Hemli, Jonathan M. MD; Patel, Nirav C. MD; Subramanian, Valavanur A. MD
It is well established that patients who require emergency intraoperative conversion to cardiopulmonary bypass during attempted off-pump coronary surgery incur significantly higher mortality and morbidity than do matched patients who undergo successful completed off-pump or on-pump revascularization.1 The incidence of emergency conversion does depend, to some extent, on the off-pump experience of the operating surgeon, and it tends to gradually decrease over time as the operator becomes more facile with the technical demands of an off-pump procedure. Although the frequency of emergency conversion seems to be inversely correlated with surgical experience, it remains unclear as to whether growing surgical skill is associated with any appreciable change in the burden of morbidity that seems inherent to emergency conversion. We sought to examine our longitudinal experience with off-pump coronary grafting to specifically determine whether patients who require emergency conversion today are at risk for similar adverse outcomes as patients who required conversion in our institution more than a decade ago.
Between January 1999 and December 2010, 4763 consecutive patients underwent isolated coronary artery bypass surgery at our institution. A primary off-pump revascularization strategy was attempted in 4415 cases (92.7%) at the discretion of the operating surgeon. All patients included in our analysis underwent coronary surgery via median sternotomy. Reoperative procedures were included in our data. Patients who had another procedure concomitant with coronary grafting (such as valve or thoracic aortic surgery) were excluded as were patients who had surgery via a nonsternotomy approach.
During the study period, 100 patients required emergency intraoperative conversion to cardiopulmonary bypass (CPB). These 100 patients were divided into two sequential groups of 50 patients each (group 1 and group 2). The profile and outcomes of the more recent 50 patients (group 2) were compared with those of the preceding 50 patients (group 1). Results were also contrasted with those of patients who had successful off-pump coronary surgery (n = 2737) during the same period and with a contemporaneous cohort of patients who had on-pump surgery as their primary treatment modality (n = 268).
Our technique of off-pump coronary revascularization has been reported previously.2 After median sternotomy, all conduits were harvested and prepared, and intravenous heparin was administered to maintain an activated clotting time longer than 300 seconds. Cardiac exposure was facilitated by the use of deep pericardial traction sutures, Trendelenburg position, and right and left table-tilt, as required, as well as with the assistance of an apical suction cardiac positioning device (Xpose; Guidant Corporation, Cupertino, CA USA). A compression or suction myocardial stabilization system (surgeon’s choice) was used during construction of the distal coronary anastomoses. During the anastomoses themselves, intracoronary shunts were used routinely, as were proximal (and variably, distal) silicone rubber vessel loops (Silastic; Dow Corning, Midland, MI USA) to maintain vascular control. A humidified carbon dioxide blower system was used to improve visualization at the operative field.
The sequence of coronary grafting was at the discretion of the operating surgeon but almost invariably entailed completing the left internal mammary artery to left anterior descending coronary artery graft first. Proximal anastomoses were performed either before or after the remaining distal anastomoses, depending on the particular situation. Composite graft construction, distal anastomotic connection devices, clampless aortic seals, or a partially occluding aortic clamp were all variably used depending on the quality of the patient’s ascending aorta, nature of the distal targets, and the individual’s coronary anatomy.
The heparin dose was usually completely reversed after completion of all grafts. Graft flow and patency were assessed using a transit time flow measurement system (Medistim VeriQ; Medistim USA Inc, Plymouth, MI USA) before chest closure, and, if necessary, further intraoperative graft interrogation was performed using indocyanine green on-table angiography (SPY, Novadaq Technologies, Inc, Mississauga, ON, Canada). All patients were maintained on dual antiplatelet therapy postoperatively (aspirin and clopidogrel) unless specifically contraindicated.
Conversion to CPB
When conversion to CPB was deemed necessary, additional heparin was administered to achieve an activated clotting time of at least 480 seconds. The distal ascending aorta was the most common site for arterial cannulation. Venous drainage was usually accomplished via a dual-stage cannula passed through the right atrial appendage. The operation was then completed either on the beating or arrested heart at the discretion of the surgeon. “Emergency conversion” was defined as the use of CPB as a consequence of hemodynamic compromise, refractory myocardial ischemia, ongoing hemorrhage, or cardiac arrest. Patients who were placed on CPB in a “semielective” fashion because of the nature of their coronary anatomy (diffusely diseased vessels, small-caliber targets, intramyocardial vessels) were excluded from this definition.
Patients were identified from our prospectively collected cardiac surgical registry, maintained in accordance with New York State Cardiac Registry Reporting System guidelines. Approval for data review was obtained from our institutional review board.
Definitions of patient demographic characteristics, preoperative variables, and postoperative outcomes are obtained from the New York State Cardiac Surgery Reporting System and can be referenced online at the New York State Department of Health Web site at http://www.health.ny.gov/forms/cardiac_surgery/. Data variables were analyzed using SPSS for Windows statistical software package, version 16.0 (SPSS Inc; Chicago, IL USA). Analysis was performed using χ2 tests for categorical variables, t tests for continuous variables, and F tests to compare variance. Statistical significance was defined as P < 0.05.
During the study period, off-pump coronary revascularization was attempted in 4415 patients, representing 92.7% of all patients undergoing coronary surgery in our institution during that time frame. One hundred patients required intraoperative emergency conversion to CPB, an overall incidence of 2.27%. In an attempt to elucidate the impact of increasing surgical experience on patient outcomes, we divided these 100 patients into two sequential groups of 50 patients each (group 1 and group 2). Group 1 had coronary surgery between January 1999 and June 2002, and group 2 had surgery some time between July 2002 and December 2010.
Preoperative characteristics of those patients who required emergency conversion are outlined in Table 1. Both patient groups were largely similar in risk profile, although a higher proportion of patients in the earlier cohort (group 1) presented with symptoms of congestive heart failure, primarily dyspnea on exertion.
The patients in group 2 were also broadly similar in profile to patients who underwent either successful off-pump (off-pump coronary artery bypass [OPCAB]) or on-pump (coronary artery bypass grafting [CABG]) coronary surgery during the same period (Table 2). Patients who had successful off-pump surgery without emergency conversion were less likely to have moderate to severely impaired left ventricular function, and they were less prone to present with symptoms referable to congestive heart failure.
During the entire 11-year study period, there was a trend in decreasing incidence of emergency conversion to CPB from 4.8% in 1999 to 1.3% in 2010, with a nadir frequency of 0.4% in 2009 (Fig. 1). Group 1 represents an incidence of emergency conversion of 2.97% (January 1999–June 2002) as compared with 1.77% for group 2 (July 2002–December 2010).
There was no significant difference in indication for emergency conversion between groups 1 and 2 (Fig. 2). Most patients in both groups required emergency institution of CPB because of hemodynamic compromise during the procedure, usually as a consequence of myocardial ischemia.
Based on multivariate regression analysis, the only significant predictor for emergency conversion to CPB was compromised left ventricular function (defined as left ventricular ejection fraction <40%) that was more strongly predictive for group 2 than for group 1 (Table 3).
As summarized in Table 4, the postoperative outcomes of all patients who required emergency conversion were largely unchanged during the entire course of the study period. Although there seems to be a slight trend toward fewer complications in the more recent group of converted patients (group 2) as compared with those converted patients operated on in the years preceding (group 1), none of these variables reached statistical significance when subjected to rigorous analysis.
On the other hand, when compared with contemporaneous patients who underwent successful OPCAB, the more recent 50 conversion patients (group 2) had significantly higher in-hospital mortality (8.0% vs 0.9%, P = 0.002) and a lower proportion of these patients were free from all complications during the postoperative period (72.0% vs 92.2%, P < 0.0001) (Table 5). The converted patients had a higher incidence of mediastinitis (6.0% vs 1.4%, P = 0.036), postoperative bleeding requiring surgical reexploration (12.0% vs 2.7%, P = 0.003), sepsis of any cause (8.0% vs 0.7%, P = 0.001), and respiratory failure (14.0% vs 2.1%, P < 0.0001). There was also a trend toward more frequent new-onset renal failure in the converted patients (4.0% vs 1.1%).
Similarly, the converted cohort (group 2) demonstrated a higher mortality rate than patients who had successful primary on-pump revascularization (CABG) during the same period (8.0% vs 2.2%, P = 0.055) (Table 5). The converted patients also seemed to have a higher incidence of sternal wound infection, postoperative hemorrhage, new renal failure, and respiratory failure than did the CABG group. Postoperative sepsis of any cause was also more common in the conversion cohort than in the CABG patients (8.0% vs 0.4%, P = 0.002).
Intraoperative conversion to CPB remains a distinct possibility in any attempted off-pump coronary revascularization procedure. In discussing conversion to CPB during off-pump surgery, it is important to distinguish between true emergency conversion and a more elective or semielective initiation of CPB. In our practice, emergency conversion is defined as a mandatory institution of CPB to salvage clinical stability of the patient as opposed to an elective or considered decision to use extracorporeal support because of unfavorable poor distal target vessels. We have restricted our analysis to those patients who required true emergency conversion to CPB in an attempt to highlight the real morbidity inherent in this process. For analysis purposes, patients in our practice who were semielectively placed on CPB support during off-pump surgery were included in the group of patients who had successful OPCAB surgery in its entirety on an intention-to-treat basis because their outcomes are largely similar, a finding that has been echoed by others.3,4
The overall incidence of emergency conversion in our study was 2.27%, representing 100 patients within an 11-year period. Our rate of conversion seems comparable to that of other available data, published emergency conversion rates varying anywhere between 1.6% and 9.5%.5–11 We did notice a decrease in the need for emergency conversion over time: from 4.8% in 1999 to 0.9% in 2009 and 1.3% in 2010 (Fig. 1). This is likely reflective of improving surgeon experience with off-pump surgery, more appropriate targeted preoperative patient selection for off-pump versus on-pump techniques, and our increasing use of available technology to facilitate a smoother, less technically demanding off-pump operation (eg, apical suction cardiac positioning devices, routine use of intracoronary shunts, etc).2 The relationship between the learning curve in OPCAB surgery and the incidence of emergency conversion has been examined by others,5,12 and our experience is indeed reflective of this.
Interestingly, although emergency conversions have become slowly less common in our institution as our experience with OPCAB techniques has continued to grow, the pattern of intraoperative conversion has largely remained unchanged over time. By far, the most common reason to initiate CPB during our OPCAB procedures was the presence of hemodynamic compromise almost invariably as a consequence of myocardial ischemia (Fig. 2). Other authors13–15 have also found this to be the most common reason for emergency conversion. Moreover, we have found that the timing of emergency conversion remains largely unpredictable irrespective of our evolving surgical experience. As demonstrated in Figure 3, emergency conversion was required for some of our patients at almost any stage of the off-pump operation, and this did not seem to be significantly different between the two successive groups of 50 converted patients each.
The only significant predictor of conversion in our patient population was “impaired left ventricular function” defined as an ejection fraction of less than 40%, and even this only reached statistical significance in our more recent group of conversions (group 2). Numerous other factors have been variably found by other authors to be predictive of conversion,1,11,16 but we did not find this to be the case (Table 3). This further highlights the point that emergency conversion during OPCAB surgery remains an unpredictable phenomenon. Moreover, interestingly, perhaps contrary to what one might otherwise expect, we did not find even significant left main stem stenosis to be a predictor of the need to emergently initiate CPB during the operation. In the setting of tight left main coronary disease, we always complete the left internal mammary artery to left anterior descending coronary artery graft first. This graft requires less aggressive cardiac manipulation, and the early restored left anterior descending coronary artery flow may help mitigate the malignancy of the left main disease and facilitate subsequent off-pump revascularization of the other coronary territories.
The increased mortality incurred by emergency intraoperative conversion to CPB is well established,17 and, indeed, we have previously described our experience with this.2 Our converted patients had an in-hospital mortality rate of 12.0% (group 1) and 8.0% (group 2) as compared with 0.9% and 2.2% for successfully completed OPCAB and CABG procedures, respectively (Fig. 4). A recent comprehensive review of issues related to conversion in off-pump surgery reported an in-hospital mortality rate in the literature ranging from 5.4% to 32.1% for emergency conversion patients as compared with 0% to 3.6% for matched unconverted OPCAB and CABG patients, respectively.18 More importantly, we have found that the mortality risk inherent in emergency conversion has largely remained unchanged over time. Although we may have demonstrated a slight trend toward improved survival between group 1 and group 2, this did not reach statistical significance, and, indeed, the in-hospital mortality rate of the more recently converted patients in our study (group 2) remains quite substantial despite our increased surgical experience and attendant improvements in perioperative care.
Patients who survive emergency conversion are still at risk for significant morbidity (Table 5; Fig. 5). Our most recent group of 50 emergency conversions (group 2) sustained significantly higher rates of mediastinitis, postoperative bleeding requiring surgical reexploration, respiratory failure, and sepsis of any cause than did a contemporaneous group of patients who had successful OPCAB surgery during the same time frame. There was also a trend toward more frequent renal failure and gastrointestinal complications in the converted patients, although these differences did not reach statistically significant levels. The converted patients also tended to develop more complications than did a time-matched cohort of on-pump (CABG) patients, although the statistical power of this comparison was not as impressive. These increased complications have all been well described in association with the need for emergency conversion.1,7–10,18,19 It is important to emphasize, however, that the incidence and pattern of complications in our patients did not differ significantly between groups 1 and 2. We have found that, not unlike mortality, the morbidity of emergency conversion remains considerable, and that it does not seem to diminish over time in parallel with the growing experience of the institution.
To our knowledge, this is one of the first reports to directly compare outcomes in emergency conversion between two distinct time groups in a single center in an attempt to delineate the impact of increasing surgical experience on the pattern of morbidity in these patients. The intrinsic limitations of the retrospective nature of our review may be somewhat offset by the fact that the same surgeons were the primary operators throughout the time frame of our study, thus potentially mitigating any selection bias in our data set. We have demonstrated that the morbidity and mortality of emergency intraoperative conversion to CPB remain substantial even after more than a decade of routine off-pump coronary surgery at our institution. Emergency conversion continues to be an unpredictable intraoperative event, can occur at any stage of the operation, and not uncommonly portends poorer results. These adverse outcomes are not mitigated by increasing surgical experience, and they remain significantly worse than comparable results of successfully completed off-pump or on-pump myocardial revascularization. Safer and more consistent bailout strategies during off-pump coronary grafting are still required to reduce the burden of emergency conversion irrespective of the experience of the surgeon.
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This is another in a series of articles from the group at Lenox Hill Hospital in New York on off-pump coronary artery bypass grafting. In this report, they looked at their experience with emergency conversion over time. From 1999 to 2010, more than 4400 patients underwent off-pump coronary artery bypass grafting. The emergency conversion rate was just over 2%. They compared their first 50 conversions with their subsequent 50 patients. Over time, the emergency conversion rate decreased from 3% to 1.8%. However, the morbidity in these patients remained essentially unchanged. The outcomes in these patients was worse than those who did not undergo conversion.
This study indicates the need for improving patient selection for off-pump coronary artery bypass grafting to avoid conversion. Safer and more consistent bailout strategies are required to reduce the morbidity of emergency conversion. The limitations of this study are its retrospective nature and the relatively small number of patients undergoing conversion. The strengths of this study are the vast experience of this group in off-pump coronary artery bypass grafting and the fact that the same surgeons were the primary operators throughout the time frame of this study.
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