Our study has 3 noteworthy findings. First, we found cardiologists in the United States currently play a substantial role in carotid stenting. Approximately one-third of the operators performing this procedure in Medicare beneficiaries are cardiologists and they are responsible for over half the total procedures. Surgeons and radiologists, in contrast, were responsible for 28% and 18% of procedures, respectively. Second, we found that cardiologists treat patients substantially different from other specialists. Not surprisingly, procedures by cardiologists more frequently involved patients with cardiac conditions or recent invasive cardiac procedures. Yet these patients also had fewer neurologic conditions, including less evidence of recent acute stroke or TIA. While it is not possible to definitively assess for symptomatic disease in these data, the differences we found across physician specialty were substantial. This finding will be important to clarify in future work since the benefits of carotid revascularization are generally less in asymptomatic patients. Finally, we found that HRRs where cardiologists performed most carotid stenting had higher population-based rates of utilization than other HRRs with similar outcomes.
Unlike many medical procedures, carotid stenting is a “cross-disciplinary” technology that is performed by operators from several, diverse physician specialties. These groups have widely different clinical opportunities and incentives for performing it, which could lead to important differences in how patients are selected for carotid stenting. For example, cardiologists care for a large number of patients with atherosclerotic heart disease. These individuals may also have coexisting carotid disease that may or may not be symptomatic. Involvement of cardiologists in the care of these patients could lead to greater recognition of carotid stenosis in the general population and subsequent referral for carotid stenting, particularly among less symptomatic patients. For example, we found that over 50% of patients who were treated by cardiologists and also underwent cardiac catheterization prior to carotid stenting had carotid and cerebral angiography performed simultaneously. This proportion was much lower among those treated by surgeons (24%), radiologists (18%) or others (34%), which suggests the possibility that routine case finding of severe carotid stenosis by cardiologists during diagnostic angiography influenced patient selection (i.e., “oculo-stenotic reflex”).
Radiologists, in contrast, typically have little involvement in the early evaluation of patients with stroke and carotid stenosis and rely primarily on referrals from other physicians. Surgeons are the only operators involved in carotid stenting that also perform carotid endarterectomy – the open-surgical procedure that is the current standard for carotid revascularization. This may make them less likely to view carotid stenting as a first-line therapy and restrict their use to select cases. Differences in educational training may also contribute to their overall decision to perform carotid stenting. For example, cardiologists routinely use standard 0.014” wire-based equipment and embolic protection devices for their coronary interventions, both of which are requirements for most carotid stenting. It is possible that these experiences make them generally more enthusiastic for the use of catheter-based techniques than surgeons or even radiologists. Prior work also supports this possibility as the involvement of cardiologists in other non-cardiac vascular interventions has been linked to higher population-based rates of utilization for those procedures.9,10
Understanding the role of physician specialty in carotid stenting is not just of theoretical interest, but importantly, has practical implications. In 2006, the Centers for Medicare & Medicaid Services (CMS) raised considerable controversy when it proposed modifying its current national coverage decision for carotid stenting to allow only physicians who perform carotid endarterectomy to determine which patients would be suitable for carotid stenting.13
Although the goal was to improve overall decision-making for carotid stenting, cardiologists and radiologists were concerned that this would have essentially left surgeons as the lone gatekeepers for its use. Although CMS subsequently retreated from this proposed modification, our findings suggest that decisions about using this procedure may indeed be linked to physician specialty. If this is true, promoting multi-disciplinary decision-making around carotid stenting could reduce unwanted variation in its adoption. The inclusion of specialists not directly involved in carotid stenting, like neurologists, may also aid in these decisions, particularly since they are more experienced in identifying stroke-related complications and managing these patients long-term.14
These issues all take on even greater relevance given current controversies surrounding the clinical effectiveness of carotid stenting relative to carotid endarterectomy. For example, the CREST investigators recently reported that both procedures led to similar rates of stroke, myocardial infarction or death in 2502 patients over a mean follow-up period of 2.5 years.11
Although this appears to suggest overall equivalence between the 2 strategies, the large International Carotid Stenting Study (ICSS) – also published earlier this year – found higher rates of this combined endpoint with carotid stenting.15
In addition, both trials found a significant interaction between age and outcomes that favored carotid endarterectomy in older patients, which is of direct relevance for the elderly Medicare population we studied. With this degree of uncertainty about the role of carotid stenting, it is likely that substantial variation in its use will continue and could intensify in regions more enthusiastic for its application.16
Our study should be interpreted with the following limitations in mind. We relied on Medicare claims data, which are inherently limited in their ability to understand the clinical context in which a procedure may be used. For example, we could not directly determine whether patients were “symptomatic” or “asymptomatic” and had to rely on the presence of claims data suggesting a recent diagnosis of acute stroke or TIA. As a result, we are unable to comment on the overall appropriateness of its use across specialties despite wide variation in its adoption. It may be that markets where cardiologists are performing most procedures are “overutilizing” carotid stenting or that markets where surgeons or radiologists predominately perform the procedure have important barriers that limit access and are “underutilizing” it. Importantly, either concern may be overcome by promoting multidisciplinary decision-making to optimize its utilization. Along similar lines, it could be argued that differences in adoption of carotid stenting across markets may be due to variation in the prevalence of carotid disease or the overall enthusiasm of physicians for carotid revascularization within in a region. Although admittedly an imperfect measure, we attempted to account for these factors in our analysis by using adjusted utilization rates for carotid endarterectomy in 2004. A stronger counterargument to this concern, however, are recent data reporting large differences in rates of carotid stenting across markets – a degree of variation that is unlikely to be explained by differences in these factors alone.3,4
Another consideration is that findings in Medicare claims data may not be generalizable to younger patients or managed care populations. Although some of these concerns could be overcome using data from clinical registries, those sources have their own limitations. They are typically limited to select high-volume centers and have no information on “at-risk” denominator populations to determine how its use by different specialties might impact on utilization. An additional complicating factor is that different professional organizations (e.g., the American College of Cardiology and the Society for Vascular Surgery) are promoting unique registries, often to their own physician constituents.17,18
This may prevent these registries from providing a comprehensive picture of carotid stenting in the United States and limits their ability to address policy-based questions across different specialties and broad patient populations. In contrast, this is a particular advantage of using Medicare claims data. Finally, the most reliable outcome we were able to evaluate was mortality, which is generally low following carotid stenting. Our ability to assess other important outcomes, like the development of stroke, is more limited within claims data.
In conclusion, cardiologists perform the majority of carotid stenting in the United States. Procedures performed by cardiologists involve patients who appear to differ in important ways from those treated by other specialists like surgeons and radiologists. HRRs where cardiologists perform most procedures have significantly higher population-based rates of utilization suggesting an important role for physician specialty in the choice to use carotid stenting. Further studies are needed to better understand the specific nature of this relationship and whether multi-disciplinary decision-making by teams of specialists could optimize the use of this innovative technology.