CLINICAL STUDIES
Operator volume and outcome of patients undergoing coronary stent placement
Adnan Kastrati, MDa,
Franz-Josef Neumann, MDa and
Albert Schömig, MDa
a Deutsches Herzzentrum and 1. Medizinische Klinik rechts der Isar, Technische Universität München, Munich, Germany
Manuscript received January 28, 1998;
revised manuscript received April 20, 1998,
accepted June 1, 1998.
Address for correspondence: Dr. Adnan Kastrati, Deutsches Herzzentrum München, Lazarettstrasse 36, 80636 München, Germany
kastrati{at}dhm.mhn.de
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Abstract
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Objectives. The aim of this study was to assess the relation between operator experience in coronary stent placement procedures and the clinical outcome of patients.
Background. The results of coronary balloon angioplasty are closely related to the experience of the operator performing the procedure. Data on the effect of operator experience on the results after coronary stent placement are missing.
Methods. The study included 3,409 consecutive patients undergoing coronary stent placement for the management of coronary artery disease. A composite end point of cardiac death, myocardial infarction and aortocoronary bypass surgery during the first 30 days after the intervention, was the primary end point and the procedural failure was the secondary end point of the study.
Results. Adverse clinical outcome occurred in 2.99% of the 3,409 patients undergoing coronary stent placement. Procedural failure was recorded in 2.08% of the patients. Operator volumes above 483 procedures were associated with a risk-adjusted adverse outcome rate of 1.70% ± 1.28%, which is significantly lower than the overall rate of 2.99%. Operator yearly volumes of under 90 procedures were associated with a risk-adjusted adverse outcome rate of 4.59% ± 1.17%, which is significantly higher than the overall rate of 2.99%. The operator experience was an independent predictor even after adjusting for the effect of other risk factors. The analysis demonstrated that an experience of at least 100 procedures is required to obtain better outcome even in patients with simple coronary lesions and that operators should perform at least 70 procedures annually to expect a better outcome in patients with both simple and complex coronary lesions.
Conclusions. Operator experience is a significant and independent predictor of the outcome of patients undergoing coronary stent placement. An experience of at least 100 procedures and an annual volume of at least 70 procedures are required to ensure a significantly better outcome after coronary stent implantation.
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Abbreviations and Acronyms
| | CABG | = aortocoronary bypass surgery | | CART | = classification and regression tree analysis | | PTCA | = percutaneous transluminal coronary angioplasty | | ROC | = receiver operating characteristic curve |
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A significant correlation has been demonstrated between operator and/or institution experience and outcome after aortocoronary bypass surgery (CABG) (1,2) or percutaneous transluminal coronary angioplasty (PTCA) (3–9). This has frequently been viewed as a confirmation of the old paradigm that "practice makes perfect" (10–14) and has served as the rationale for initiating the public release of operator-specific data about cardiovascular procedures and advocating the use of "scorecards" (15,16) or practice profiles of individual operators (17). The relationship between experience and outcome has been the basis for establishing volume standards to be met by hospitals and single operators before granting clinical privileges in PTCA (18–20). Coronary stent placement is an established treatment option for symptomatic coronary artery disease (21–24). It is increasingly used in most centers and frequently constitutes more than 60% of the total number of coronary interventions (25,26). Stent implantation is sometimes considered as a "great leveler" of physician expertise in coronary interventions (13) because it may prevent the early complications in bailout situations after PTCA (27,28). However, data about the relationship between experience and outcome for coronary stent placement are not available and, accordingly, no recommendations about the required volume of procedures for sufficient expertise have been formulated (26,29). It has been pointed out that assessment of the value of strategies of medical care, including the value provided by individual practitioners, has become a reality which requires the development of efficient and fair methods of assessment (30).
The primary objective of the study was to assess the relation between operator experience in coronary stent placement procedures and early (30-day) clinical outcome of the patients. As a secondary objective, the relation between operator experience and procedural success was also assessed.
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Methods
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Patient population.
The study population consisted of all 3,409 patients with symptomatic coronary artery disease who underwent coronary stent placement at our institution from May 1992 through September 1997. Excluded were only 73 patients who had been in cardiogenic shock or under mechanical ventilation before stent placement procedure.
All patients received heparin and aspirin intravenously before the intervention. The stent implantation attempt was always preceded by conventional balloon angioplasty. Hand-crimped, slotted tube stents on balloons of different sizes were used in almost all cases. Balloon size and pressure were at the operator’s discretion. A more detailed description of the technique has been published previously (28). All patients with successful stent placement were given anticoagulant or antiplatelet therapies (23) to prevent stent thrombosis.
Data collection, definitions and end points.
All data were entered into a database. The physician who performed the intervention entered the data related to the procedure. Other clinical, laboratory, quantitative angiographic and follow-up data were entered by physicians not involved in the intervention. Data completeness and consistency were systematically checked by the software and by a physician in charge of database supervision. The entered name of the operator was checked for consistency with that on the signed report of the intervention.
During the hospital stay electrocardiograms were recorded daily. Routine enzyme measurements were performed up to 12 h after the procedure and on recurrence of symptoms thereafter. Patients with suspected ischemia underwent repeat coronary angiography. Patients were followed as outpatients or contacted by phone 1 month after the intervention.
Off-line quantitative angiographic analysis was carried out using a validated automated edge detection software (CMS, Medics Medical Imaging Systems, Nuenen, The Netherlands) (31,32) by operators unaware of procedural and clinical outcome of the patient.
The complexity of target lesions was assessed using the modified American College of Cardiology/American Heart Association scoring system (33). Complex lesions were defined as lesions of type B2 or C. Left ventricular function was assessed qualitatively; the diagnosis of reduced left ventricular function required the presence of hypokinesia in at least one or two segments. Procedural success was defined as placement of the stent at the desired position with less than 30% residual stenosis. Clinical events were recorded continuously. All deaths were considered due to cardiac causes unless an autopsy established a noncardiac cause. The diagnosis of acute myocardial infarction was based on typical chest pain lasting more than 30 min, abnormal Q waves not present on the baseline electrocardiogram, or an increase in the creatine kinase concentration to twice the upper limit of normal. The diagnosis of recurrent myocardial infarction was also based on an increase of more than 30% in the serum level of creatine kinase. All CABGs, emergent or nonemergent, carried out in the first 30 days and involving the stented vessel were also recorded as cardiac events.
The primary end point of the study was the clinical outcome of patients at 30 days after the procedure. A combined end point of cardiac death, myocardial infarction and CABG was defined as the clinical outcome measure. This was also the most frequent end point used in similar studies with PTCA. Operator experience was defined in two ways: the overall number of interventions performed by an operator up to the day of the actual procedure, and the yearly number of interventions. The secondary end point of the study was procedural failure.
Statistical analysis.
For the purpose of this study, the lesion responsible for the adverse outcome (failed procedure, subacute occlusion) or one lesion at random was selected for analysis in patients with multilesion interventions. Continuous variables are expressed as mean ± standard deviation (SD). Descriptive monovariate analysis for continuous data was made using two-sided unpaired t-test. No attempt was made to control for potential deviations from normal distribution since the main analysis consisted in multivariate methods. Discrete variables were analyzed using chi-square test with Yate’s correction if appropriate. Operator experience, both as overall and as yearly volume of procedures, was analyzed as a continuous and ordinal variable as well. Since there are no previous studies on this subject to offer meaningful cutoff points for operator experience, transformation in an ordinal form was performed by subdividing in volume ranges defined by the 20th percentiles.
Multivariate analysis was based on multiple logistic regression models. The predictive accuracy of the model was assessed by means of the area under receiver operating characteristic (ROC) curve. Bootstrapping with 1,000 replications of the original sample data was used for model validation and the slope shrinkage parameter between training and test samples was assessed (34). The primary end point of the study was analyzed in three steps. First, a multiple logistic model was built with baseline clinical and lesion characteristics. Using the logistic regression equation, the expected rate for an adverse outcome was obtained for each volume range. Next, the risk-adjusted rate for a volume range was calculated as the ratio between the observed and expected rate, multiplied by the overall rate for the outcome in the entire study population. The 95% confidence intervals (CI) for this risk-adjusted rate were obtained, as previously described (35). In a second step, operator experience was entered into the multiple logistic regression model and tested for independent effect on the outcome. Finally, independent predictors of an adverse outcome were entered in a classification and regression tree (CART) analysis to allow for risk stratification based on baseline characteristics and indices of experience. In all three steps, analyses were repeated for both measures of operator experience and overall and yearly volume of procedures. Multiple logistic regression analysis was also applied for assessing the secondary end point of procedural failure. For all statistical analyses the S-Plus Version 4 software (Mathsoft, Inc., Seattle, WA) was used. Statistical significance was defined as p values <0.05.
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Results
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Primary end point analysis.
All interventions described in this study were performed by 10 operators. The number of operators that contributed to each total volume quintile was 10, 9, 6, 4 and 3 for the 1°, 2°, 3°, 4° and 5° quintile, respectively. The composite end point of cardiac death, nonfatal myocardial infarction or CABG during the first 30 days after the procedure was reached by 102 patients (2.99%). More specifically, 38 patients (1.1%) died of cardiac causes, 40 patients (1.2%) sustained nonfatal myocardial infarction and 24 (0.7%) underwent a successful CABG. During the same period one more death occurred due to massive organ bleeding under anticoagulation therapy. The cause of death was verified at autopsy. The event rates for the different volume ranges were 2.97%, 3.82%, 3.50%, 3.08% and 1.60% in the 1°, 2°, 3°, 4° and 5° quintiles, respectively. Demographic, clinical, angiographic and operator volume data of the study population divided according to the occurrence or nonoccurrence of cardiac events are presented in Table 1. In addition, because almost all stent placement procedures involved crimping stents onto the balloon by hand, a separate analysis was made for the event of stent loss. Sixty-three out of a total of 5,833 stents were lost either in a peripheral artery (55 stents) or in the coronary circulation (8 stents). The incidence of this event was 1.1% in patients without adverse clinical outcome and 1.0% in those with adverse clinical outcome (p = 0.8). All demographic, clinical and lesion-related variables listed in Table 1 were entered into the multivariate model. Unstable angina, reduced left ventricular function and complex lesions were associated with higher probability of an adverse outcome. Figure 1 presents the risk-adjusted cardiac event rates and their 95% CIs for the different volume ranges (quintiles), with the overall observed rate as reference. Only the highest quintile corresponding to a volume range of more than 483 stent implantations with the contribution of 3 operators presents a risk-adjusted rate of 1.70% ± 1.28% that is significantly lower than the reference level (p < 0.05). The rates for the lower four quintiles of operator volume are at or above the overall rate. Logistic regression analysis was repeated including operator volume as an additional independent variable. The area under the ROC curve for the model was 0.71. Figure 2 presents the odds ratios (OR) of the independent correlates of the composite end point. The most potent risk factor were complex lesions with an OR of 2.91 (95% CIs, 1.48 to 5.71), followed by unstable angina (2.0 [1.25 to 3.22]) and reduced left ventricular function (1.62 [1.07 to 2.47]). Greater operator experience was associated with a decreased risk: a volume of 500 procedures presented an OR of 0.64 (0.43 to 0.96) with respect to a volume of 100 procedures. The independent predictors derived from the multiple logistic regression model were entered into a CART analysis (Fig. 3). This analysis demonstrates that while an experience of 100 or more procedures is sufficient for a significant decrease in the risk of adverse outcome in patients with simple lesions, much more experience (450 procedures or more) is needed to improve the outcome of patients with complex lesions.
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Figure 1 Plot of the risk-adjusted cardiac event rate (death, myocardial infarction or bypass surgery) for different operator volume ranges of stent placement procedures. Data presented as means and 95% confidence intervals. Dotted line shows the event rate for the entire population (2.99%).
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Figure 2 Graph presenting odds ratios and their 95% confidence intervals for independent risk factors for an adverse outcome as identified by multivariate analysis. For operator volume of stenting procedures, the comparison is between 500 and 100 procedures. Odds ratios are displayed on a logarithmic scale.
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Figure 3 Graph presenting the CART model constructed with the independent risk factors for an adverse outcome. The area of a circle is proportional to the size of the subgroup. Numbers within the circles indicate adverse outcome rate in percent for the given subdivision. The analysis identifies at each level two subgroups with a statistically significant difference in adverse outcome rate. The number of operators that contributed to procedures corresponding to the seven terminal nodes of the CART was 8, 9, 10, 3, 3, 10 and 10, respectively (from the left to the right). Pts = patients; Proc. = procedures; LV Func. = left ventricular function. Note the effect of operator volume of stenting procedures on the outcome of patients.
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The same type of analyses was carried out for the relationship between the operator yearly volume of procedures and outcome. The number of operators that contributed to each yearly volume quintile was 10, 7, 4, 3 and 2 for the 1°, 2°, 3°, 4° and 5° quintile, respectively. The event rates for the different yearly volume ranges were 4.44%, 2.91%, 2.72%, 1.94% and 2.79% in the 1°, 2°, 3°, 4° and 5° quintiles, respectively. Figure 4 shows that risk-adjusted event rate of 4.59% ± 1.17% for the yearly volume range of <90 procedures is significantly higher than the overall event rate of 2.99% (p < 0.05). All operators contributed to this quintile. The risk-adjusted rates for the other higher quintiles are at or under the reference level. Yearly volume of procedures resulted also in an independent determinant of outcome in logistic regression analysis in addition to lesion complexity, unstable angina and reduced left ventricular function. A yearly volume of 140 procedures was associated with an OR of 0.63 (0.44 to 0.89) as compared with a volume of 70 procedures/year. The area under the ROC curve for the model was 0.71. The CART analysis (Fig. 5) indicates that a yearly volume of 70 procedures or more is necessary to ensure a better outcome for both simple and complex lesions.
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Figure 4 Plot of the risk-adjusted cardiac event rate (death, myocardial infarction or bypass surgery) for different operator yearly volume ranges of stent placement procedures. Data presented as means and 95% confidence intervals. Dotted line presents the event rate for the entire population (2.99%).
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Figure 5 Graph presenting the CART model constructed with the independent risk factors for cardiac events. The area of a circle is proportional to the size of the subgroup. Numbers within the circles indicate adverse outcome rate in percent for the given subdivision. The analysis identifies at each level two subgroups with a statistically significant difference in adverse outcome rate. The number of operators that contributed to procedures corresponding to the six terminal nodes of the CART was 9, 9, 10, 9, 9 and 9, respectively (from the left to the right). Pts = patients; Proc./Y = procedures per year; LV Func. = left ventricular function. Note the effect of operator yearly volume of stenting procedures on the outcome of patients.
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Secondary end point analysis.
Stent placement attempt failed in 71 procedures (2.08%). In 45.1% of these patients, procedural failure was complicated by an adverse outcome (vs. 2.1% for patients with successful procedures). The most frequent reason for failure was the inability to reach the target lesion with stent (79% of the cases). The observed failure rate in the five volume ranges followed the same trend as the cardiac event rate with 2.4%, 1.9%, 2.8%, 2.2% and 1.2% in the 1°, 2°, 3°, 4° and 5° quintile, respectively. Table 2 presents the univariate analysis of the patients’ characteristics according to the procedural outcome. The multiple logistic model for procedural failure demonstrated that complex lesions (p = 0.007), small vessel size (p = 0.028) and low operator volume (p = 0.032) were the only independent risk factors.
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Discussion
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This study found a direct relationship between operator experience in performing coronary stent placement and the clinical outcome of the patients after this intervention. In addition, we demonstrated that a risk-stratification protocol, based on baseline clinical and angiographic characteristics, is possible for patients undergoing coronary stent implantation. This protocol helps in matching patient risk profile with the minimal operator experience required to achieve acceptable results.
Methodological issues.
The primary end point of the study was the clinical outcome of patients at 30 days after stent placement procedure. The time frame for the end point assessment was therefore 30 days, far beyond the hospital stay period which is the usual interval of similar studies with PTCA (3,5–9,36,37). Extension of the observation period has recently been advocated for outcome studies after PTCA (38). This may be particularly important for stent implantation where thrombotic vessel occlusion, the most frequent cause of an adverse clinical outcome, may also occur after the first week (39). Therefore, the assessment of 30-day outcome reduces the risk of missing major adverse events after the intervention. Adopting the composite end point of death, myocardial infarction and CABG as the principal outcome measure, we rendered our results comparable with most of the previous PTCA studies (4–6,8). Due to the subjective nature of the definition of procedural failure (30), it was used only as a secondary end point in this study. The baseline characteristics of the patients were taken into consideration specifically since the complex interrelation between operator experience and the risk profile of the patient undergoing the intervention has been recognized (14). The only way to assess the individual skills of the operator is to assess outcomes after adjusting for the influence of other risk factors. The results of the present study are further strengthened by the fact that we included in the multivariate analysis not only the conventional patient and lesion characteristics as others have done previously (6,8), but also lesion-related quantitative variables. With current digital angiographic equipment, these data are readily available to most of the operators during the examination.
Comparison with previous findings for PTCA.
This study concurs with previous reports for PTCA (37,40–44) in finding that baseline patient and lesion characteristics are the major determinants of outcome after stent implantation procedure. Not unexpectedly, complex lesions, unstable angina and reduced left ventricular function were associated with more adverse events during the first 30 days. Although weaker than the abovementioned factors, operator experience showed a significant independent role in the patient outcome. Both measures of operator experience and overall caseload and yearly volume of procedures were correlated with a better outcome. Our analysis indicated that a clear effect is achieved with more than 480 procedures, which corresponds to the highest quintile and with a yearly volume of more than 90 procedures. These relatively high levels may be explained by the characteristics of our population. More than 70% of the lesions were complex and almost half of them of type C. The CART analysis confirmed also that much more experience is needed to achieve a better outcome in patients with complex lesions.
Very useful information is provided by our CART analysis based on patient and lesion characteristic in addition to operator experience measures. The risk for an adverse outcome showed a clear difference between complex and simple lesions and, within either group, operator experience was the most powerful predictor of outcome. This analysis showed that even for patients with simple lesions, the operator should have performed a minimum number of 100 procedures to ensure acceptable results. This number coincides with that of 125 procedures recommended by the American College of Cardiology and American Heart Association as the standard to be met to achieve clinical competence during training in performing PTCA (18,19). In addition, our CART analysis demonstrated that a minimum of 70 stenting procedures per year need to be carried out to maintain good performance even for patients with complex lesions. This figure is not different from that of 75 procedures per year recommended by the American College of Cardiology and American Heart Association to maintain competency in PTCA (18,19). In summary, until more definitive data are provided, the current guidelines for PTCA (18,19) should be considered valid for coronary stent placement as well. This study also suggests that the fulfillment of the abovementioned experience criteria may be considered only as a first step in the policy of improving the outcome of patients undergoing coronary stent placement. To facilitate achieving this goal, the risk profile of the patient should be matched with adequate experience, reserving more experienced operators for more complex lesions. Obviously, not all characteristics of B2- or C-type lesions impart the same risk to stent placement procedure. Our data in this regard constitute only the first steps toward the definition of a complete strategy that appropriately matches the patient’s risk with the interventionalist’s experience.
Limitations.
Coronary stenting has been associated with the reduction of cardiac events as compared to PTCA (21,22). This remarkable advantage brings about a paradoxical side effect: the low number of events increases the risk for the clinical investigation to be underpowered (12,30,45). Our study is also affected by this risk. To overcome this pitfall, the use of outcome at 6 months with much more events has been advocated for interprovider comparison (38). Before adopting such an end point a logical link should be demonstrated between long-term outcome and operator experience. Thus, larger scale investigations are needed in the future to confirm the validity of the relationship found in this study. It has been recognized, however, that despite their imperfections, the statistical models are equal to or better than expert clinicians in predicting patient outcome (30).
Coronary stenting may be considered an immature technology. The superiority of combined antiplatelet therapy in comparison to older anticoagulation schemes has already been demonstrated (23,46). Further improvements in poststenting treatment are currently sought in therapies aiming at the platelet fibrinogen receptor blockade (47) or in stent-coating techniques (48). A yet to be proven role has been advocated for high balloon pressure and intravascular ultrasound guidance (49,50). A variety of stent designs is available on the market. Because it has yet to be determined how these evolving concepts and techniques affect clinical outcomes, Lindsay et al. (38) recommended that comparisons must be made to concurrent experience. Another difficulty in including such variables in the analysis comes from the close association between adverse outcome and procedural failure in our study: 45.1% of failures ended with an adverse event. The reason for failure in the overwhelming majority of cases in this study was the inability to reach the target lesion with stent before any of the abovementioned changes could have been applied. Similar difficulties have also prevented the studies of outcome after PTCA (3,4,7–9) from including in the analysis advances in guiding catheters, guiding wires or balloon catheters which have certainly had a role in improving procedural results.
Finally, health care carriers are interested to know not only the individual but also the institutional providers with better outcomes after coronary interventions. The latter issue can not be answered by the present study based on a single-center experience. With the current widespread use of coronary stent placement, time has come for undertaking multicenter studies to address the relationship between institution volume of procedures and outcome after stenting.
Conclusions.
This study demonstrated that parallel to other patient and lesion characteristics, operator experience exerts an independent influence on the outcome of patients undergoing coronary stent placement. These results suggest that the current guidelines of the American College of Cardiology and the American Heart Association for achieving and maintaining competency in PTCA, that is, 125 procedures performed during the training period and 75 procedures performed annually, are also valid criteria for coronary stent placement. In addition, the findings of this study may help to identify patient’s risk before the intervention and appropriately match it with the operator experience required for achieving the most favorable outcome.
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