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Ann Thorac Surg 2004;78:26-33
© 2004 The Society of Thoracic Surgeons
a Bristol Heart Institute, University of Bristol, Bristol, United Kingdom
b Health Services Research Unit, London School of Hygiene and Tropical Medicine, London, United Kingdom
Accepted for publication October 28, 2003.
* Address reprint requests to Dr Ascione, Bristol Heart Institute, University of Bristol, Bristol Royal Infirmary, Bristol BS2 8HW, UK, UK
e-mail: r.ascione{at}bristol.ac.uk
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Abstract |
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METHODS: Data for consecutive patients undergoing off-pump coronary artery bypass grafting were collected prospectively. Patients satisfying at least one of the following criteria were classified as high-risk: age older than 75 years, ejection fraction less than 0.30, myocardial infarction in the previous month, current congestive heart failure, previous cerebrovascular accident, creatinine greater than 150 µmol/L, respiratory impairment, peripheral vascular disease, previous cardiac surgery, and left main stem stenosis greater than 50%. Early morbidity, 30-day mortality, and late survival were compared.
RESULTS: From April 1996 to December 2002, 686 high-risk patients underwent off-pump coronary artery bypass grafting revascularization. Operations by five consultants (416; 61%) and four trainees (239; 35%) were the focus of subsequent analyses. Nine visiting or research fellows performed the other 31 operations. Prognostic factors were more favorable in trainee-led operations. On average, consultants and trainees grafted the same number of vessels. There were 18 (4.3%) and 5 (1.9%) deaths within 30 days, and 14 (3.4%) and 5 (1.9%) myocardial infarctions in consultant and trainee groups, respectively. After adjusting for imbalances in prognostic factors, odd ratios for almost all adverse outcomes implied no increased risk with trainee operators, although patients operated on by trainees had longer postoperative stays and were more likely to have a red blood cell transfusion. Kaplan-Meier cumulative mortality estimates at 24-month follow-up were 10.5% (95% confidence interval, 7.7% to 14.2%) and 6.4% (95% confidence interval, 3.8% to 10.9%) in consultant and trainee groups, respectively (hazard ratio = 0.60 [95% confidence interval, 0.37 to 0.99]; p = 0.05).
CONCLUSIONS: Off-pump coronary artery bypass grafting surgery in high-risk patients can be safely performed by trainees.
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Introduction |
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| This article has been selected for the open discussion forum on the CTSNet Web site: http://www.ctsnet.org/discuss
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The reproducibility of a surgical technique is of paramount importance for its widespread use. Off-pump oronary artery bypass grafting (OPCAB) has gained popularity during the last decade, and many reports have highlighted its potential to improve in-hospital outcome compared with conventional techniques [1–5]. Off-pump coronary artery bypass grafting surgery represents a technique that differs substantially from the principles of coronary artery bypass grafting (CABG) using cardiopulmonary bypass. It requires the construction of anastomoses on a moving coronary target that are as effective as anastomoses constructed on a stationary target. Lack of experience of the technique may put off even the most experienced conventional CABG surgeon from using OPCAB surgery and dissuade experienced OPCAB surgeons from teaching the technique to trainees.
The effectiveness of OPCAB surgery has been investigated at our institution in both elective and high-risk patients [6–10]. As the results of our randomized controlled trials in elective patients became apparent [6–9], we started to perform an increasing proportion of operations in high-risk patients using OPCAB, assuming that such patients had even more to gain than elective patients from the less invasive technique. This policy was then extended to our cardiothoracic training program.
The gradual increase in the use of OPCAB surgery in high-risk patients and the introduction of OPCAB to the teaching program have been monitored but not formally evaluated. Therefore, this study aimed to compare early and mid-term clinical outcomes for OPCAB operations performed in high-risk patients by consultants and trainees.
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Material and methods |
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Data were extracted for patients with one or more of the following baseline characteristics, which are widely accepted as risk factors for early mortality or morbidity: age older than 75 years, ejection fraction less than 0.30, myocardial infarction (MI) in the previous month, current congestive cardiac failure, previous cerebrovascular accident, creatinine greater than 150 µmol/L, respiratory impairment (current chronic obstructive airways disease or asthma), peripheral vascular disease, and previous cardiac surgery requiring a sternotomy. Patients with one or more of these factors that are widely accepted to confer an increased risk of perioperative morbidity and mortality were designated as being at high risk.
Data for all high-risk patients undergoing OPCAB were extracted from the database. Salvage and emergency operations were excluded from the analysis because trainees rarely carry out such operations. Operations performed by locum surgeons or clinical assistants, by visiting or research clinical fellows (<2-year visit/placement; VRFs), and by consultants who had performed fewer than 25 OPCAB operations in high-risk patients, ie, who have not adopted OPCAB as their preferred technique, were also excluded from the main analyses. Two trainees with national training numbers performing OPCAB operations on high-risk patients were in years 3 to 6 of the UK specialist training program in cardiothoracic surgery (1 year of which is often spent conducting research). The 2 other trainees were supernumerary and were at an equivalent stage in their training. The data set for main analyses included consecutive high-risk OPCAB operations for the 4 trainees, whether supervised or unsupervised, and the selected consultants. Consultant supervision was defined as a case in which the consultant was scrubbed and acted as first assistant [11]; operations by trainees were defined as unsupervised when the consultant responsible was not scrubbed.
Anesthetic and operative technique
Anesthetic technique and heparin management were standardized and have previously been reported [8]. The target vessel was exposed and snared above the anastomotic site using a 4-0 Prolene suture (Ethicon, Somerville, NJ) with a soft plastic snugger. The coronary artery was then opened, an intracoronary shunt was systematically used, and the anastomosis performed using a pressure stabilizer [12].
Postoperative management
At the end of surgery, patients were transferred to the intensive care unit. Patients were extubated as soon as they met the following criteria: hemodynamic stability, no excessive bleeding (<80 mL/h), normothermia, and consciousness with pain control. Fluid management postoperatively consisted of 5% dextrose infused at 1 mL · kg–1 · h–1, with additional colloidal plasma volume substitute or blood to maintain normovolemia and hematocrit greater than 24%. Potassium and magnesium deficiencies were promptly treated as necessary to maintain electrolyte balance within the normal range [6].
Clinical data collection, monitoring, and definitions
Data characterizing perioperative clinical outcomes were collected prospectively and entered into the Patient Analysis and Tracking System. Thirty-day mortality was defined as any death that occurred within 30 days of operation, whether in or out of hospital, or deaths of patients in hospital after this if they had remained in hospital until death. Perioperative myocardial infarction, ST-segment changes, pacing, and arrhythmias were recorded and defined as previously reported [6]. Need for postoperative hemodynamic support included use of inotropic agents, intraaortic balloon pump, ventricular assist device, or pulmonary catheter. Pulmonary complications included chest infection, ventilation failure, reintubation, and tracheostomy [9]. Postoperative blood loss was defined as total chest tube drainage [8]. Neurologic complications included permanent and transient strokes [13]. Renal complications included acute renal failure requiring hemodialysis. Finally, infective complications included septicemia and sternal and leg wound infections as defined by positive culture and requiring antibiotic therapy [9].
Deaths and dates of death were obtained by linking the clinician for patients in the study population with the UK national population register (through the National Health Service Stategic Tracing Service). Data for all patients in this study were successfully linked to the National Health Service Stategic Tracing Servicedatabase.
Management of discharge
We aim to discharge patients undergoing coronary artery bypass surgery on the fifth postoperative day. The suitability of patients to be discharged either home or to another hospital for further convalescence is made according to a local protocol. The decision to discharge is based on a satisfactory routine checkup on day 4 consisting of clinical examination, complete blood count, urea and electrolytes, electrocardiograph, and chest roentgenograph. If the patient is medically unfit on day 5, hospitalization is prolonged and further investigations may be performed depending on the clinical status.
Statistical analysis
More variables are available in the Patient Analysis and Tracking System database than are described here. A decision about the key variables of interest to investigate was made before conducting any analyses. All variables selected at the outset for investigation are listed in Tables 1 and 2.
All operation notes for patients who underwent CABG during the study period were reviewed to identify acute conversions from OPCAB to conventional CABG because of electrical or hemodynamic instability. These operations were then coded as OPCAB for the analyses, ie, according to the principle of intention-to-treat.
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When there were sufficient events, analyses also considered whether trainee operations were (1) supervised or unsupervised, or (2) performed by VRFs. These additional analyses are only reported if there appeared to be discrepancies in the findings for these subgroups. The large number of prognostic variables and outcomes of interest resulted in many statistical comparisons. No correction was made for multiple comparisons. However, our interpretation of the findings takes into account the consistency of the findings and their magnitude, as well as their statistical significance.
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Results |
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Thirty-day mortality rates were 4.3% (18 deaths) and 1.7% (4 deaths) for the consultant and trainee groups, respectively. Perioperative MI occurred in 14 (3.4%) and 2 (0.8%) patients in the consultant and trainee groups, respectively. The frequency of most early adverse outcomes was lower or the same in trainee-led operations as in consultant-led operations.
Unadjusted and adjusted ORs comparing the odds of an outcome in consultant-led versus trainee-led operations are shown in Table 4. Most OR estimates were close to unity or were substantially less than unity, indicating a tendency for outcomes to be at least as good in trainee as consultant OPCAB operations. Estimates tended to shift upward with adjustment for confounding, reflecting the fact that patients operated on by trainees had, on average, a more-favorable risk profile. Total postoperative stay longer than 7 days was an exception, with patients operated on by trainees having a higher risk of this outcome, a finding that remained significant after adjustment for confounding.
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We also investigated the risk of outcomes among operations performed by VRFs. These analyses had very low power because there were few such operations, and analyses were not possible for some outcomes because there were no events. The risk of adverse outcomes tended to be higher in operations performed by VRFs than for operations performed by consultants (ORs ranging from 0.8 to 2.8 except for perioperative MI OR, 9.3), but the increased risk was only statistically significant for perioperative MI (p = 0.001); perioperative MIs were recorded in 3 of 31 (9.7%) operations performed by VRFs.
The median postoperative follow-up times for the consultant and trainee groups were 1.7 years and 1.6 years, respectively. Kaplan-Meier cumulative mortality curves for the two groups are shown in Figure 2. The curve for operations led by trainees was consistently below that for consultant operations, and the unadjusted hazard ratio was 0.62 (95% confidence interval, 0.37 to 1.04; p = 0.07). Cumulative mortality after 2-year follow-up was estimated to be 10.5% (95% confidence interval, 7.7% to 14.2%) and 6.6% (95% confidence interval, 3.8% to 11.2%) for consultant and trainee groups, respectively. The hazard ratio shifted slightly toward unity with adjustment for confounding equal to 0.70 (95% confidence interval, 0.37 to 1.33; p = 0.28).
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Comment |
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The dilemma is particularly evident with respect to teaching trainees to perform OPCAB surgery. The effectiveness of OPCAB compared with conventional surgery has been demonstrated by randomized controlled trials, and the applicability of the technique has been supported by large cohort studies of consecutive and high-risk groups of patients [1–10]. However, widespread acceptance of a new surgical technique also depends on its reproducibility and on the feasibility of teaching the technique to the next generation of surgeons. A recent survey [16] from several cardiothoracic training centers in the United States demonstrated that only 22% of residents had performed 20 or more OPCAB procedures during their training, and only 12% had performed 20 or more complete myocardial revascularizations using OPCAB. Of these, only 4% had performed OPCAB circumflex coronary artery revascularization. Trainees' experience of high-risk OPCAB procedures may be further reduced because of the overriding responsibility of the consultant to ensure the safety of patients.
The feasibility of teaching OPCAB surgery has already been described but mostly in the context of elective patients [17–19]. This study summarizes the experience of a single institution in training cardiothoracic residents to perform OPCAB surgery in consecutive high-risk patients. There were three main findings: (1) there was no evidence that patients operated on by trainees had greater mortality or morbidity than patients operated on by consultants—if anything, the effect estimates suggested that adverse outcomes were less frequent in trainee-led operations; (2) there was little evidence of any difference in outcome for supervised and unsupervised trainees; and (3) the rates of adverse outcomes are relatively low for a high-risk group.
It is unlikely that the study has missed an increased risk of adverse outcomes in trainee-led operations because of chance. However, it is important to consider other limitations of the study when interpreting the findings.
The study was necessarily observational both for logistical and ethical reasons. It is very difficult to characterize fully the way in which cases are selected by consultants for trainees. Consultant surgeons are likely to use more subtle clinical signs to select cases for trainees than can be readily documented by formal data collection. Residual confounding is therefore likely to be present, both because the preoperative risk of a poor outcome was inadequately characterized and because analyses can never perfectly adjust even for prognostic factors that are measured. These limitations would cause the effect estimates to be biased in favor of trainee-led operations. However, it is unlikely that residual confounding could reverse the true direction of association.
Two important pieces of information were not documented in the database, namely conversions from OPCAB to conventional CABG surgery and trainee to consultant designation as main operator. We reviewed all operation notes during the study period to identify conversions from OPCAB to conventional CABG surgery, and such operations are included in the analysis. A change in the designation of trainee to consultant for especially high-risk patients, or patients who had poor outcomes because of complications, would have led to the performance of trainees being overestimated. We do not believe that this occurred to a significant extent. Neither consultants nor trainees could recall any such instances, and, consequently, we conclude that changes in designation in the main operator were extremely rare.
However, trainees did volunteer that during the early stages of their training, the consultant may have intervened in one of two ways. The consultant may have taken over in a planned manner to perform one or more anastomoses, for example a circumflex artery graft; trainees thought that this might have occurred up to five times. Alternatively, the consultant may have intervened to show the trainee how to position the heart to gain access to carry out the required distal anastomosis; in these instances, the trainee then performed the anastomosis.
It is possible that knowledge of the main operator could have caused bias in the management of patients. For example, this might be the reason why patients operated on by unsupervised trainees had a longer than average postoperative hospital stay, ie, these patients were kept in the hospital for an extra day or two, to be on the safe side. It is notable that the finding for postoperative stay contradicts findings for other outcomes; it is difficult to understand why postoperative stay should be longer when the adverse effects that usually lead to longer stays tended to be less frequent. It is also possible that knowledge of operator could have caused bias in the recording of outcome data. However, none of the health-care staff were aware during the study that the comparison was going to be made, and strict local guidelines are used to make decisions about perioperative clinical management. These features of the study would have minimized the opportunity for bias.
Therefore, there are two main explanations for trainees appearing to have better results: (1) residual confounding (see above); and (2) the learning curve for OPCAB, which includes elements both of learning the best way to apply the technique and learning the manual dexterity; the first of these elements mainly applied to the consultants as they developed the technique before starting to teach it to trainees. The findings probably arise from a combination of these explanations, all of which would have caused bias favoring trainee-led operations.
We had no strong hypothesis about supervised versus unsupervised trainee-led operations. On the one hand, supervised trainees are likely to be less experienced but consultants are on hand to advise in the event of a difficulty. On the other hand, unsupervised trainees, although more experienced, may have been more reluctant to call on consultants for help. The findings tend to support the latter interpretation. Operations performed by VRFs, almost all supervised, appeared to be associated with an increased risk of adverse outcomes, especially for perioperative MI. Although data were only available for a few operations, this finding needs further investigation.
In conclusion, our data show that high-risk OPCAB procedures can be safely performed by cardiothoracic trainees in a recognized training program. Early and mid-term outcomes are as good for patients operated on by these trainees as for patients operated on by consultants. The small amount of information available for trainees who are not in a recognized training program raises concern about the potential dangers of informal training. We strongly recommend the development of recognized teaching programs to address these concerns and the organization of dedicated audit systems.
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