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Supply and Distribution of Vascular Access Physicians in the United States: A Cross-Sectional Study

Background: As functioning permanent vascular access (arteriovenous fistulas [AVF] or arteriovenous grafts [AVG]) is crucial for optimizing patient outcomes on hemodialysis, the supply of physicians placing vascular access is key. We investigated whether area-level demographic and healthcare market attributes were associated with the distribution and supply of AVF/AVG access physicians in the U.S. Methods: A nationwide registry of physicians placing AVF/AVG in 2015 was created using data from the US Renal Data System and the American Physician Association’s Physician Masterfile. We linked the registry information to the Area Health Resource File to assess the supply of AVF/AVG access physicians and their professional attributes by Hospital Referral Region (HRR). Bivariate analysis and Poisson regression were performed to examine the relationship between AVF/AVG access physician supply and demographic, socioeconomic, and health resources conditions of HRRs. Setting: All 50 states. The main outcome was supply of AVF/AVG access physicians, defined as the number of physicians performing AVF and/or AVG placement per 1,000 prevalent end stage kidney disease patients. Results: The majority of vascular access physicians were aged 45-64 (average age=51.6), male (90.8%), trained in the U.S. (76.4%), and registered in a surgical specialty (73.6%). The supply of physicians varied substantially across HRRs. The supply was higher in HRRs with higher % White population (ß=0.44; SE=0.14; p=0.002), lower unemployment rates (ß=-10/74; SE=3.41; p=0.002), and greater supply of primary care physicians (ß=0.18; SE=0.05; p=0.001) and nephrologists (ß=15.89; SE=1.22; p<0.001). Conclusions: Geographic variation was observed in the supply of vascular access physicians.

explain disparities in vascular access and outcomes in the U.S. and should be the subject of further study and improvement.
Vascular access is a lifeline to the health and wellbeing for patients receiving chronic hemodialysis; it is also one of the most challenging and expensive aspects of hemodialysis care. 1,2 The Medicare fee-for-service program spent an estimated $2.8 billion for services related to dialysis vascular access, half of which was dedicated to inpatient vascular access procedures or complications. 3 In 2004, the Centers for Medicare and Medicaid Services (CMS) established the Fistula First Breakthrough Initiative (FFBI) in the wake of initial evidence on arteriovenous fistulas' (AVF) durability, low rates of infection and thrombosis, and low patient mortality. [4][5][6] The FFBI set a goal of optimizing vascular access that prioritized AVF over arteriovenous grafts (AVG) and central venous catheters. 7,8 While there has been demonstrated improvement in fistula frequency since FFBI implementation, newer research reveals similar patency outcomes between AVF and AVG in the U.S. [9][10][11][12] When providers and patients discuss vascular access options, currently the choice of fistula or graft relies heavily on patient-level factors such as age, race, and vessel caliber.
Despite careful considerations of these factors, over 30% of fistulas have primary failure, and over 60% need interventions prior to use. 13 Potential explanations for the poor primary and secondary outcomes of AVF and AVG have continued to focus on patient factors that are often unmodifiable. An important but under-studied factor in the choice and success of vascular access is the physician who creates the fistula or graft. Earlier studies have examined the role of surgeons in vascular access outcomes. A single-site German study found the surgeon performing the procedure was the strongest predictor of AVF failure in the early 1990s. 14 O'Hare and colleagues found Veterans Health Administration surgeons' practice patterns in 2000-2001 were associated with type of vascular access use. 15 Using data from 12 countries in the Dialysis and Practice Patterns Study, Saran et al. showed surgical training was key to fistula placement and survival, 16 consistent with Goodkin and colleagues' (2010) review that vascular access surgery performed by surgeons who had placed a greater number of AVFs during their training was associated with higher odds of AVF placements and a reduced risk of AVF failure. 17 While this heterogenous group of studies suggests that physicians who create vascular access may be an important driver of vascular access utilization and outcomes, no study has examined systematically the supply, location, and types of physicians creating vascular access across the U.S. We sought to demonstrate the feasibility of developing a nationwide registry using existing data sources. In this paper, we described the methodology toward developing such a registry and examined the geographic distribution and characteristics of physicians placing permanent vascular access -specifically, AVF and AVG. Furthermore, we investigated arealevel socioeconomic status (SES) and market attributes in relation to the distribution of AVF/AVG access physicians (i.e., physicians who performed AVF and/or AVG placement).
Lastly, we examined whether the distribution of AVF/AVG access physicians was related to the supply of primary care physicians and nephrologists in the local area.

Methods
To test the feasibility of creating a nationwide registry of physicians who performed AVF and AVG placements, we conducted a cross-sectional observational study based on patient-level 2015 Medicare claims data. We then matched this claims-based registry to the American Physician Association (AMA) Physician Masterfile and Area Health Resource File to identify vascular access physicians' demographic, specialty, training and certification attributes, practice locations, and area attributes of their practice locations. Variables were aggregated to the hospital referral region (HRR) level. HRRs are a common measure of healthcare markets; their boundaries are based on actual patient utilization, reflecting the geographic extent of healthcare markets for tertiary care. 18 They are appropriate for studies of end stage renal disease (ESRD) services because ESRD patients receive a significant amount of specialized non-dialysis care in tertiary settings. 19 Data Sources The primary source of data was the U.S. Renal Data System (USRDS), which is the national repository of data and trends on all ESRD patients and providers. Patient information came from the Medical Evidence Report (CMS Form 2728), which ESRD providers are required to complete any time a patient begins or re-enters dialysis treatment or kidney transplantation services. Medicare claims data included inpatient, outpatient, and physician/supplier claims and were used to identify physicians who performed vascular access procedures. Data from these files were merged to construct a comprehensive baseline patient record and aggregated to the HRR level for covariate adjustment.
We used the AMA Physician Masterfile to ascertain professional and practice attributes of physicians performing AVF and/or AVG placement procedures. The AMA Physician Masterfile contains demographic, medical training, certification and specialty information on >1,168,000 physicians (>98% coverage), members and non-members, residents and medical students in the U.S. We used information in the Area Health Resources File (AHRF) to measure demographic and socio-economic conditions of HRRs. AHRF contains a comprehensive array of county-level population statistics, including population counts, healthcare resources, personal income, and land area statistics. We converted county-level demographic and socio-economic statistics from AHRF to the zip code-level based on land-area weighting, and then aggregated zip code-level information to HRRs. Zip code assignments to HRRs were obtained from publicly available geographic boundary files from the Dartmouth Atlas of Health Care.

Participants
The primary subjects of the study were physicians who performed AVF and/or AVG

Measurement
The main outcome in this study was the supply of AVF/AVG access physicians, defined as the number of physicians performing AVF and/or AVG placement per 1,000 prevalent ESRD patients, across HRRs in 2015.
Linked NPI and AMA Physician Masterfile were used to obtain the following physician characteristics: age, sex, zip code, MD vs. DO, indicator of U.S. vs. international medical graduates, year of graduation (before 1980, 1980-1989, 1990-2000

Statistical Analyses
Descriptive statistics (counts, mean, 95% confidence interval, and proportions) were used to examine the dispersion and distribution of AVF/AVG access physician supply and professional characteristics (e.g., specialty, year of practice). Choropleth maps were generated to display the regional (HRR) supply of AVF/AVG access physicians across the U.S. We then employed multivariate analysis using Poisson regression in General Linearized Models (GLM) to examine the relationship between AVF/AVG access physician supply (response variable) and demographic/socioeconomic conditions of HRRs.
All analyses used SAS statistical software (version 9.4; Statistical Analysis Institute Inc.) and R (version 3.5.0; R Foundation for Statistical Computing). Statistical significance was defined as p<0.05. The average age of AVF/AVG access physicians was 51.6 years, with the majority in the 45-64 age group (57.5%) ( Table 1). An overwhelming majority of the physicians were male (90.8%), had an MD degree (95.5%), and received their medical school education in the U.S.

Supply, Geographic Distribution, and Characteristics of AVF/AVG Access Physicians
(76.4%). The distribution of physicians by medical school graduation year was skewed towards recent decades, consistent with the distribution by number of years in practice, where nearly a third of physicians (32.4%) had less than 10 years of practice experience with an average length of practice of 17.4 years. In terms of medical specialty, 44.8% of physicians identified vascular surgery as their primary specialty, 38.8% more in other surgical specialties (thoracic/cardiac, transplant, general, urology, and others), and the remainder were in nephrology (2.7%), radiology (0.1%), and other medical specialties (13.5%).
A substantial portion of vascular access physicians performed only one type of vascular access placement, rather than both. We suspected that this practice pattern may be associated with personal and professional attributes. Thus, for comparison, The practice volume of vascular access physician varied substantially. The large majority of physicians (N=3,494; 73.5%) performed less than 10 procedures in 2015. Of the 1,258 physicians performing more than 10 procedures in 2015, 1,197 performed both AVF and AVG, 61 performing AVF only, and none performing AVG only. In general, high-volume physicians were older, more likely to be male, graduated from the medical school in earlier years, and had more years of practice than physicians who performed less than 10 vascular access procedures in 2015. With few exceptions (1.7%), all of them had a surgical background and 68.4% were vascular surgeons (detailed results available from the corresponding author).

Analysis of the Vascular Access Physician Supply
As a framework for assessing factors that were associated with the distribution of AVF/AVG access physicians, we drew on location theory. The theory assumes generally that physicians are economic agents acting in their own self-interest and that their choice of practice location reflects personal preferences for income as well as environmental and professional considerations. [20][21][22] Based on these assumptions, we expected there would be a greater supply of AVF/AVG access physicians in HRRs with a higher degree of urbanicity, higher % White population, higher % population with a high school diploma, and better economic conditions, because these HRR conditions meant there would be a higher demand for vascular access placement and the living conditions would be more favorable to physicians. Similarly, AVF/AVG access physicians may be more likely to practice in HRRs with a higher supply of primary care physicians and nephrologists, because there would be more patient referrals and better medical support for vascular access services. Figure 3 displays the distribution of AVF/AVG access physician supply by HRR characteristics. As expected, the supply of AVG/AVG access physicians was significantly higher in HRRs with a higher % White population (r=0.22, p<0.001), a higher % population with high school diploma (r=0.25, p<0.001), a higher level of per capita income (r=0.15, p=0.01); and the supply was significantly lower in HRRs with a higher % population in poverty (r=-0.24, p<0.001) and a higher unemployment rate (r=-0.23, p<0.001). Consistent with our expectation, the supply of AVF/AVG access physicians was also positively associated with the supply of primary care physicians (r=0.36, p<0.001) and nephrologists (r=0.53, p<0.001) in HRRs. There was no difference in AVF/AVG access physician supply by the level of urbanicity (r=-0.03, p=0.63).
In adjusted analysis (Table 2) We also compared the location differences between AVF only, AVG only, and AVF and AVG physicians and between high (>10 procedure) and low-volume (<=10 procedures) physicians (detailed results are available from the corresponding author). In general, physicians performing AVF only and AVG only were more likely to be located in HRRs with higher % White population, better economic conditions (i.e., lower poverty, lower unemployment rates, and higher per capita income), and higher supply of primary care physicians and nephrologists.
In multivariate analysis, the only factor significantly associated with the geographic distribution of high-volume (>10) physicians, who were more likely to perform both AVF and AVG, was the supply of nephrologists, suggesting their practice location was determined primarily by referral sources.

Discussion
In this nationally representative study, we developed a novel registry of 4,752 vascular access physicians by systematically matching Medicare-claims data on ESRD and pre-ESRD patients with the AMA Physician Masterfile for the year 2015. We found the majority of physicians creating fistulas and grafts were vascular surgeons or general surgeons. While the majority of these physicians created both types of access, surprisingly 31.1% created only AVF while 6.1% created only AVG. A minority of vascular access physicians performed more than 10 placements in 2015 and they almost exclusively were surgeons. Furthermore, the registry demonstrated marked variation in the supply of vascular access physicians across the country.
Additionally, area-level characteristics were associated with the supply of vascular access physicians.
Building on the methodology in Shahinian et al., 23 our study is one of the first to detail the demographic and specialty profiles of vascular access physicians across the 50 states of the U.S. It is interesting to note that the vast majority of vascular access physicians are male, likely reflecting the general gender distribution of physicians in surgical specialties. Similar to previous studies using U.S. and international data, 16,17 we found wide variation in specialty among vascular access physicians. Physicians in surgical specialties -primarily those in vascular surgery (45%) and general surgery (29%) -constituted the vast majority (84%) of physicians who performed AVF and/or AVG placement procedures in 2015. In contrast, a Dialysis Outcomes and Practice Patterns Study (DOPPS) showed that, in 2003, 61% of vascular access surgeries in the U.S. were conducted by vascular surgeons and 31% by general surgeons. 16 The different findings suggest a possibility that there may be increasing involvement of medical specialists in vascular access creation over time, and a shift in the professional background of vascular access physicians from surgical to medical specialties.
Our findings reveal a stark contrast in the role of medical specialists in vascular access creation. According to our analysis, nephrologists constituted only 3% of vascular access physicians in the U.S. This rate was lower than those observed in other countries. Approximately 30% of vascular access surgeries in Japan were conducted by nephrologists; in Italy, nephrologists were the primary vascular access operators in 85% of hemodialysis facilities. 16 Such practice pattern differences may be due to medical training and credential issues across countries and may continue to evolve with the introduction of percutaneous AVF creation technique. Moreover, the medical specialties represented in our study were considerably diverse -including pulmonary critical care medicine, internal medicine, critical care medicine, anesthesiology, cardiovascular disease, family practice, and vascular and interventional radiology -and a small percentage of those physicians claims surgery as their second specialty. This level of diversity appears unique in the U.S. and has important implications regarding the quality of vascular access placement in consideration of a small number of placement procedures they tended to perform as well as three prior research findings. First, research indicated that adequate training in vascular access creation predicted vascular access patency in hemodialysis patients. 16,17 Second, a surgeon's prior volume of AVF placement was shown to be associated with successful AVF maturation. 23 Third, U.S. surgeons had less vascular access training than surgeons in other countries. 17 Medical specialists, conceivably, may receive even less training on vascular access placement; they may not focus a significant portion of their practice on the procedures; and the volume they performed tended to be small. If so, a sizable number of the vascular access procedures performed in the U.S. may not produce reliable and sustainable access for hemodialysis patients. To verify this concern, future research linking the professional background and specialty of vascular access physicians to quality and outcomes of vascular access procedures is needed. Surprisingly, we found nearly half of vascular access physicians performed only AVF or AVG placement in 2015. The reasons for this specialization is not clear, and cannot be plausibly attributable to the measured physician characteristics such as gender, country of medical graduation, and others. In a comparison of high and low-volume vascular access physicians we found high-volume physicians were more likely to performed both AVF and AVG and were more likely to be in areas with lower % White population and poorer economic conditions. These findings suggest that the degree of specialization may be related to local practice patterns or reimbursement. Future research is needed to further investigate and corroborate the findings.
Our analysis showed substantial geographic variation in physician supply.
Comparatively, HRRs in the South (particularly, Georgia, Alabama, Mississippi, and Louisiana), Appalachia, and Midwest/Plains regions were underserved by physicians who performed vascular access procedures for hemodialysis patients. Besides such regional differences, the distribution of vascular access physicians was associated with demographic, economic, and health resources conditions of HRRs in a manner that is consistent with the prediction of the location theory. [20][21][22] Specifically, we found vascular access physicians were more likely to locate in HRRs with a higher % White population and less likely to be in HRRs with higher unemployment rates.
These findings are interesting in light of the persistent geographic and demographic variation in fistula prevalence despite the overall success of the FFBI initiative. Geographic variation has been observed across the 18 End Stage Renal Disease Networks. Hemodialysis patients in New England have the highest rates of initiation with AVF, while patients in the Southeast have the lowest rates. [24][25] At the county level, variation in incident and baseline prevalent AVF rates was associated with the concentration of poverty in a dialysis center's county. 26 Moreover, racial and ethnic variation has been observed in AVF utilization. Using CMS Medical Evidence Report data, Arce et al. found that Hispanics were less likely to use AVF for first outpatient HD compared with non-Hispanics. 27 In a different study, Zarkowsky et al.
showed that Black and Hispanic patients were less likely to initiate hemodialysis with an AVF than White patients despite being younger and having fewer comorbidities and that the differences were not explained by factors related to healthcare such as medical insurance status and pre-dialysis nephrology care. 28 In consideration of all this previous evidence, the findings of our study suggest that the supply of vascular access physicians may be a crucial and previously overlooked factor. Specifically, the tendency of vascular access physicians to practice in socially and economically advantaged areas may explain the persistent evidence that minority dialysis patients have a lower frequency of functional AVF and AVG than White patients. If this is confirmed by further research that directly links the supply of vascular access physicians to prevalence and success of AVG and AVG placement, improvement in the medical school curriculum and training of vascular access as well as change in reimbursement policy would be needed to increase the supply of vascular access physicians in underserved areas and disadvantaged patient populations.
We also found that vascular access physicians, particularly those who performed a high number of procedures, tended to practice in HRRs where there was abundant supply of primary care physicians and nephrologists. This finding suggests that solving maldistribution of vascular access physicians needs to consider the overall distribution of medical resources, as indicated by supply of primary care physicians and nephrologists.
Our study was limited by its cross-sectional design. We have demonstrated the feasibility of reliably identifying vascular access physicians, their professional attributes and practice information using ESRD and pre-ESRD claims data and existing secondary datasets. Future