BEATING HEART SURGERY

BEATING HEART SURGERY

Lilly, R Eric

Coronary artery bypass grafting (CABG) remains the most common heart operation with over 300,000 procedures performed annually in the United States.1 Inherent to the successful widespread acceptance of surgical coronary arterial revascularization was the introduction of Gibbon’s heart lung machine in 1966. Since then, cardiopulmonary bypass (CPB) has been applied routinely. Recently coronary surgical efforts have been refocused towards lessinvasive procedures including a move away from routine use of CPB for CABG with operations performed on the beating heart. Mid-term data including randomized trials have examined these newer procedures both in terms of coronary graft patency and in terms of complications when compared to conventional CABG with CPB. Results from these studies are mixed. This paper will review the status of surgical coronary revascularization and examine the controversy surrounding routine use of “beating-heart” CABG.

CONVENTIONAL CORONARY ARTERY BYPASS SURGERY

The overwhelming majority of CABG procedures in the United States and Canada are still performed on an arrested heart while the patient is supported by CPB.2 The process of initiating CPB requires that the patient be anticoagulated, usually with heparin, and that the ascending aorta be extensively manipulated. Aortic manipulation is often extreme with placement of an arterial cannula in the ascending aorta for return of oxygenated blood from the CPB circuit, crushing the ascending aorta by cross-clamping, and performing often multiple arteriotomies for suturing the proximal anastomoses for the coronary bypass grafts. While on CPB, the patients’ platelets adhere to the circuit, inflammatory mediators become activated and coagulation factors are diluted, all leading to often a severe coagulopathy and deleterious systemic inflammatory effects.3 Arterial cannulation is most commonly performed in the in the ascending aorta prior to the origin of the great vessels. Atheromatous plaque from the aorta or debris from the CPB circuit may embolize distally with resultant stroke or end-organ dysfunction. Despite these potential deleterious affects, CPB remains safe for most patients as evidenced by reported mortality and stroke rates in the 1-3% range.

Reliance on CPB, however, has traditionally limited the possibility for surgical revascularization in many patients with significant co-morbidities. Severe atherosclerotic changes in the ascending aorta are associated with unacceptably high stroke and mortality rates when routine CPB is applied.4 Additionally, patients with severe pulmonary disease or recent stroke also exhibit unacceptable clinical outcomes. These patients have remained essentially unserved by conventional CABG.

BEATING HEART SURGERY

To serve high-risk patients with surgical coronary revascularization and to make CABG surgery less invasive, the trend in cardiac surgery has been away from routine use of CPB in CABG and toward decreased manipulation of the ascending aorta. The use of off-pump coronary surgery (OPCAB) has soared in the United States over the past 7 years. Some centers perform CABG exclusively off-pump on the beating heart. Nationwide, 25% of all CABGs are performed without CPB.

CABG performed on the beating heart is an old procedure: the first reported procedure occurred in St Petersburg, Russia, in 1964.5 This technique has been and remains the standard operation for surgical coronary revascularization in developing countries as reported by many series published throughout the 1990s.6-8 The routine use of OPCAB in these countries is less about patient outcomes and more about the relative high cost of the CPB circuit.

In the United States, industry-driven technological advancement of off-pump surgical equipment has in large part driven the popularity of this technique. Suction devices that are applied to the epicardial surface allow easy manipulation of the heart and acceptable surgical exposure to all coronary artery distributions without compromising the patients’ hemodynamics. Additional stabilization devices are applied to the epicardium around the target coronary vessel requiring surgical bypass that create a relatively motionless field thereby allowing successful coronary anastomoses. (Figure 1)

Two basic techniques for beating heart surgical revascularization have gained acceptance. Mid-CAB is a procedure that employs a left anterior thoracotomy through which the left internal mammary artery (LIMA) is taken down from the chest wall. Commercially available retractors facilitate the LIMA harvest. The pericardium is then opened overlying the left anterior descending coronary artery (LAD) and the LIMA to the LAD anastomosis is performed on the beating heart with the aid of a stabilizer. The Mid-CAB approach is typically limited to a single LAD-LIMA anastomosis and therefore is reserved for patients with left anterior descending coronary artery disease only. This technique has been unpopular, in part because of the pain associated with a thoracotomy which is often significantly greater than that of a full sternotomy, and because of the rarity of single vessel LAD disease patients being referred for surgical revascularization.

Most patients referred for CABG have multivessel and often complex coronary artery disease. The beating heart technique designed to serve these patients has been termed OPCAB. This procedure involves a standard midline chest incision with a complete sternotomy. Patients are heparinized with generally half the dose required for CPB. Both pleural spaces are widely opened to facilitate displacement of the heart within the chest cavity allowing exposure of all coronary artery distributions without kinking of the great vessels or compression of the right ventricle as the heart is manipulated. Stabilizers are utilized to facilitate the coronary anastomoses which are performed on the beating heart. A bloodless field is accomplished with the aid of a carbon dioxide/saline mister to irrigate the field, silastic tapes placed around the artery to control blood loss, and often intra-coronary shunts which are removed prior to completion of the anastomosis. A period of regional heart ischemia is induced as the vessel being bypassed is occluded. Patients are monitored with SVO2 Swan-Ganz catheters and trans-esophageal echocardiography to assess regional heart function. The OPCAB procedure is the most commonly performed beating heart surgery.

The principal advantage of OPCAB is the avoidance of CPB. To accomplish coronary bypass grafts, ascending aortic arteriotomies are generally required. Historically these anastomoses are facilitated by placement of a partial occlusion clamp on the side of the ascending aorta. This clamp crushes the aorta and therefore allows for the possibility of atheromatous plaque to be dislodged and embolize distally. New devices have become commercially available that allow coronary grafts to be anastomosed to the ascending aorta without these crushing clamps. These devices have included the St Jude Symmetry Aortic Connector and the Guidant Heart String device. (Figure 2) The full benefits of OPCAB versus conventional CABG for reduction of post-operative complications depend on this clamp-less approach.

RESULTS

The most common complication of conventional CABG is the occurrence of atrial fibrillation.9 Post-operative length of stay is generally prolonged and marked hemodynamic consequences can occur particularly in patients with baseline marginal left ventricular function. Extensive effort has been applied to the understanding and prevention of atrial fibrillation in conventional CABG resulting in the routine clinical use of beta- Blockers and careful attention to electrolyte shifts in the post-operative setting. Despite all clinical efforts, the reported incidence of atrial fibrillation remains as high as 40% in some series.9 Advanced patient age is the most commonly associated factor for its occurrence.

A recent meta-analysis of observational studies sought to examine the potential benefit of OPCAB for prevention of atrial fibrillation when compared to conventional CABG in older patients.10 Eight observational studies performed in patients greater than 70 years of age published between 1999 and 2003 were evaluated. These included 3017 patients of whom 764 underwent OPCAB and 2253 conventional CABG. The overall incidence of atrial fibrillation was 22% in the OPCAB group versus 28% in the conventional CABG group. These data translated into a significant, albeit small, reduction in atrial fibrillation in OPCAB using the random effect model. The authors postulate that a diminished inflammatory response and decreased global myocardial as well as atrial injury during OPCAB versus conventional CABG likely contribute to this risk reduction.

Multiple trials have addressed the question of whether all coronary revascularization surgery should be done routinely without the assistance of CPB. A meta-analysis of 18 randomized studies comparing routine coronary surgery versus OPCAB published prior to August 2003(11) examined the combined end-point of mortality, stroke and myocardial infarction in 1584 patients of which 783 were randomized to OPCAB and 801 to conventional CABG. The use of OPCAB in these patients reduced the relative risk of this combined end-point by 27% at 2 weeks, 25% at 1 month, 45% at 3 months and 34% at 1 year. None of these reductions reached statistical significance secondary to the relatively small number of events noted. An additional five randomized trials in 2004 included an additional 1,164 patients.12 These studies reported an occurrence of the same combined end-point as above (i.e., mortality, stoke and myocardial infarction) to be 2.9% in the on-pump CABG group as compared to 3.9% in the OPCAB group.

Additional data examining the routine use of on-pump CABG versus conventional CABG have been derived from the New York State Cardiac Reporting System.” Between 1997 and 2000, 9,135 patients underwent OPCAB, compared to 59,044 who underwent conventional CABG in New York State. Risk adjusted mortality was 2.02% for OPCAB as compared to 2.16% for conventional CABG (p=0.390). Patients with surgery performed off-pump exhibited lower post-operative lengths of stay (5 vs. 6 days, p

Inherent to successful coronary artery bypass surgery is the patency of the bypass grafts. An important randomized trial in 2004 compared the two surgical strategies in terms of bypass graft patency. This was a single center and single surgeon trial.” A total of 200 consecutive patients were randomized either to conventional on-pump CABG or to OPCAB regardless of their coronary anatomy or left ventricular function. Post-operatively, patients underwent coronary angiography prior to discharge from the hospital and at 1-year of follow-up. Patients with severe atherosclerosis or renal insufficiency were excluded from angiography. Prior to discharge from the hospital, 93.4% of patients underwent angiography with graft patency of 99% for OPCAB patients as compared to 97.7% for conventional CABG. Coronary angiography was again performed at 1-year. At this time, 93.6% of grafts were patent in the OPCAB group, as compared to 95.8% in the conventional CABG group. Neither of these differences reached statistical significance. Additionally, no difference was found between the groups in the incidence of death, myocardial infarction, stroke, recurrent angina, readmission for cardiac events or need for post-operative percutaneous intervention.

A second randomized trial again examined graft patency at an intermediate-term follow-up.16 This study randomized 104 patients to either OPCAB or conventional CABG. Postoperative coronary angiography was performed at 3 months. In the conventional CABG group 127 of 130 grafts (98%) were patent as compared to 114 of 130 grafts (88%) patent in the OPCAB group (p=0.002).

These studies, while small, have called into question the routine use of OPCAB for surgical coronary revascularization. The potential benefits of OPCAB which include a diminished rate of perioperative atrial fibrillation, a small but diminished stroke risk and diminished perioperative bleeding appear to be off-set by an increased risk of gastrointestinal complications and a significantly decreased long-term graft patency and patient survival. Routine application of off-pump CABG as a replacement for on-pump conventional CABG is obviously in question. The Veterans Affairs Outcomes Following Myocardial Revascularization: On and Off Cardiopulmonary Bypass (ROOBY) study which is accruing patients will likely illuminate this controversy.12

As evidenced by this discussion, a controversy exists within the surgical community surrounding the routine use of off-pump techniques for coronary revascularization in low-risk patients. There is no controversy, however, concerning application of these techniques to high-risk patients. The use of CPB clearly predisposes all patients to some element of lung injury as manifested by reductions in FEV1 and increased oxygen requirements in post-CPB patients. These changes may render marginal patients ventilator dependent. The OPCAB technique has been shown to attenuate this reduction in lung function.17 OPCAB has also been associated with a decreased need for blood transfusions compared to conventional CABG.11 As such, OPCAB should be applied to patients with blood dyscrasias or in those whom blood transfusion is not an option (e.g., Jehovah’s Witness).

Patients who most clearly benefit from OPCAB, compared to conventional CABG, are those with severe atherosclerotic changes of the ascending aorta in whom use of CPB is not possible or associated with an unacceptably high stoke risk.4 Clinical trials have generally excluded patients with severe atherosclerotic changes in their aortas, given this high stroke risk. While these high-risk patients may benefit from OPCAB, there remains a defined risk of stroke even in typical patients undergoing CABG without severe atherosclerotic changes of the aorta. The most surprising result from the randomized trials has been the lack of a significantly greater stroke risk reduction in patents performed off-pump as compared to those in whom conventional CPB was utilized. The absence of a greater reduction is likely attributable to the lack of routine use of clamp-less CABG in these trials. Broad application of clamp-less techniques for coronary anastomoses such as afforded by the Guidant Heart String device likely will further reduce stoke in OPCAB patients.

Patients with recent stroke often exhibit worsening neurologic status when routine cardiopulmonary bypass is applied. Diminished cerebral autoregulation after stroke, non-pulsatile flow during CPB and the potential conversion to a hemorrhagic stroke secondary to the extreme heparinization required for CPB all contribute to these potential neurologic changes. The general practice is to reserve CPB in stroke patients until six weeks have elapsed after their event. OPCAB revascularization is a valuable strategy for these high-risk patients.

FUTURE DIRECTION

Refinements in surgical revascularization will rely on techniques that allow CABG to be performed on the beating heart without the necessity of a sternotomy. This may be accomplished using an approach similar to the Mid-CAB technique except that both the right and left internal mammary arteries are harvested from the chest wall employing bilateral thoracoscopic incisions. Coronary anastomoses may then be performed under direct vision or with the aid of robotic technology. The principle advantage of this technique is that both internal mammary arteries may be harvested without incising the sternum, compromising the blood supply to the sternum, and thereby increasing the risk of mediastinitis. While use of bilateral IMAs has been demonstrated to add a survival benefit to patients undergoing CABG,18 routine use is generally avoided in obese or diabetic patients because of a marked increased risk of infectious complications in these patients in particular. Further refinements in robotic technology will be required prior to widespread acceptance of this technique.

SUMMARY

Off-pump beating heart coronary revascularization is a valuable surgical technique for high-risk patients, particularly those with severe atherosclerotic changes of the aorta, COPD, recent stroke, or for those in whom blood administration is contraindicated. Advances in clampless surgical techniques should further the benefit of OPCAB versus conventional CABG mostly in terms of stroke risk reduction. For now, routine use of OPCAB for all surgical revascularization procedures remains in question.

REFERENCES

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14. Song HK, Petersen RJ, et al. Safe evolution towards routine off-pump coronary artery bypass. Eur J Cardiothor Surg 2003;24:947-52.

15. Puskas JD, Williams WH, et al. Off-pump vs conventional coronary artery bypass grafting:. JAMA 2004;291;1841-9.

16. Khan NE. De Souxa A, et al. A randomized comparison of off-pump and on-pump multivessel coronary artery bypass surgery. NEJM 2004;350:21-8.

17. Staton GW, Williams WH, et al. Pulmonary outcomes of off-pump vs on-pump coronary artery bypass surgery in a randomized trial. Chest 2005;127:892-901.

18. Lytle BW, Blackstone EH, et al. Two internal thoracic artery grafts are better than one. J Thorac Caridiovasc Surg 1999;117;855-72.

R. ERIC LILLY, MD

R. Eric Lilly, MD, is Assistant Professor of Surgery, Brown Medical School, and Cardiothoracic Surgeon, Department of Surgery, the Miriam Hospital.

CORRESPONDENCE:

R. Eric Lilly, MD

Thoracic & Cardiovascular Surgical Center, Inc.

208 Collyer Street

Providence, RI 02904

phone: (401) 793-5930

e-mail: elilly@lifespan.org

Copyright Rhode Island Medical Society Jan 2006

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