Asian Cardiovasc Thorac Ann 2008;16:226-230
© 2008 Asia Publishing EXchange Ltd
Predictors of Emergency Conversion to On-Pump During Off-Pump Coronary Surgery
Arman Hovakimyan, PhD,
Vilen Manukyan, PhD,
Sarkis Ghazaryan, PhD,
Merouzhan Saghatelyan, MD,
Lusine Abrahamyan, MD1,
Hagop Hovaguimian, MD
Nork-Marash Medical Center Yerevan, Armenia
1 Department of Health Policy, Management and Evaluation, University of Toronto, Toronto, Canada
For reprint information contact: Arman Hovakimyan, PhD Tel: 374 10 578 369 Fax: 374 10 578 369 Email: hovakimyan_ah{at}yahoo.com, Armenakyan Str. 13, Yerevan-0047, Armenia.
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ABSTRACT
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The purpose of this study was to determine predictors and evaluate outcomes of emergency conversion to cardiopulmonary bypass during planned off-pump coronary artery bypass grafting. From January 2001 to November 2005, of 467 consecutive patients aged 
60 years who underwent off-pump coronary surgery, 17 (3.6%) were converted to cardiopulmonary bypass. Those converted to an on-pump technique had significantly higher rates of postoperative cerebrovascular accident (17.6% vs 1.1%), intraaortic balloon pumping (5.9% vs 0%), and red blood cell transfusion (82.4% vs 57.3%), as well as prolonged intensive care unit stay (52.9% vs 25.2%), ventilation time (25% vs 5.3%) and hospital stay (64.7% vs 31.3%) compared to patients whose operation was completed off-pump. Multivariable logistic regression identified left ventricular ejection and left main stenosis as significantly associated with conversion. The rate of emergency conversion to cardiopulmonary bypass during planned off-pump coronary surgery was acceptable, but patients who required conversion had less favorable early outcomes than those who remained off-pump.
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INTRODUCTION
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Off-pump coronary artery bypass (OPCAB) has gained popularity because of many clinical and economic benefits over conventional coronary artery bypass grafting (CABG).1–3 However, occasionally during beating heart surgery the patient must be placed on cardiopulmonary bypass (CPB) to safely complete the operation.4 The reported incidence of conversion to CPB ranges from 1.1% to 22%.4–9 Several studies have shown that patients converted to CPB have higher mortality and complication rates than those who successfully completed OPCAB, while others demonstrated no increase in mortality.10–14 Predictors and outcomes in the converted patients have not been investigated in detail, thus the aim of this study was to determine the predictors and evaluate outcomes of emergency conversion to CPB during planned OPCAB.
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PATIENTS AND METHODS
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From January 2001 to November 2005, 467 consecutive patients aged 60 years underwent OPCAB in our institution. Our experience with OPCAB began in 1999. Before January 2001, we performed OPCAB without any categorical preferences. Following an internal evaluation of stroke rates after CABG with CPB determined that the incidence of postoperative cerebrovascular accident was higher in patients over 60 years of age, our strategy in this group has been to start the operation with a plan to perform it without CPB. Patients 
60-years old with atherosclerosis of the aorta and previous cerebrovascular accident also had OPCAB, but as they were small in number, they were not included in the study. During the study period, OPCAB was performed in approximately 30% of all procedures for isolated myocardial revascularization. Written informed consent was obtained from all patients included in the study. At the start of data collection, we did not have an established institutional review board; however, approval to conduct the study and access the data was obtained from the Medical Board of the hospital prior to initiating the study. The preoperative variables are shown in Table 1
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Emergency conversion was defined as use of CPB at any time during OPCAB. Impaired left ventricular function was defined as ejection fraction < 50%, prolonged ventilation time was > 24 hours of mechanical ventilation, prolonged intensive care unit stay was > 48 hours, and prolonged hospital stay was > 10 days after the operation. Other preoperative variables and complications were defined according to the Society of Thoracic Surgeons database definitions.
All operations were performed through a median sternotomy. For OPCAB, after the pericardium was opened, pericardial sutures were placed to facilitate pericardial retraction for cardiac elevation and exposure. A single dose of heparin (350 units·kg–1) was administrated. To reduce the amplitude of ventricular wall movement, a suction-type mechanical stabilizer (Octopus, Medtronic, Minneapolis, MN, USA) was employed. Internal vascular control was achieved with intracoronary shunts (FloCoil Shunt, CardioThoracic Systems Inc., Cupertino, CA, USA) or bulldog clamps. A blower/mister using CO2 (flow rate, 3–5 L·min–1) or a micro-sucker system was applied to obtain a bloodless surgical field. The proximal anastomosis on the ascending aorta was constructed using 6/0 polypropylene continuous sutures and a partial-occlusion clamp at a controlled systolic pressure of 60–70 mm Hg. If significant aortic calcification precluded safe clamp placement, proximal anastomoses were made at the side of the internal thoracic artery, or rarely, the right subclavian artery. At the end of the procedure, protamine was administered, and the chest was closed. Usually, the left internal thoracic artery was implanted first. The occluded or highly stenotic artery was bypassed before the collateral artery. Distal anastomoses were usually constructed before proximal anastomoses. Whether each proximal anastomosis was constructed after the respective distal anastomosis, or all proximal anastomoses were placed after completion of all distal anastomoses, depended on the operating surgeon. Hypotension resulting from cardiac manipulation was corrected by volume infusion, patient positioning or intravenous inotropic agents, to avoid conversion. Intraluminal coronary shunts, external pacing, pursestring sutures prior to the procedure and a "dry ready" CPB machine were deployed as necessary. A perfusionist remained on standby during all OPCAB procedures.
For on-pump CABG, patients were heparinized with an initial dose of 350 units·kg–1, periodically supplemented with additional doses to maintain an activated clotting time > 400 sec. Conventional CABG was performed with standard cannulation for CPB (ascending aortic and dual-stage cannulation of the right atrium), moderate systemic hypothermia and intermittent ischemic fibrillatory arrest. Distal anastomoses were undertaken with or without an aortic cross clamp, depending on the surgeon’s preference. All proximal anastomoses were carried out with an aortic side-biting clamp in place. At the end of the procedure, heparin was reversed with protamine.
Data were obtained retrospectively from our surgical database and patients’ charts, and analyzed using SPSS version 11.5 software (SPSS Inc, Chicago, IL, USA) and Stata version 7.0 for Windows (Stata Corporation, College Station, TX, USA). Results of continuous variables are expressed as mean ± standard deviation. Results of categorical variables are expressed as frequencies and percentages. Categorical data were compared using Pearson’s chi-square test or Fisher’s exact test, as appropriate. Distributions of continuous data were observed, and t tests or Mann-Whitney tests were performed accordingly for comparisons of groups. Logistic regression analysis was used to determine important independent predictors of conversion to CPB. To examine the independent effect of an individual variable while controlling for the others, multiple logistic regression analysis was conducted using a forward stepwise selection method. We tested our model’s discrimination via the receiver operating characteristic curve, and assessed the calibration of the prediction model (Hosmer-Lemeshow goodness-of-fit statistic).
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RESULTS
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During the study period, 450 operations were completed off-pump, and 17 patients required CPB. The overall conversion rate was 3.6%. Patients were converted from OPCAB to CABG with CPB because of hemodynamic instability in 13 (76.5%), cardiac arrest in 2 (11.8%) and arrhythmia in 2 (11.8%). Fifteen patients were converted during attempts at coronary anastomosis: left anterior descending coronary system in 9 (60%), circumflex in 5 (33.3%) and right coronary artery system in 1 (6.7%). Impaired ventricular function, left main stenosis, and urgent or emergency surgery were more prevalent in converted patients than in the OPCAB group (Table 1
). The overall complications rate was similar between groups (Table 2). However, some significant differences in individual complications were identified. Converted patients had higher incidences of postoperative cerebrovascular accident, intraaortic balloon pumping, transfusion of red blood cells, prolonged intensive care unit stay, ventilation and hospital stay compared to OPCAB patients. Converted patients had almost 5-times higher hospital mortality, 3-times more renal failure, and almost twice the rates of reexploration for bleeding and wound infection than OPCAB patients; however, none of these differences reached statistical significance. All 18 pre and intraoperative variables were entered into univariate analysis using logistic regression. Multivariate logistic regression analysis revealed 2 independent predictors of conversion to CPB: left ventricular ejection fraction and left main stenosis (Table 3). The final model demonstrated relatively strong discriminatory ability (area under receiver operating characteristic curve, 0.77). The Hosmer-Lemeshow goodness-of-fit statistic was not significant ( p = 0.48), indicating no significant departure from a perfect fit. The annual rates of conversion showed a tendency to decrease over time: 5.5%, 6.3%, 3.8%, 2.5%, 1.1% in the 1st–5th years (2001–2005) of experience (Figure 1).
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DISCUSSION
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Off-pump operations have become increasingly popular because of their perceived potential to reduce the morbidity of CABG. Recent results of OPCAB in low and high-risk patients showed better results than operations using CPB.15–17 Nevertheless, there are inevitably situations when it is necessary to go on CPB during OPCAB surgery. Our overall conversion rate of 3.6% is within the reported range of 1.1% to 22%.4–9 Hemodynamic instability, electrical disturbances and anatomic problems are well-reported reasons fo conversion.4,5 The most frequent cause of conversion in our study was hemodynamic instability (76.4%), while cardiac arrest and arrhythmia necessitated conversion in the other cases. Probably all reasons for conversion to CPB ultimately result in secondary hypotension that may cause brain, myocardial or renal injury.
Fifteen patients in our study were converted during attempted coronary anastomosis; the majority on the left anterior descending coronary and circumflex arteries. Only 6.7% of conversions occurred during right coronary grafting, similar to the findings of Patel and colleagues.5 We also think that grafting the left coronary artery system first in most patients allowed adequate perfusion of the myocardium while the right coronary was occluded for grafting.
There are few reports addressing the predictors of conversion. Edgerton and colleagues4 noted that independent predictors for conversion were surgeon’s experience, previous CABG and congestive heart failure. Legare and colleagues10 determined the only predictive factor to be high body mass index, whereas Patel and colleagues5 could find no predictors of conversion to CPB. Ejection fraction and left main stenosis were independent predictors for emergency conversion to CPB during OPCAB in our study. Patients with left main coronary artery stenosis have always caused anxiety. Dewey and colleagues18 showed that left main coronary disease is not a contraindication to off-pump techniques. The procedure is well tolerated in these patients according to Vassiliades and colleagues19 because the left internal mammary artery is grafted early to the left anterior descending artery before any substantial manipulation of the heart. To the best of our knowledge, there have been no studies showing left main disease as a risk factor for conversion to CPB. Our small sample size did not allow making valid conclusions, but this hypothesis could be further checked in larger studies.
Our annual rate of conversion showed a decreasing trend, which we think may be explained by the improved experience of the entire surgical team. Edgerton and colleagues4 and Reeves and colleagues20 found a correlation between surgeon experience and the need for conversion. Although our annual rates of emergency conversion decreased throughout the study, the overall rate of conversion is within the current range. We found a high rate of adverse outcomes in patients converted to CPB, as shown by others.10–13 Clinically important differences were observed between the groups with respect to hospital mortality, renal failure, reexploration for bleeding and wound infection, we believe that statistical significance would be reached if we had a larger sample size.
The results of our study should be interpreted with caution in view of several limitations. In accordance with our OPCAB policy, we included only patients aged 60 years and older. Furthermore, this was a retrospective study with a limited number of patients and a small number of outcome events, which could limit the power of the analysis and increase type II error. The patients in each group were operated on by 2 surgeons and 3 anesthesiologists; however, we collected all available data of OPCAB patients operated on at a single cardiac surgical center within the past 5 years. The results might be included in a meta-analysis, considering the fact that conversion is a rare event in OPCAB surgery. It was concluded that the rate of emergency conversion to CPB during planned OPCAB surgery was acceptable, although patients who required conversion had less favorable early outcomes than those who remained off-pump.
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ABSTRACT
INTRODUCTION
