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There is wide geographic variation in the use of coronary revascularization in the United States. Rates are closely related to rates of coronary angiography. We assessed the relationship between coronary angiography and coronary artery revascularization by procedure type (coronary artery bypass graft surgery, CABG, or percutaneous coronary intervention, PCI).
Using Part B claims for a 20% sample of the Medicare population, we calculated population based rates of testing and treatment by region, using events identified in Part B claims as the numerator and the total number of Medicare beneficiaries residing in the area as the denominator, adjusting for regional differences in demographic characteristics using the indirect method.
Cardiac catheterization rates varied substantially across regions, from 16/1000 to 77/1000 Medicare beneficiaries. The relationship between coronary angiography rates and total coronary revascularization rates was strong (R2 = 0.84). However, there was only a modest association between coronary angiography rates and CABG rates (R2 = 0.41) with the suggestion of a threshold effect. The association between coronary angiography rates and percutaneous coronary intervention rates was strong (R2 = 0.78) and linear.
The diagnostic-therapeutic cascade for coronary artery disease differs by therapeutic intervention. For CABG, the relationship is modest and there appears to be a testing threshold beyond which additional tests do not result in additional surgeries; for PCI the relationship is very tight and no threshold appears to exist. Given the results of recent studies of medical versus invasive management of stable coronary disease, patients living in high diagnostic intensity regions may be getting more treatment than they want or need.
There is wide geographic variation in of the use of coronary artery revascularization across the United States1,2. Population based rates of coronary artery bypass graft surgery (CABG) in the Medicare population vary nearly 5 fold, while rates of percutaneous coronary interventions (PCI) vary more than 6 fold3. The wide variation in rates is not explained by differences in demographic characteristics or disease prevalence as measured by acute myocardial infarction (AMI) rate3. However, the rates of revascularization are highly correlated with rates of coronary angiography4; we found that, in the Medicare population in1996, the correlation between coronary angiography and revascularization was 0.93 (R2 = 0.87) and the relationship appeared to be almost perfectly linear3.
What might drive rates of coronary angiography? Generally, there is a correlation between the capacity to perform the diagnostic test and the likelihood that a test will be performed. There are moderate correlations between the population based supply of cardiac catheterization laboratories1,3 as well as the supply of invasive and interventionalist cardiologists3 and the coronary angiography rate. Looking further upstream, populations living in regions where more imaging stress tests are performed are also more likely to undergo coronary angiography5. This finding of a tight linkage between the diagnostic test and the therapeutic intervention has been called the ‘diagnostic-therapeutic cascade’6.
Previous investigations of the diagnostic-therapeutic cascade for coronary disease have considered the overall rate of revascularization and not examined the relationship between coronary angiography and CABG separately from the relationship between coronary angiography and PCI. However, since these studies were performed, the population based rates of CABG have decreased substantially while those of PCI have continued to increase7. This paper first evaluates whether a tight relationship between diagnostic and therapeutic intensity still exists and then evaluates this relationship separately for the two interventional alternatives.
We used Medicare Part B (physician and supplier) claims data for a 20% random sample of Medicare beneficiaries to identify coronary angiography and revascularization procedures occurring during 2005. Procedures were defined using the appropriate CPT codes (CABG:33510-33536; PCI: 92980-92984, 92995-92996; coronary angiography: 93508-93529, 93539-93540, 93545). The Part B file is a line item file, potentially containing more than one line item and associated CPT code for each procedure. We defined a procedure as any line item containing a CPT code of interest and counted only one procedure of each type for each person per day, identified using a unique patient identification code and procedure date. If a single individual had, for instance, PCI and CABG on the same day, both would be counted. If an individual had two PCIs on different days, both would be counted. If an individual had two PCIs on the same day, we would only count one as we could not distinguish between two procedures or a single procedure using more than one PCI code. We added together all procedures of a given type to define the numerators of population based rates.
We used the Medicare Denominator file to identify all beneficiaries eligible for Part B at mid-year 2005 (about 5.4 million beneficiaries). Because not all managed care plans submitted claims during this time period, we excluded beneficiaries who were enrolled in managed care plans (approximately 17% of the total Part B population). The count of these individuals eligible for Part B and not managed care enrolled on June 30, 2005, defined the denominators of population based rates.
Both the Part B and Denominator files contain beneficiary age, sex, race, and ZIP code of residence. Each beneficiary was assigned to a Hospital Referral Region (HRR) based on his/her ZIP code of residence. Hospital Referral Regions are small geographic areas representing referral patterns for tertiary care, including cardiac surgery8. In order to account for differences in the distribution of important demographic variables across HRRs, we calculated the adjusted rates of coronary angiography, total revascularization, CABG and PCI for each HRR by dividing the number of events by the total number of beneficiaries and standardizing to the age/sex/race distribution of the total Medicare population using the indirect method9. This method calculates the number of events that would be expected to occur in each HRR if the national (standard population) age/sex/rates applied; the observed number of events is then divided by the expected number of events and the resulting O/E ratio is multiplied by the national rate (standard population rate) to produce the adjusted rate.
We used weighted least squares regression methods10, weighting by the inverse of the variance, to assess the relationship between rates of coronary angiography and rates of revascularization, assessing departures from linearity by introducing squared, logarithmic and inverse terms into regression models; because the models containing squared terms fit best, these are the results we report. We tested the final models for heteroscedasticity and independence of errors using graphical methods and a formal statistical test of model specification11. There were two extreme outliers that distorted the relationship between coronary angiography rates and rates of PCI; to be conservative, we report the regression model results with these outliers deleted. We report the strength of the relationship between coronary angiography and revascularization rates using the R2 statistic; this statistic can be interpreted as the proportion of variation in revascularization rates explained by variation in coronary angiography rates.
The Institutional Review Board at Maine Medical Center approved this study. The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
Table 1 gives the demographic characteristics of the study sample of approximately 5.4 million Medicare beneficiaries and the characteristics of those receiving one of the procedures of interest. While 13% of the study population was aged 85 or older, only 7% of those receiving cardiac catheterization were in the oldest age group; rates of PCI were much higher than rates of CABG in all groups, but proportionately even higher in older patients, women, and black patients than in their comparator groups. For example, while there are 12.3 PCIs for each 4.7 CABGs in the youngest patients aged 65-69, a ratio of 2.6, there are 7.9 PCIs for each 1.6 CABGs, a ratio of 4.9.
Coronary angiography rates were highly variable across HRRs, with a mean rate of 37.2 procedures per 1000 and a range of 16.2 to 77.0 per 1000 Medicare beneficiaries (Figure 1). CABG was also variable, with a mean of 4.9 procedures per 1000 and rates varying by a factor of nine (range 1.1-10.1). PCI was much more frequently performed with a mean of 13.2 procedures per 1000 and varied by a factor of nearly eleven (range 4.2-44.7).
Coronary angiography rates were highly positively associated with revascularization rates (Figure 2; R2 = 0.84). The relationship was positive with some suggestion of a flattening of the regression line at higher angiography rates (negative squared term coefficient, p = 0.02).
Considering CABG and PCI separately, the relationships were quite different. Angiography rates were only modestly associated with CABG rates (Figure 3; R2 = 0.41). The relationship had a modest slope at lower angiography rates; however, there was a suggestion of a threshold effect, such that increasing angiography rates beyond 50 procedures per 1000 beneficiaries yielded much smaller increases in CABG rates (negative squared term coefficient, p <0.0001). The relationship between angiography rates and PCI rates was much stronger and linear (Figure 3, R2 = 0.78). (See Appendix Table 1 for all model details.)
Table 2 give estimated results from the final models. On average, an increase from 20 to 30 per 1000 in the coronary angiography rate resulted in an estimated increase of 4.6 per 1000 in the revascularization rate; an increase from 60 to 70 in the angiography rate resulted in an increase of 3.3 in the revascularization rate. For CABG, in areas with low angiography rates, each increase of 10 per 1000 in the angiography rate resulted in an increase of about 1 per 1000 in the CABG rate, while at higher angiography rates, the estimated CABG rate increased much less if at all; the increase from 60 to 70 resulted in no increase in CABG rate. However, an increase of 10 in the coronary angiography rate was associated with an estimated increase of 3.4 in the PCI rate regardless of the angiography rate.
Coronary angiography rates were strongly correlated with population based rates of revascularization. However, when comparing coronary angiography individually to the two interventional options very different patterns emerged. Compared with the modestly sloped coronary angiography-CABG relationship with a suggestion of a threshold effect, the coronary angiography-PCI relationship was steep and linear.
The simple explanation for the tight relationship between coronary angiography and overall revascularization rates is that there is a large reservoir of undiagnosed coronary artery disease, more of which is discovered in areas with higher coronary angiography rates. However, we have previously shown that rates of hospitalization for acute myocardial infarction are uncorrelated with revascularization rates (R2 = 0.03) 3. The different relationship between angiography and PCI versus CABG further suggests that disease burden alone is unlikely to be the primary factor explaining these findings. Coronary disease burden could only explain these relationships if there are extremely different distributions of coronary artery disease severity and anatomy between populations, unaccounted for by age, sex, and race.
Indications for CABG are fairly specific12,13 and most studies suggest that CABG is currently performed for combinations of anatomy and function for which surgical revascularization improves long term survival, at least in comparison to older medical therapy, or for patients with multi-vessel disease to relieve symptoms and improve quality of life. The modest relationship between angiography and CABG suggest that while more cases are found for surgical intervention as coronary angiography rates increase, beyond a certain point increasing rates do not identify additional patients for which CABG is the preferred intervention. However, the picture for PCI is quite different. There is a much more positive relationship between coronary angiography and PCI rates and there is no place on the coronary angiography/PCI curve that suggests that all patients with lesions that could undergo a PCI have been identified. What accounts for the apparent lack of a threshold for intervention with a catheter?
When we look for coronary artery disease in an elderly population, we find it. However, the coronary angiography-CABG relationship suggests that the ‘marginal yield’ of severe coronary artery disease falls with higher rates of coronary angiography. Therefore, it appears that the marginal disease found at very high rates of angiography is predominantly non-surgical one- and two-vessel disease. Unlike CABG, there are almost no absolute indications for PCI 14, the exception being primary PCI for AMI. Thus, it must be patient and/or physician preferences for PCI versus medical management that drives the tight diagnostic-therapeutic relationship for PCI.
The limited data that exists suggests that patients’ and physicians’ preferences for treatment of coronary artery disease may be in conflict. Only one randomized trial of patient preferences for treatment of coronary artery disease has been performed15. In this study, patients on the waiting list for revascularization in Ontario Canada, a geographic region that had lower rates of intervention than that in any HRR in the United States, were randomized to usual care versus an interactive decision aid designed to assist patients in making complex medical decisions. Patients in the decision aid arm were 28% more likely to choose medical treatment over either PCI or CABG than those in the control arm. Lin and colleagues found that cardiologists, despite acknowledging the lack of evidence of mortality and AMI prevention benefit stated they would provide PCI for minor lesions unrelated to symptoms16. The authors (and the cardiologists interviewed) attributed these findings to the ‘occulostenotic reflex’17, the impulse to fix a stenosis even if it is unlikely to cause future problems. Cardiologists also reported a bias towards intervention because of a belief, despite evidence18,19,20,21 that an open artery provided benefit and because of concerns about ‘regret’ if they failed to act and a patient suffered a subsequent event. Combined, these studies suggest that patients with coronary artery disease may be more risk averse than cardiologists when considering treatment options.
Other factors may also play a role in the tight, linear relationship between angiography and PCI. Often, the same cardiologist is both diagnostician and the interventionalist. Potential issues related to ‘self-referral’ are compounded by clinically significant intra-observer and inter-observer variability in interpretation of angiograms 22 and the need to make rapid, rather than measured, decisions on whether and where to intervene. This propensity to intervene has been given a clear push by private insurers. Since the early 1990s, insurance companies have encouraged the use of ‘ad hoc’ angioplasty through explicit payment policies that discourage staged procedures - a diagnostic event followed by an interventional event. This policy may have made sense from the standpoint of reducing episode based costs; however, it may have had the unanticipated consequence of increasing the numbers of PCIs performed.
Our study has some limitations. First, we limited our analysis to the aged Medicare population. However, we have no reason to believe that patterns of care across geographic regions vary systematically by age or insurance status. Variation in Medicare rates is reflected in younger populations (www.bcbsm.com/atlas,23). We are unable to assess the indication for testing nor of the results of the coronary angiography and indications for revascularization. However, this is only an important limitation if disease patterns vary remarkably across geographic areas and we have previously found little evidence of large disparities in disease distribution across geographic regions.
The tight relationship between diagnostic intensity and therapeutic intensity with PCI raises a fundamental challenge to those managing patients with coronary artery disease. There is established evidence of benefit of PCI in unstable coronary syndromes24, and current guidelines recommend such a strategy in these patients25. However, several recent studies have suggested that patients with stable coronary disease who undergo PCI may receive no marginal benefit, or worse, face increased risk of death and AMI compared to those managed medically26,27,28. We would contend that unstable patients are more likely to receive intervention no matter where they live, but the marginal patients in high rate areas are likely to be those who are less symptomatic in whom the benefit is much less well defined. Our models suggest that, in areas with high rates of coronary angiography, at most 0.2 patients with serious coronary anatomy eligible for CABG would be identified for each 10 angiograms performed. Thus, the ‘cost’ of, at most, 0.2 appropriate CABGs would be 10 additional angiograms and 3.4 additional PCIs. Given the ‘diagnostic-therapeutic yield’, a fundamental question needs to be asked: in the low and moderate risk patient, do we even want to know their anatomy?
Funding sources: This research was supported in part by National Heart, Lung, and Blood Institute grant #R01HL080437 and the Foundation for Informed Medical Decision Making. The funders had no role in the design of the study, analysis or interpretation of the data. Dr. Lucas had full access to all the data in the study and takes full responsibility for the integrity of the data and the accuracy of the analysis.
Conflict of interest disclosures: Dr. Malenka reports receiving research funding from Guidant Endovascular Systems.
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