without radial artery compression.  A reduction in digital arterial pressure of >40 mmHg reflects a clinically significant change in pressure 15.   One study proposed that an increase in systolic-diastolic velocities in the ulnar artery after radial artery compression suggests a good compliance of the artery, to accommodate the entire blood flow from the brachial artery.  The presence of reverse flow at the superficial palmar branch indicates a communicating palmar arch16. 

    

Post-operative complications of radial artery harvest such as ischemia, dysesthesia, or compartment syndrome can be avoided if proper attention is concentrated on: anatomic landmarks during dissection, including the protection of sensory innervation and control of the tiny branches of the radial artery.  The mobile wad of three, the biceps tendon, the radial styloid (and pulse), and the tendon of the flexor carpi radialis muscle are the superficial landmarks that define the skin incision.

 

 

 

A linear incision is made a few fingerbreadths from the wrist and elbow crease.  The fascia over the mobile wad of three is divided between the brachioradialis and flexor carpi radialis muscles.  Special attention is made in order to avoid dividing the lateral antebrachial cutaneous nerve (usually can be kept to the lateral side of the fascial division).

 

 

Retraction of the brachioradialis and flexor carpi radialis muscles reveals the radial artery17.  The radial artery is dissected as a pedicle including its two surrounding veins and some fat tissue18.  Arterial and venous branches are divided by either using hemoclips or by using an ultrasonic dissector (Harmonic scalpel; Ethicon).

One study demonstrated less conduit spasm when using this ultrasonic dissection technique than with hemoclip application19.  When dissection of the radial artery is completed, vascular clamps are applied at the proximal and distal ends.  The radial artery and surrounding veins are divided and a 5-0-prolene suture is sewn around the proximal and distal stumps of the radial artery.  A 4-0 silk tie can be tied around the stump including the surrounding vein stumps gently in order to avoid necrosis of the vessels.  Collateral circulation can be confirmed by observing the pulsating stumps and by the presence of backflow with the initial release of the vascular clamp.  Paparverine is then injected into the radial artery and left to soak in the paparverine solution until it is needed for proximal anastomosis to the internal mammary artery17. 

 

Wound closure is performed in three layers after adequate hemostasis is achieved.  There is a greater risk of compartment syndrome in the harvest of the radial artery then in the harvest of the greater saphenous vein.  A hemovac drain is placed prior to wound closure.  The deep fascia is closed with a few interrupted sutures using 4-0 vicryl sutures.  Running stitches are then used to close the subcuticular layer and the skin.

 

Newer methods of radial artery harvest are emerging, including endoscopic techniques.  This technique involves two small (2cm) incisions proximal to the wrist and distal to the elbow.  A tourniquet is applied to the upper extremity to achieve a bloodless field20. 

 

Right Gastroepiploic Artery

The first report of right gastroepiploic artery (RGEA) use was in 1973.  However, it was not widely used until the late 1980’s.   Short-term patency results have been excellent.  Right gastroepiploic artery used both as a free and pedicled graft was used mostly for grafts to the right coronary system, although sometimes used for left coronary system.

 

Studies of 200 patients, (16 re-operations) showed early angiographic patency rates (152 cases) of 95% and 40 angiographic studies were performed at 1-5 years (mean=2) also demonstrated a 95% patency rate.  Even though the RGEA appeared to be of adequate size when compared with the target artery, it’s flow and dimensions were not as consistent as those of the internal mammary artery.  A RGEA of small dimension and reduced flow should be used as a free graft, which will provide for a larger diameter at the distal anastomosis.  Spasm has shown to be a more frequent, more intense problem with RGEA than with the IMA.  In addition, its response to chemical stimulators also varies. 2   A comparative study of blood flow characteristics in RGEA and saphenous vein conduits showed that there were no flow differences between the right gastroepiploic artery and saphenous vein grafts when implanted into the same coronary bed in comparable groups of patients.  Waveform shape of the RGEA grafts was characterized by markedly higher velocity curves12.  RGEA grafts can be reserved for patients with no other available conduit, young patients in whom an all arterial operation (with a totally occluded right coronary artery - to avoid competitive flow) is desired, or in diabetic patients in whom one would want to avoid the use of bilateral IMAs13.

 

Inferior Epigastric Artery

In response to the search for additional arterial conduit, the use of the inferior epigastric artery (IEA) has been examined.  Comparisons between intra-operative findings and pre-operative duplex scanning showed that diameter measurements were quite accurate when compared with surgical findings.  The study concluded that a length of 5cm and a diameter of 2mm predict a usable inferior epigastric artery graft.  Limitations to the use of an IEA graft include: increased time of surgery (poor visualization, of the origin of the artery, and difficulty following it distally into the rectus muscle) as well as a higher than average wound complication rate.  Studies of 19 patients showed a 10-day angiographic 97% patency rate and an 89% patency rate at 6 months.  More long-term follow up data is needed to determine the future role of IEA grafts in coronary artery bypass surgery2. 

 

Additional Arterial Conduits

Studies involving the use of the splenic artery as a bypass conduit actually showed a 90% patency rate a 1-2 years post-operatively.  However, the because of its technical difficulty and high rate of atherosclerotic changes (up to 42% at autopsy), popularity of splenic artery use is very low.

    

Left gastric and gastroduodenal arteries have also been considered, but are infrequently used due to their consistently small caliber and short length. 

 

  

 

 

Histologically studies of the intercostal arteries show favorable histologic properties when compared with internal mammary arteries, but have no clinical use as thus far1.

 

Non-Autologous Conduits

Venous Conduits

Umbilical vein: Glutaraldehyde preserved umbilical vein grafts are suited to withstand arterial pressure because of their circumferential elasticity.  They are readily available and easily stored.  However, certain disadvantages limit the use of the graft in coronary bypass.  Because the graft is stored in glutaraldehyde (altering antigenicity) and is a foreign material, it is potentially immunogenic.  Intimal dissection occurs easily.  Patency is low: 50% short-term patency. 

 

Greater Saphenous Vein Homografts:  Good caliber and availability make GSV homografts an appealing conduit.  However, ABO compatibility affects patency in fresh grafts.  Treatment with glutaraldehyde reduces antigenicity, but makes the vessel unusable.  Antigenicity is less in cryopreserved homografts, but graft failure is higher due to deteriorated intima.  Patency rate studies range from 65% at 2 weeks, 41% at 7.5 months, and after 12 months, none of the grafts examined were normal.  GSV homografts are to be used as a last resort until better preservation techniques are developed1.

 

Arterial Conduits

Bovine Internal Mammary Artery:  Excellent results of the internal mammary artery use has led to an interest in bovine internal mammary artery use in coronary artery bypass.  Low patency rates at a mean of 10 months after surgery (<16%) precludes the use of bovine internal mammary arteries in bypass sugery1.

 

Synthetic Conduits

 

The appealing characteristics of synthetic grafts are non-immunogenicity, easy availability and storage, less risk of kinking (stiffness), less turbulent flow (uniform diameter, lack of branches).  One study using polytetrafluoroethylene (PTFE) as a coronary conduit to be 86% at 1 week, 64% at 1 year, 32% at 2 years, 21% at 3 years, and 14% at 45 months.  Another similar study showed a 59% patency rate at a mean follow-up of 1 year in 14 patients.  Although PFTE is a convenient alternative conduit, limitations including its tendency for infection and the development of neointima preclude it from becoming a widely used alternative conduit for coronary artery bypass surgery1. 

 

Conclusions

The internal mammary artery and the greater saphenous vein are the primary conduits of choice in coronary artery bypass grafting.  However, in the age of increasing numbers of repeat coronary revascularization, approximately 15% of patients undergoing CABG are in need of alternative conduits.  The desire for alternative arterial conduit will continue to raise questions regarding the future use of radial and other arterial grafts.   Additional early and late examination of biochemical and microscopic properties of alternative grafts as well as angiographic analysis of the patency rates are needed in order to provide a better foundation for clinical decision making.  

 

References:

  1. Canver, Charles C. Conduit Options in Coronary Artery Bypass Surgery. CHEST 1995; 108:1150-55
  2. Morgenstern DA, Mills NL. New Conduits for Coronary Artery Bypass. Coronary Artery Disease 1993; 4:677-681
  3. Geha, Alexander S. Choice of Grafts-Criteria for Selection of Coronary Bypass Conduit. Cleveland Clinic Quarterly 1978; 45(1): 46-49
  4. Doty, Donald B. Cardiac Surgery Operative Technique. St. Louis, MO. 1997: 278-307
  5. Slaughter MS, Gerchar DC, Pappas PS. Modified Minimally Invasive Technique for Greater Saphenous Vein Harvesting. Annals of Thoracic Surgery 1998; 65:571-2
  6. Green, George E. Coronary Bypass Surgery Highlights Blood Vessel Biology. Annals of Thoracic Surgery 1986; 41:235-236
  7. Loop, Floyd D. Internal Thoracic Artery Grafts: Biologically Better Coronary Arteries. New England Journal of Medicine 1996; 334(4): 263-5
  8. Wendler O, Hennen B, Marwirth T, et al. T Grafts with the Right Internal Thoracic Artery to Left Internal Thoracic Artery versus the Left Internal Thoracic Artery and Radial Artery: Flow Dynamics in the Internal Thoracic Artery Main Stem. Journal of Thoracic and Cardiovascular Surgery 1999; 118:841-8
  9. Fiore AC, Naunheim KS, Dean P, et al. Results of Internal Thoracic Artery Grafting Over 15 Years: Single versus Double Grafts. Annals of Thoracic Surgery 1990; 49:202-9
  10. Harlan BJ, Starr A, Harwin FM. Manual of Cardiac Surgery; Second Edition. New York. 1995: 97-104
  11. Loop FD, Lytle BW, Cosgrove DM, et al. Influence of the Internal Mammary Artery on 10 year Survival and Other Cardiac Events. New England Journal of Medicine 1986; 314:1-6
  12. Louagie YA, Jamart J, Buche M, et al. Intraoperative Hemodynamic Assessment of Gastroepiploic Artery and Saphenous Vein Grafts: A Comparative Study. Journal of Thoracic and Cardiovascular Surgery 1999; 118:330-8
  13. Mills NL, Hockmuth DR, Everson CT et al. Right Gastroepiploic Artery Used for Coronary Artery Bypass Grafting: Evaluation of Flow Characteristics and Size. Journal of Thoracic and Cardiovascular Surgery 1993; 106:579-86
  14. Manesse E, Sperti G, Suma H, et al. Use of the Radial Artery for Myocardial Revascularization. Annals of Thoracic Surgery 1996: 62:1076-83
  15. Moores HK, Wolk SW, Lampman RM, et al. The Use of Non-Invasive Vascular Studies for Pre-Operative Evaluation of Radial Artery Conduits for Coronary Artery Bypass Grafting. SJMH 1997
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