Consensus Statement on the Management of Nonthrombotic Iliac Vein Lesions From the VIVA Foundation, the American Venous Forum, and the American Vein and Lymphatic Society.

A nonthrombotic iliac vein lesion is defined as the extrinsic compression of the iliac vein. Symptoms of lower extremity chronic venous insufficiency or pelvic venous disease can develop secondary to nonthrombotic iliac vein lesion. Anatomic compression has been observed in both symptomatic and asymptomatic patients. Causative factors that lead to symptomatic manifestations remain unclear. To provide guidance for providers treating patients with nonthrombotic iliac vein lesion, the VIVA Foundation convened a multidisciplinary group of leaders in venous disease management with representatives from the American Venous Forum and the American Vein and Lymphatic Society. Consensus statements regarding nonthrombotic iliac vein lesions were drafted by the participants to address patient selection, imaging for diagnosis, technical considerations for stent placement, postprocedure management, and future research/educational needs.

Evaluating the Effectiveness of Systemic Heparin During Arteriovenous Fistula Creation by Emulating a Target Trial.

Most of the 800,000 people living with end-stage kidney disease in the United States rely on a functioning vascular access to provide life-sustaining hemodialysis, yet one-third of arteriovenous fistulas experience early failures. Determining the safety and effectiveness of systemic heparin during fistula creation could improve the quality and quantity of life for these vulnerable patients. In this paper, a pragmatic randomized trial was emulated to assess the effect of systemic heparin administration (vs. none) during radiocephalic arteriovenous fistula creation on early bleeding and thrombosis using data from two international multicenter randomized trials performed between 2014 and 2019. Marginal risks were estimated using inverse probability weighted parametric survival analysis and confidence intervals were generated with bootstrapping. A total of 914 patients were enrolled and 61% received systemic heparin; median (IQR) age was 58 (49, 67) years and 45% were on hemodialysis at enrollment. No difference in the risk of bleeding events was observed, with a risk difference (95% CI) at 14 days of -0.1% (-1.6, 1.4). The risk of access thrombosis was lower in the heparin group, with a risk of 3.7% (2.6, 4.8) after heparin and 5.3% (3.4, 7.4) without heparin at 14 days (risk ratio 0.72, 95% CI 0.50, 0.98).

Impact of Cerebral Embolic Protection Devices on Disabling Stroke after Transcatheter Aortic Valve Replacement: Updated Results from the STS/ACC TVT Registry.

Background: Cerebral embolic protection devices (EPDs) were developed to mitigate the risk of stroke during transcatheter aortic valve replacement (TAVR), but their benefit remains unproven. In the PROTECTED-TAVR trial, EPD use did not reduce periprocedural stroke (primary study outcome) but led to a 62% reduction in the secondary endpoint of disabling stroke. Given these results, the impact of EPDs during TAVR remains unclear. Methods: We used STS/ACC TVT registry data to examine the association between EPD use and a proxy for disabling stroke among transfemoral TAVR patients between 1/2018-6/2023. The primary outcome was in-hospital disabling stroke-defined as stroke associated with either in-hospital death or discharge to a non-home location. We evaluated the association between EPD use and disabling stroke using instrumental variable (IV) analysis with site-level preference for EPD use as the instrument-a quasi-experimental approach that can support causal inference. In addition, we performed a propensity-score based comparison using overlap weighting as a secondary analysis. Results: The study population consisted of 414,649 patients of whom 53,389 (12.9%) received an EPD. The unadjusted rate of in-hospital disabling stroke was 0.7% among the EPD group and 0.9% in the no EPD group. EPD use was associated with a reduction in disabling stroke in both IV analysis (RR 0.87; 95% CI: 0.73-1.00) and propensity-weighted (PW) analysis (OR 0.79; 95% CI: 0.70-0.90) but was not associated with a reduction in non-disabling stroke. In subgroup analyses, the benefit of EPD was greater among those with versus without prior stroke (interaction p<0.05 for IV and PW). Conclusions: In the largest study to date, among patients undergoing TAVR, EPD use was associated with a small, borderline significant reduction in stroke associated with death or a discharge to a non-home location (a proxy for disabling stroke) that is likely to be causal in nature. Taken together with previous mechanistic and clinical studies, these findings provide credible evidence that EPDs benefit patients undergoing TAVR.

Using Separate Single-Outcome Risk Presentations Instead of Integrated Multioutcome Formats Improves Comprehension in Discrete Choice Experiments.

INTRODUCTION: Despite decades of research on risk-communication approaches, questions remain about the optimal methods for conveying risks for different outcomes across multiple time points, which can be necessary in applications such as discrete choice experiments (DCEs). We sought to compare the effects of 3 design factors: 1) separated versus integrated presentations of the risks for different outcomes, 2) use or omission of icon arrays, and 3) vertical versus horizontal orientation of the time dimension. METHODS: We conducted a randomized study among a demographically diverse sample of 2,242 US adults recruited from an online panel (mean age 59.8 y, s = 10.4 y; 21.9% African American) that compared risk-communication approaches that varied in the 3 factors noted above. The primary outcome was the number of correct responses to 12 multiple-choice questions asking survey respondents to identify specific numbers, contrast options to recognize dominance (larger v. smaller risks), and compute differences. We used linear regression to test the effects of the 3 design factors, controlling for health literacy, graph literacy, and numeracy. We also measured choice consistency in a subsequent DCE choice module. RESULTS: Mean comprehension varied significantly across versions (P < 0.001), with higher comprehension in the 3 versions that provided separated risk information for each risk. In the multivariable regression, separated risk presentation was associated with 0.58 more correct responses (P < 0.001; 95% confidence interval: 0.39, 0.77) compared with integrated risk information. Neither providing icon arrays nor using vertical versus horizontal time formats affected comprehension rates, although participant understanding did correlate with DCE choice consistency. CONCLUSIONS: In presentations of multiple risks over multiple time points, presenting risk information separately for each health outcome appears to increase understanding. HIGHLIGHTS: When conveying information about risks of different outcomes at multiple time points, separate presentations of single-outcome risks resulted in higher comprehension than presentations that combined risk information for different outcomes.We also observed benefits of presenting single-outcome risks separately among respondents with lower numeracy and graph literacy.Study participants who scored higher on risk understanding were more internally consistent in their responses to a discrete choice experiment.

2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS Guideline for the Management of Lower Extremity Peripheral Artery Disease: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines.

AIM: The "2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS Guideline for the Management of Lower Extremity Peripheral Artery Disease" provides recommendations to guide clinicians in the treatment of patients with lower extremity peripheral artery disease across its multiple clinical presentation subsets (ie, asymptomatic, chronic symptomatic, chronic limb-threatening ischemia, and acute limb ischemia). METHODS: A comprehensive literature search was conducted from October 2020 to June 2022, encompassing studies, reviews, and other evidence conducted on human subjects that was published in English from PubMed, EMBASE, the Cochrane Library, CINHL Complete, and other selected databases relevant to this guideline. Additional relevant studies, published through May 2023 during the peer review process, were also considered by the writing committee and added to the evidence tables where appropriate. STRUCTURE: Recommendations from the "2016 AHA/ACC Guideline on the Management of Patients With Lower Extremity Peripheral Artery Disease" have been updated with new evidence to guide clinicians. In addition, new recommendations addressing comprehensive care for patients with peripheral artery disease have been developed.

2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS Guideline for the Management of Lower Extremity Peripheral Artery Disease: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines.

AIM: The "2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS Guideline for the Management of Lower Extremity Peripheral Artery Disease" provides recommendations to guide clinicians in the treatment of patients with lower extremity peripheral artery disease across its multiple clinical presentation subsets (ie, asymptomatic, chronic symptomatic, chronic limb-threatening ischemia, and acute limb ischemia). METHODS: A comprehensive literature search was conducted from October 2020 to June 2022, encompassing studies, reviews, and other evidence conducted on human subjects that was published in English from PubMed, EMBASE, the Cochrane Library, CINHL Complete, and other selected databases relevant to this guideline. Additional relevant studies, published through May 2023 during the peer review process, were also considered by the writing committee and added to the evidence tables where appropriate. STRUCTURE: Recommendations from the "2016 AHA/ACC Guideline on the Management of Patients With Lower Extremity Peripheral Artery Disease" have been updated with new evidence to guide clinicians. In addition, new recommendations addressing comprehensive care for patients with peripheral artery disease have been developed.

Comparative Effectiveness of Intravascular Ultrasound Versus Angiography in Abdominal and Thoracic Endovascular Aortic Repair: Systematic Review and Meta-Analysis.

The effectiveness of intravascular ultrasound (IVUS) with angiography compared with angiography guidance alone in treating aortic conditions, such as dissections, aneurysms, and blunt traumatic injuries, remains unclear. This systematic review and meta-analysis evaluates the current literature for IVUS use during thoracic endovascular aortic repair (TEVAR) and abdominal endovascular aortic repair (EVAR). A comprehensive search of MEDLINE, EMBASE, and Cochrane CENTRAL databases was conducted in March 2024 adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Studies comparing outcomes of TEVAR/EVAR with and without IVUS were identified. The outcomes of interest included contrast volume, fluoroscopy and procedural time, perioperative endoleak, and reinterventions and all-cause mortality during follow-up. Data with 95% confidence intervals (CIs) were extracted. Pooled analysis was performed using a random-effect model. Subgroup analysis was performed stratified by the condition being treated. Risk of bias was assessed using the Newcastle-Ottawa Scale for observational studies. A total of 4,219 patients (n = 2,655 IVUS and n = 1,564 non-IVUS) from 9 observational studies were included. The IVUS group exhibited a reduction in contrast agent volume (weighted mean difference -34.65 mL, 95% CI -54.73 to -14.57) and fluoroscopy time (weighted mean difference -6.13 minutes, 95% CI -11.10 to -1.15), with no difference in procedural time. The perioperative type I and III endoleak occurrences were similar (risk ratio 2.36, 95% CI 0.55 to 10.11; risk ratio 0.72, 95% CI 0.09 to 5.77, respectively). Reintervention and mortality during follow-up were comparable (hazard ratio 0.80, 95% CI 0.33 to 1.97; hazard ratio 0.75, 95% CI 0.47 to 1.18, respectively). All the included studies had small risks of bias. In conclusion, this meta-analysis provides evidence that IVUS enables the safe deployment of TEVAR/EVAR with reduced contrast agent and radiation exposure.

Community Socioeconomic Status, Acute Cardiovascular Hospitalizations, and Mortality in Medicare, 2003 to 2019.

BACKGROUND: Socioeconomically disadvantaged communities in the United States disproportionately experience poor cardiovascular outcomes. Little is known about how hospitalizations and mortality for acute cardiovascular conditions have changed among Medicare beneficiaries in socioeconomically disadvantaged and nondisadvantaged communities over the past 2 decades. METHODS: Medicare files were linked with the Centers for Disease Control and Prevention's social vulnerability index to examine age-sex standardized hospitalizations for myocardial infarction, heart failure, ischemic stroke, and pulmonary embolism among Medicare fee-for-service beneficiaries ≥65 years of age residing in socioeconomically disadvantaged communities (highest social vulnerability index quintile nationally) and nondisadvantaged communities (all other quintiles) from 2003 to 2019, as well as risk-adjusted 30-day mortality among hospitalized beneficiaries. RESULTS: A total of 10 942 483 Medicare beneficiaries ≥65 years of age were hospitalized for myocardial infarction, heart failure, stroke, or pulmonary embolism (mean age, 79.2 [SD, 8.7] years; 53.9% female). Although age-sex standardized myocardial infarction hospitalizations declined in socioeconomically disadvantaged (990-650 per 100 000) and nondisadvantaged communities (950-570 per 100 000) from 2003 to 2019, the gap in hospitalizations between these groups significantly widened (adjusted odds ratio 2003, 1.03 [95% CI, 1.02-1.04]; adjusted odds ratio 2019, 1.14 [95% CI, 1.13-1.16]). There was a similar decline in hospitalizations for heart failure in socioeconomically disadvantaged (2063-1559 per 100 000) and nondisadvantaged communities (1767-1385 per 100 000), as well as for ischemic stroke, but the relative gap did not change for both conditions. In contrast, pulmonary embolism hospitalizations increased in both disadvantaged (146-184 per 100 000) and nondisadvantaged communities (153-184 per 100 000). By 2019, risk-adjusted 30-day mortality was similar between hospitalized beneficiaries from socioeconomically disadvantaged and nondisadvantaged communities for myocardial infarction, heart failure, and ischemic stroke but was higher for pulmonary embolism (odds ratio, 1.10 [95% CI, 1.01-1.20]). CONCLUSIONS: Over the past 2 decades, hospitalizations for most acute cardiovascular conditions decreased in both socioeconomically disadvantaged and nondisadvantaged communities, although significant disparities remain, while 30-day mortality is now similar across most conditions.

Device regulation and surveillance in vascular care: Challenges and opportunities.

Cardiovascular devices are essential for the treatment of cardiovascular diseases including cerebrovascular, coronary, valvular, congenital, peripheral vascular and arrhythmic diseases. The regulation and surveillance of vascular devices in real-world practice, however, presents challenges during each individual product's life cycle. Four examples illustrate recent challenges and questions regarding safety, appropriate use and efficacy arising from FDA approved devices used in real-world practice. We outline potential pathways wherein providers, regulators and payors could potentially provide high-quality cardiovascular care, identify safety signals, ensure equitable device access, and study potential issues with devices in real-world practice.

Association Between Diagnosis-to-Limb Revascularization Time and Clinical Outcomes in Outpatients With Chronic Limb-Threatening Ischemia: Insights From the CLIPPER Cohort.

BACKGROUND: The extent and consequences of ischemia in patients with chronic limb-threatening ischemia (CLTI) may change rapidly, and delays from diagnosis to revascularization may worsen outcomes. We sought to describe the association between time from diagnosis to endovascular lower extremity revascularization (diagnosis-to-limb revascularization [D2L] time) and clinical outcomes in outpatients with CLTI. METHODS AND RESULTS: In the CLIPPER cohort, comprising patients between 66 and 86 years old diagnosed with CLTI betweeen 2010 and 2019, we used Medicare claims data to identify patients who underwent outpatient endovascular revascularization within 180 days of diagnosis. We described the risk-adjusted association between D2L time and clinical outcomes. Among 1 130 065 patients aged between 66 and 86 years with CLTI, 99 221 (8.8%) underwent outpatient endovascular lower extremity revascularization within 180 days of their CLTI diagnosis. Among patients with D2L time <30 days, there was no association between D2L time and all-cause death or major lower extremity amputation. However, among patients with D2L time >30 days, each additional 10-day increase in D2L time was associated with a 2.5% greater risk of major amputation (hazard ratio, 1.025 [95% CI, 1.014-1.036]). There was no association between D2L time and all-cause death. CONCLUSIONS: A delay of >30 days from CLTI diagnosis to lower extremity endovascular revascularization was associated with an increased risk of major lower extremity amputation among patients undergoing outpatient endovascular revascularization. Improving systems of care to reduce D2L time could reduce amputations.

Opportunities and Limitations of Renal Denervation: Where Do We Stand?

Hypertension is a primary contributor to cardiovascular disease, and the leading risk factor for loss of quality adjusted life years. Up to 50% of the cases of hypertension in the United States remain uncontrolled. Additionally, 8%-18% of the hypertensive population have resistant hypertension; uncontrolled pressure despite 3 different antihypertensive agents. Recently, catheter-based percutaneous renal denervation emerged as a method for ablating renal sympathetic nerves for difficult-to-control hypertension. Initial randomized (non-sham) trials and registry analyses showed impressive benefit, but the first sham-controlled randomized controlled trial using monopolar radiofrequency ablation showed limited benefit. With refinement of techniques to include multipolar radiofrequency, ultrasound denervation, and direct ethanol injection, randomized controlled trials demonstrated significant blood pressure improvement, leading to US Food and Drug Administration approval of radiofrequency- and ultrasound-based denervation technologies. In this review article, we summarize the major randomized sham-controlled trials and societal guidelines regarding the efficacy and safety of renal artery denervation for the treatment of uncontrolled hypertension.

Why selective screening for asymptomatic carotid stenosis is currently appropriate: a special report.

INTRODUCTION: Two of the main reasons recent guidelines do not recommend routine population-wide screening programs for asymptomatic carotid artery stenosis (AsxCS) is that screening could lead to an increase of carotid revascularization procedures and that such mass screening programs may not be cost-effective. Nevertheless, selective screening for AsxCS could have several benefits. This article presents the rationale for such a program. AREAS COVERED: The benefits of selective screening for AsxCS include early recognition of AsxCS allowing timely initiation of preventive measures to reduce future myocardial infarction (MI), stroke, cardiac death and cardiovascular (CV) event rates. EXPERT OPINION: Mass screening programs for AsxCS are neither clinically effective nor cost-effective. Nevertheless, targeted screening of populations at high risk for AsxCS provides an opportunity to identify these individuals earlier rather than later and to initiate a number of lifestyle measures, risk factor modifications, and intensive medical therapy in order to prevent future strokes and CV events. For patients at 'higher risk of stroke' on best medical treatment, a prophylactic carotid intervention may be considered.

Optimizing antithrombotic therapy in patients with coexisting cardiovascular and gastrointestinal disease.

Balancing the safety and efficacy of antithrombotic agents in patients with gastrointestinal disorders is challenging because of the potential for interference with the absorption of antithrombotic drugs and for an increased risk of bleeding. In this Review, we address considerations for enteral antithrombotic therapy in patients with cardiovascular disease and gastrointestinal comorbidities. For those with gastrointestinal bleeding (GIB), we summarize a general scheme for risk stratification and clinical evidence on risk reduction approaches, such as limiting the use of concomitant medications that increase the risk of GIB and the potential utility of gastrointestinal protection strategies (such as proton pump inhibitors or histamine type 2 receptor antagonists). Furthermore, we summarize the best available evidence and potential gaps in our knowledge on tailoring antithrombotic therapy in patients with active or recent GIB and in those at high risk of GIB but without active or recent GIB. Finally, we review the recommendations provided by major medical societies, highlighting the crucial role of teamwork and multidisciplinary discussions to customize the antithrombotic regimen in patients with coexisting cardiovascular and gastrointestinal diseases.

Modelling costs of interventional pulmonary embolism treatment: implications of US trends for a European healthcare system.

AIMS: Catheter-directed treatment (CDT) of acute pulmonary embolism (PE) is entering a growth phase in Europe following a steady increase in the USA in the past decade, but the potential economic impact on European healthcare systems remains unknown. METHODS AND RESULTS: We built two statistical models for the monthly trend of proportion of CDT among patients with severe (intermediate- or high-risk) PE in the USA. The conservative model was based on admission data from the National Inpatient Sample (NIS) 2016-20 and the model reflecting increasing access to advanced treatment from the PERT™ national quality assurance database registry 2018-21. By applying these models to the forecast of annual PE-related hospitalizations in Germany, we calculated the annual number of severe PE cases and the expected increase in CDT use for the period 2025-30. The NIS-based model yielded a slow increase, reaching 3.1% (95% confidence interval 3.0-3.2%) among all hospitalizations with PE in 2030; in the PERT-based model, increase would be steeper, reaching 8.7% (8.3-9.2%). Based on current reimbursement rates, we estimated an increase of annual costs for PE-related hospitalizations in Germany ranging from 15.3 to 49.8 million euros by 2030. This calculation does not account for potential cost savings, including those from reduced length of hospital stay. CONCLUSION: Our approach and results, which may be adapted to other European healthcare systems, provide a benchmark for healthcare costs expected to result from CDT. Data from ongoing trials on clinical benefits and cost savings are needed to determine cost-effectiveness and inform reimbursement decisions.

Clinical Outcomes and Predictors of Advanced Therapy for the Management of Right Heart Thrombus.

BACKGROUND: The role of advanced therapies (systemic thrombolysis, catheter-based treatment, and surgical thrombectomy) for the management of right heart thrombus is poorly defined. In this study, we assessed the clinical predictors and outcomes of advanced therapy compared with anticoagulation alone for the acute management of right heart thrombus. METHODS: In this observational cohort study, we analyzed consecutive patients who were treated for right heart thrombus. The primary end point was 90-day all-cause mortality. Clinical predictors of utilizing advanced therapy were assessed with multivariable logistic regression. Propensity score matching was utilized to compare adjusted outcomes between patients receiving advanced therapies versus anticoagulation alone. RESULTS: A total of 345 patients were included in the study. Advanced therapy was utilized in 13.6% (N=47) of patients, of which 25.5% (N=12/47) was systemic thrombolysis, 23.4% (N=11/47) was endovascular thrombectomy, and 53.2% (N=25/47) was surgical thrombectomy. Younger age (odds ratio, 0.98 [95% CI, 0.96-0.99]) and concurrent pulmonary embolism (odds ratio, 5.36 [95% CI, 2.48-12.1]) predicted utilization of advanced therapy. In propensity score-matched analysis, there was no difference in 90-day mortality (hazard ratio, 0.46 [95% CI, 0.17-1.22]), in-hospital mortality (odds ratio, 0.64 [95% CI, 0.17-2.19]), or length of stay (ß, -4.39 [95% CI, -14.0 to 5.22]) between advanced therapy and anticoagulation. CONCLUSIONS: Among a diverse cohort of patients with right heart thrombus, outcomes did not differ between those who underwent advanced therapy and anticoagulation alone. Important predictors for utilizing advanced treatment included younger age and the presence of a concurrent pulmonary embolism. Future studies assessing advanced therapy in larger and broader patient populations are necessary.

Intravascular Ultrasound Use in Peripheral Arterial and Deep Venous Interventions: Multidisciplinary Expert Opinion From SCAI/AVF/AVLS/SIR/SVM/SVS.

Percutaneous revascularization is the primary strategy for treating lower extremity venous and arterial disease. Angiography is limited by its ability to accurately size vessels, precisely determine the degree of stenosis and length of lesions, characterize lesion morphology, or correctly diagnose postintervention complications. These limitations are overcome with use of intravascular ultrasound (IVUS). IVUS has demonstrated the ability to improve outcomes following percutaneous coronary intervention, and there is increasing evidence to support its benefits in the setting of peripheral vascular intervention. At this stage in its evolution, there remains a need to standardize the use and approach to peripheral vascular IVUS imaging. This manuscript represents considerations and consensus perspectives that emerged from a roundtable discussion including 15 physicians with expertise in interventional cardiology, interventional radiology, and vascular surgery, representing 6 cardiovascular specialty societies, held on February 3, 2023. The roundtable's aims were to assess the current state of lower extremity revascularization, identify knowledge gaps and need for evidence, and determine how IVUS can improve care and outcomes for patients with peripheral arterial and deep venous pathology.

Treatment Variability Among Patients Hospitalized for Chronic Limb-Threatening Ischemia: An Analysis of the 2016 to 2018 US National Inpatient Sample.

BACKGROUND: Little is known about treatment variability across US hospitals for patients with chronic limb-threatening ischemia (CLTI). METHODS AND RESULTS: Data were collected from the 2016 to 2018 National Inpatient Sample. All patients aged ≥18 years, admitted to nonfederal US hospitals with a primary diagnosis of CLTI, were identified. Patients were classified according to their clinical presentation (rest pain, skin ulceration, or gangrene) and were further characterized according to the treatment strategy used. The primary outcome of interest was variability in CLTI treatment, as characterized by the median odds ratio. The median odds ratio is defined as the likelihood that 2 similar patients would be treated with a given modality at 1 versus another randomly selected hospital. There were 15 896 (weighted n=79 480) hospitalizations identified where CLTI was the primary diagnosis. Medical therapy alone, endovascular revascularization ± amputation, surgical revascularization ± amputation, and amputation alone were used in 4057 (25%), 5390 (34%), 3733 (24%), and 2716 (17%) patients, respectively. After adjusting for both patient- and hospital-related factors, the median odds ratio (95% CI) for medical therapy alone, endovascular revascularization ± amputation, surgical revascularization ± amputation, any revascularization, and amputation alone were 1.28 (1.19-1.38), 1.86 (1.77-1.95), 1.65 (1.55-1.74), 1.37 (1.28-1.45), and 1.42 (1.27-1.55), respectively. CONCLUSIONS: Significant variability in CLTI treatment exists across US hospitals and is not fully explained by patient or hospital characteristics.