Surgery For Coronary Artery Disease

Peter A. Seirafi, M.D.,
Assistant Professor of Surgery, Division of Cardiothoracic
Surgery at the University of Florida / Jacksonville

 
In the four decades since the first surgical coronary revascularization procedures were performed, dramatic advancements have been made on multiple fronts in the understanding and treatment of coronary artery disease. These advances range from the study of atherosclerosis, where we now know that this is a dynamic, multifactorial process, to the most recent developments in percutaneous and surgical revascularization. Large studies from the 1970's, comparing medical and surgical therapy, helped define which groups of patients benefited from coronary artery bypass grafting (CABG). In general, those patients with left main disease, and patients with significant two- and three- vessel disease with depressed ventricular function were found to have improved long term survival with surgery. Further studies indicated that use of the left internal mammary artery (LIMA) to the left anterior descending (LAD) artery, as well as obtaining complete revascularization, significantly impacted on long term survival. The introduction of percutaneous transluminal coronary angioplasty (PTCA) and stenting have led to a dramatic increase in the number of percutaneous coronary interventions (PCI) performed, from 32,300 in 1983 to 400,000 in 1995.

The increased number of PCI's has had a significant impact on the patient population undergoing surgical revascularization. Edwards et al1, analyzing The Society of Thoracic Surgeons National Database of patients receiving CABG between 1980 and 1990, found an increase in the mean age from 58 to 64 years, a higher proportion of women, more reoperations, worsening ventricular function, and an increased number of emergent and urgent operations. Simply put, patients undergoing coronary surgery are "sicker" than ever before. This is of significant concern because operative risk for CABG is largely determined by patient age, comorbid conditions, reoperation, and emergent status. Poor ventricular function remains important, although less ominous than in the past. With improvement in perioperative management, including myocardial protection, the overall mortality for isolated CABG remains at approximately 3%, despite the increasingly complex patient population.

Some of the major themes in recent years in the surgical treatment of coronary artery disease have been the use of multiple arterial conduits, minimally invasive bypass surgery, and the use of transmyocardial laser for patients with angina who are not revascularization candidates. This article will provide an update on these specific developments for the surgical treatment of ischemic heart disease.

Arterial Conduits

The most important determinant of late outcome is the type of bypass conduit used. Loop et al2, in a sentinel article in 1986, found that the use of the left internal mammary artery (LIMA) in grafting the left anterior descending artery (LAD) had a significantly positive impact on long term survival. In fact, the LIMA - LAD graft is a more important predictor of survival than progressive coronary artery disease. In addition, lower hospital mortality is associated with mammary artery grafting. The success of the LIMA as a conduit is thought to be due to release of endothelium-derived nitric oxide and prostacyclin, which inhibit smooth muscle proliferation and atherosclerosis, thus promoting graft longevity. These reports led to an increased use of mammary artery grafting, such that in 1997, ~80% of patients undergoing first time bypass surgery received a single mammary artery. Most commonly, CABG is performed by grafting the LIMA to the LAD, and using saphenous vein grafts (SVG) to the other obstructed arteries. At 10 years, the patency rate of the LIMA is ~90%, and that of SVG is ~50%.

Given that a LIMA - LAD graft prolongs survival and reduces cardiac events, and that saphenous vein conduits perform less well, surgeons became interested in using additional arterial conduits to reduce the rate of reoperation and to enhance long term survival. The most logical choice was the right internal mammary artery (RIMA). In comparing bilateral vs. single mammary artery patients, Lytle et al3 reported that bilateral mammary grafting had decreased risks of reoperation and late mortality. Patients with moderate to severe ventricular dysfunction who had bilateral mammary grafting had a significantly lower reintervention rates, although no benefit on survival was demonstrated. However, Sergeant et al4, in a series of 9600 patients, reported that the return of angina and survival after CABG were not influenced by use of multiple arterial grafts. Concern about sternal wound infection, especially in diabetic and obese patients, as well as the patency and technical issues in using the RIMA, have limited its use. In 1997, only about 8% of patients received bilateral mammary grafts.

Aside from the mammary artery, the most widely used arterial conduit for CABG is the radial artery. Introduced in 1970's, the radial artery graft was rapidly found to be unsatisfactory, with extremely poor patency rates. This was thought to be due to arterial spasm. Renewed interest in the radial occurred with the discovery of a few patients from the 1970's who were found to have patent radial artery grafts 20 years postoperatively. With further study, improved harvesting techniques, and antispasmodic medications, the radial artery was reintroduced in the1990's, and the results have improved markedly. The patency rate for radial artery grafts has been encouraging, 92% at 1 year and 84% at 5 years. Thus, the radial artery has gained widespread acceptance as the second choice for arterial grafting after the mammary artery.

Prior to radial artery harvest, it is mandatory to assess adequacy of ulnar collateral circulation to the hand. Acute hand ischemia is an extremely rare event when assessment of collateral circulation is done. The most common complication after radial harvest is dysesthesia of the arm and hand (8-10%), which usually resolves.

Additional arterial conduits include the gastroepiploic and inferior epigastric arteries. Generally, these conduits are used only if there is a need for additional conduits, as these are technically more difficult and spasm remains a concern.

The bulk of medical evidence indicates that the use of the LIMA- LAD is a critical component for long term survival. Additional arterial conduits are most useful in younger patients, as this group is most likely to benefit from the decreased rate of cardiac events and reoperation, and from the potential for enhanced survival. Multiple arterial conduits are also useful in patients who have inadequate saphenous veins or lower extremity ischemia. Further long-term studies are needed to accurately assess the proper role of these conduits.

Minimally Invasive Cardiac Surgery

During the past decade, the medical and lay press has given a great deal of attention to all types of "minimally invasive" or "minimal access" surgery, especially beating heart surgery. Introduced in the 1960's, beating heart surgery never gained widespread acceptance in this country, due to further developments and refinements in cardiopulmonary bypass as well as myocardial protection. The performance of a microsurgical anastomosis in a motionless heart became the gold standard operation for bypass surgery with resultant low mortality, documented graft patency, and superior patient survival.

In recent years, there has been renewed interest in beating heart surgery, mainly based on the avoidance of the potentially deleterious effects of cardiopulmonary bypass. It is well known that bypass initiates a series of physiologic derangements including activation of systemic inflammatory response, which can affect many end organs. Elderly patients, as well as those with severe lung, kidney, and cerebrovascular disease, have increased complication rates when placed on cardiopulmonary bypass. It was mainly for those reasons, as well as for patients with isolated left anterior descending disease, that minimally invasive bypass surgery had appeal and was `rediscovered' in the United States. Beating heart surgery had been widely used for more than 20 years, mostly in South America, with very good results. Initial acceptance of beating heart surgery in this country was limited due to concern over the ability to correctly perform the critical left internal mammary to left anterior descending anastomosis on a beating heart, as well as the difficulty of performing a complete revascularization.

The term minimally invasive coronary bypass surgery is difficult to define. Is it the surgical approach, i.e. length of the incision, or is it the avoidance of cardiopulmonary bypass, or both? The International Society for Minimally Invasive Cardiac Surgery defines four approaches:

  1. Minimally Invasive Direct Coronary Artery Bypass (MIDCAB): This procedure is done on a beating heart through a small anterior thoracotomy. In general, it only offers access to the left anterior descending and diagonal coronary arteries. It is not widely used due to limited exposure, difficulty in harvesting the mammary artery, and the applicability to patients with only single vessel disease, which generally represents <5% of surgical cases. Furthermore, the pain from thoracotomy may be greater than from sternotomy.
  2. Off Pump Coronary Artery Bypass (OPCAB): By far, this is the most commonly used approach. The procedure is performed by conventional median sternotomy, which permits harvest of the internal mammary artery and access to all coronary arteries.
  3. Port-Access CABG: In this procedure, the bypass grafting is performed through small incisions, but with conventional cardiopulmonary bypass. The connections to the heart-lung machine are made through the femoral vessels rather that through the open chest. This approach is time consuming, technically difficult, and poses risk to patients with lower extremity arterial or venous insufficiency. It has not been widely applied.
  4. Robotic Coronary Artery Bypass: With this technique, instruments are introduced through small incisions in the chest and manipulated by robotic arms controlled by the surgeon, who is seated at a computer console. The use of robotic coronary endoscopic surgery is in the initial stage of development. Early results are positive and the development of complete multi-vessel endoscopic CABG is on the horizon.

With the improvement in stabilization devices which hold the coronary arteries nearly motionless while the heart continues to beat, there has been a dramatic increase in the number of OPCAB's performed. It is estimated that in 1999-2000, 25-30% of all coronary bypass operations were OPCAB's, up from 5% in 1996-1997. Contraindications to OPCAB include cardiogenic shock and persistent hemodynamic instability during the manipulation of the heart required to expose the coronary arteries. Other relative contraindications depend on surgeon comfort and experience, and include patients with obesity, cardiomegaly, need for lateral wall revascularization, and pectus excavatum. Those patients with coronary arteries that are small, intramyocardial, or calcified remain a challenge. It is generally agreed that the quality of the revascularization should not be sacrificed for the sake of a minimally invasive procedure.

There is a wealth of data claiming equivalent if not superior results of OPCAB over conventional CABG. One of the difficulties in interpreting this data is that there is no prospective, randomized trial comparing the two approaches. Furthermore, patient selection varies widely. Conventional bypass is considered the gold standard, with well established long term graft patency rates and survival. Recent published studies of OPCAB patients by centers with large experience show very good early graft patency rates. Puskas et al5, reported a 98.8% overall graft patency rate, and 100% for mammary grafts, in 167 patients studied after OPCAB. In general, the majority of published studies now show at least equivalent early graft patency rates as compared to conventional CABG. Concern about the completeness of revascularization remains, as most studies show that patients in the OPCAB groups receive fewer grafts. More recent reports indicate that increasing surgical experience and improved coronary stabilizers allow complete revascularization of all obstructed arteries.5

Overall morbidity and mortality rates vary in the literature, but complications such as reoperation for bleeding, stroke, atrial fibrillation, mediastinitis, renal failure, myocardial infarction, and death do not consistently differ between OPCAB and conventional CABG patients. Arom et al6, in a retrospective study comparing the two groups, found significantly decreased mortality rate with OPCAB in the highest risk group of patients. Interestingly, this paper also points out with one year follow up there was a trend toward increased recurring angina and interventional procedures in the OPCAB group. Whether due to anastomotic difficulties or incompleteness of revascularization, this remains an obvious concern.

Most studies comparing the two groups show that OPCAB patients are extubated earlier, have reduced blood transfusion requirements, shorter intensive care unit stay, shorter hospital stay, and lower overall cost. However, Bull et al7, found no significant difference in hospital stay or costs between groups. Potential reductions should be interpreted in light of the fact that OPCAB patients tend to receive fewer grafts than conventional CABG patients do.

An important issue which has received much public attention in recent years is cognitive impairment following cardiopulmonary bypass. Diegeler et al8 showed postoperative cognitive impairment in 90% of patients undergoing conventional CABG and no impairment in the OPCAB group. This suggests that cognitive impairment is strongly associated with cardiopulmonary bypass, possibly due to microemboli, and that avoidance of bypass might reduce neuropsychologic impairment following CABG.

The goal of minimally invasive coronary bypass surgery is to achieve a complete revascularization with less pain and shorter length of stay with patient safety and long-term outcome that is identical to (or better than) conventional CABG. Patient care should not be compromised in a rush to perform the latest procedure. While the early results of beating heart surgery are encouraging, more study of these surgical procedures is needed.

Transmyocardial Laser Revascularization (TMR)

In addition to improving long-term survival, one of the most important benefits of complete revascularization is relief of angina. As many as 6 million Americans suffer from angina. With appropriate risk modification, medical therapy, and, when necessary, percutaneous and surgical revascularization, the vast majority of patients improve. However, there are patients who continue to have severe refractory angina despite maximal medical therapy, and whose coronary disease in not amenable to revascularization. For this group of patients, treatment options have traditionally been limited, with refractory angina continuing to frustrate both patient and physician.

In recent years, there has been extensive study of transmyocardial laser revascularization (TMR). In this procedure, a laser is used to create transmural channels from the epicardium to the endocardial surface of the left ventricular wall, thereby allowing perfusion of the ischemic myocardium with oxygenated ventricular blood. The two most commonly used lasers, the CO2 and the holmium:YAG, use thermal ablation to create transmyocardial channels. It is now generally accepted that the success of this procedure, i.e. the relief of angina, is not due to patency of these channels, as most studies show occlusion of the channels with necrotic debris within a few weeks after the procedure.

Leading theories of the mechanism of TMR in the relief of angina include stimulation of angiogenesis and myocardial neural ablation. The myocardial injury and inflammation caused by TMR have been shown to elevate the levels of certain angiogenic growth factors, causing angiogenesis and neovascularization. However, most studies do not demonstrate a consistent increase in regional myocardial blood flow or ejection fraction after TMR, although there have been reports of improvement in regional wall motion during dobutamine stress testing9. Denervation of the ischemic myocardium has also been proposed as a mechanism for the effect seen with TMR10. It is possible that more than one mechanism may be responsible for the anti-anginal effect.

Currently patients with medically refractory Class III/IV angina who are not candidates for CABG or percutaneous revascularization, may be candidates for TMR. Clinical trials demonstrate a consistent improvement in angina in patients with refractory Canadian Class III and IV angina, who are not candidates for traditional revascularization procedures. Two large studies, by Frazier11 and Allen12, found significant improvements in angina and quality of life at one year, compared to medical therapy. Angina had improved by at least two Canadian Classes in > 70% of TMR patients. However these, and other studies have failed to demonstrate a significant survival benefit with TMR, and DeCarlo et al13 found a significant return of angina with longer follow up. Therefore, while initial clinical trials are encouraging, further study of TMR is needed, especially in the areas of sustained angina relief and long term survival.

TMR is being increasingly applied in combination with CABG, with TMR being performed in those regions where bypass grafting is not feasible. An additional exciting area of study is direct administration of angiogenic growth factors as well as gene therapy in conjunction with TMR.

Summary

  1. Patients undergoing coronary revascularization today are "sicker" than in the past.
  2. Use of multiple arterial conduits during coronary revascularization may lead to fewer reinterventions and prolonged survival.
  3. Beating heart surgery has shown excellent early graft patency rates, and the ability to achieve complete revascularization is increasing with surgeon experience.
  4. In the short term, TMR significantly improves angina in >70% patients who are not candidates for percutaneous or surgical revascularization.

References

  1. Edwards FH, Clark RE, Schwartz M. Coronary artery bypass grafting: The Society of Thoracic Surgeons National Database experience. Ann Thorac Surg 1994;57:12-9.
  2. Loop FD, Lytle BW, Cosgrove DM, Stewart RW, et al. Influence of the internal mammary artery graft on 10-year survival and other cardiac events. N Engl J Med 1986;314:1-6.
  3. Lytle BW, Blackstone EH, Loop FD, et al. Two internal thoracic artery grafts are better than one. J Thorac Cardiovasc Surg 1999;117:855-872.
  4. Sergeant P, Blackstone E, Meyns B. Is return of angina after coronary artery bypass grafting immutable, can it be delayed, and is it important? J Thorac Cardiovasc Surg 1998;116:440-444.
  5. Puskas JD, Thourani VH, Marshall J, et al. Clinical outcomes, angiographic patency, and resource utilization in 200 consecutive off-pump coronary bypass patients. Ann Thorac Surg 2001;71:1477-84.
  6. Arom KV, Flavin TF, Emery RW. Et al. Safety and efficacy of off-pump coronary artery bypass grafting. Ann Thorac Surg 2000;69:704-710.
  7. Bull DA, Neumayer LA, Stringham JC, et al. Coronary artery bypass grafting with cardiopulmonary bypass versus off-pump cardiopulmonary bypass grafting: does eliminating the pump reduce morbidity and cost? Ann Thorac Surg 2001;71:170-5.
  8. Diegeler A, Hirsch R, Schneider F, et al. Neuromonitoring and neurocognitive outcome in off-pump versus conventional coronary artery bypass operation. Ann Thorac Surg 2000;69:1162-1166.
  9. Cooley DA, Frazier OH, Kadipasaoglu KA, et al. Transmyocardial laser revascularization: clinical experience with twelve-month follow-up. J Thorac Cardiovasc Surg 1996;111:791-9.
  10. Kwong KF, Kanellopoulos GK, Nickols JC, et al. Transmyocardial laser treatment denervates canine myocardium. J Thorac Cardiovasc Surg 1997;114:883-90.
  11. Frazier OH, March RJ, Horvath KA. Transmyocardial revascularization with carbon dioxide laser in patients with end-stage coronary disease. N Engl J Med 1999;341:1021-8.
  12. Allen KB, Dowling RD, Fudge TL, et al. Comparison of transmyocardial revascularization with medical therapy in patients with refractory angina. N Engl J Med 1999;341:1075-6.
  13. DeCarlo M, Milano AD, Pratali S, et al. Symptomatic improvement after transmyocardial laser revascularization: how long does it last? Ann Thorac Surg 2000;70:1130-33.
October, 2001/ Jacksonville Medicine

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