The Benefits of Off-Pump Coronary Bypass

Direct coronary revascularization was 1st attempted in 1958 on the beating heart. ¹ With the development and refinement of techniques for cardiopulmonary bypass and myocardial protection, the obvious advantages of a quiet and bloodless field for precise microvascular anastomosis to perform coronary bypass were recognized and widely adopted. The substantial benefits of coronary artery bypass grafting in terms of improved survival and freedom from adverse cardiac events in patients with coronary artery disease have been clearly established. ² Nevertheless, cardiopulmonary perfusion has been shown to induce a systemic inflammatory response, which is the suspected cause of postoperative complications, including multiorgan dysfunction. During the past few years, the development of technology for off-pump coronary bypass grafting has led to a resurgence of interest in beating heart surgery in the hope of improving outcomes and lowering costs, especially in high-risk patients. In the United States, off-pump procedures have grown in application to an estimated 20%–25% of coronary artery bypass (CAB) procedures in 2001. Proponents have continued to examine the efficacy of off-pump coronary bypass (OPCAB) procedures in comparison with conventional CAB, but controversy exists on the selection of patients who are most likely to benefit from the procedure and on the claims of improved outcomes.

In this issue, Bottio and associates ³ attempt to answer this question by means of a prospective study of all patients undergoing isolated coronary bypass at their institution during 2001. The authors have made a thorough attempt to identify all variables that may have influenced outcomes, but it is important to note that their patients were not randomized for treatment. Among the 324 patients, 216 had on-pump CAB and 108 patients had off-pump CAB. The selection of the procedure was made by the surgeon and the patient in accordance with undefined criteria. Although the authors attempt to show the preoperative similarity between patients in each treatment group (see their Table I), a selection bias toward single- and double-vessel disease (64%) is apparent among the OPCAB group and a bias toward triple-vessel disease (51%) is seen in the on-pump CAB group (see their Table II). This corresponded to significantly fewer bypass grafts per patient in the OPCAB group (1.5 vs 2.25). Moreover, the reduced requirement for grafts in the OPCAB group was associated with a higher percentage of patients (85%) who received all arterial conduits.

In addition to the inherent bias in patient selection in their study, the authors also claim a difference in treatment strategy between the 2 groups, wherein older patients in the OPCAB group underwent “incomplete” revascularization, while those in the on-pump group underwent full revascularization. The long-term efficacy of this difference in treatment strategy was not analyzed, since only short-term hospital follow-up was provided. Prior reports have shown that incomplete revascularization is the third most significant factor in reducing reoperation-free survival. ⁴

Despite the differences in patient selection and treatment strategy, the authors can show no survival advantage for patients having OPCAB, in comparison with conventional CAB. Three prospective randomized trials have reported mortality rates of 0–2%, with no significant difference in early death between off-pump and conventional CAB. ⁵ Cleveland ⁶ analyzed the risk-adjusted mortality rate from the national STS database of 118,440 patients and found a higher mortality rate for conventional CAB (2.9%) compared with off-pump CAB (2.3%), but a patient selection bias is recognized in such an analysis. Some proponents of OPCAB have shown a survival advantage in large series of patients from single-institution reports, whereas others have failed to show any difference in mortality rates when careful case-matching has been performed. Therefore, controversy remains whether off-pump surgery actually reduces the mortality rate associated with CAB procedures.

The present study does demonstrate that experienced surgeons can develop technical proficiency in off-pump revascularization. Angiographic follow-up was performed on 80% of off-pump CAB patients in this study. Graft stenosis was uncommon (3.2%). Only a few studies have provided postoperative angiographic validation for OPCAB. ⁵ In the Patency Outcomes and Economics of MIDCAB (POEM) trial, ⁷ no difference in LIMA-to-LAD patency was found in comparing off-pump with on-pump bypass patency at 6 months (95.1% vs 95.7%). Puskas ⁸ reported 98.8% graft patency among 421 grafts in 163 OPCAB patients prior to discharge. Nevertheless, others have reported technical problems unique to OPCAB, including anastomotic stenosis, graft occlusion, and distal intimal injury from temporary occlusive snares. Concern has been expressed that off-pump CAB may induce a hypercoagulable state. ⁹ Despite the reassurance of early angiographic patency, a perioperative hypercoagulable state may affect late graft closure, especially in vein grafts. Kim ¹⁰ reported a reduced patency rate of 68% in saphenous vein grafts at 1 year after OPCAB.

Proponents of OPCAB have examined the impact of this technology on perioperative morbidity, length of stay, and hospital costs. In the present study, the authors used univariate analysis to examine postoperative morbidity in patients who underwent off-pump and on-pump CAB. There was no significant difference in the incidence of postoperative bleeding, myocardial infarction, acute renal failure, sternal dehiscence, neurologic events, atrial fibrillation, or prolonged respiratory assistance between these 2 groups. The authors did find a significant increase in intra-aortic balloon use and a longer intensive care unit stay for on-pump CAB patients. However, when the analysis combined all morbidity end-points, a derived cumulative morbidity score showed a significant advantage for off-pump patients. It is unclear, in the present study, whether differences in patient selection, surgical strategy, or other variables may have influenced this cumulative outcome score.

Most reports of OPCAB surgery have shown a significant decrease in perioperative blood transfusion and mediastinal blood loss, compared with on-pump CAB. ⁵ Most series have reported earlier extubation in off-pump patients, compared with on-pump. Re duced postoperative bleeding, hemodynamic stability, and institution-specific policies all affect the duration of intubation after CAB. Conflicting reports do not conclusively establish that off-pump surgery will lessen perioperative renal complications. The incidence of sternal infection is not decreased by OPCAB. ⁵

In most reports, perioperative myocardial infarction is not significantly different between on-pump and off-pump CAB. Several studies have shown a decrease in myocardial enzyme spill for off-pump patients, but the clinical relevance of this is undetermined. Myocardial stunning has been reported with OPCAB and may require treatment with vasopressors. Prevention of ischemia with distal coronary perfusion devices, intracoronary shunts, and β-blockers has been effective. In some reports, OPCAB decreases the incidence of postoperative atrial fibrillation, but in others there is no difference. Atrial fibrillation remains a significant postoperative problem that affects length of stay and postoperative rehabilitation. In the present study, off-pump surgery decreased length of stay in the intensive care unit. In the majority of studies, hospital length of stay was reduced 1 to 4 days for off-pump patients. This may be attributed to patient selection and earlier extubation, as well as to an aggressive hospital discharge policy.

Because most strokes are believed to result from atheroemboli, avoidance of aortic cannulation, aortic clamping, and cardiopulmonary bypass should reduce the risk of perioperative stroke. Although most reported studies of off-pump CAB have been unable to show a reduction in perioperative strokes, several studies have provided evidence of stroke reduction with OPCAB. ^5,11 Using multivariate analysis, Patel ¹² identified cardiopulmonary bypass as a risk factor for perioperative stroke. Stamou ¹³ demonstrated, by propensity analysis, that on-pump patients were 1.8 times more at risk than off-pump patients. Kobayashi ¹⁴ reported only 1 (0.5%) perioperative stroke (attributed to postoperative atrial fibrillation) among 208 patients who underwent all arterial bypass using OPCAB techniques. Stroke prevention, especially in the elderly, is one of the most important unresolved issues in cardiac surgery today.

The safety and success of coronary artery bypass surgery has been validated by 35 years of scientific discovery and innovation. The development of off-pump coronary bypass techniques has disrupted conventional wisdom concerning the optimal method for myocardial revascularization. Nevertheless, the process of change has been productive in focusing attention on alternative methods to improve outcomes in specific patient groups. Off-pump surgery offers the advantages in low-risk patients of decreased cost, reduced length of stay, and avoidance of transfusion. In elderly, high-risk patients, OPCAB may reduce the risk of stroke, renal failure, prolonged respiratory assistance, and perhaps death. Cardiopulmonary bypass appears to offer advantages in patients with congestive heart failure, cardiomegaly, acute myocardial infarction, or coronary arteries that are technically difficult because of diffuse disease or inaccessible location.

In conclusion, we must ask, Which patients benefit most from off-pump coronary surgery? In what group of patients is OPCAB detrimental? Are complications truly avoided in high-risk patients? Are patient outcomes really improved in low-risk patients? Most of these questions remain unresolved and will require additional study with carefully designed, randomized clinical trials to establish the appropriate application for each method.