Putting the Vascular Back Into Cardiovascular Research

Despite the clinical burden of acute limb ischemia, its epidemiological characterization is limited. This deficit is especially striking compared with our understanding of the epidemiology of ST-segment–elevation myocardial infarction (STEMI), a vascular emergency of the coronary arteries. Although important differences in the pathogenesis and epidemiology of these diseases exist, the overlap is notable. STEMI and acute limb ischemia of atherothrombotic origin share vascular risk factors, and acute MI is a risk factor for embolic acute limb ischemia. Additionally, both STEMI and acute limb ischemia require time-critical treatment, and both account for significant cardiovascular morbidity and mortality. Approximately 500 000 STEMIs occurred in the United States in 2001 compared with acute limb ischemia in 213 000 patients. Inpatient costs associated with STEMI and acute limb ischemia also are similar, ranging from $14 304 to $23 678 per hospitalization for STEMI and $6000 to $45 000 per hospitalization for acute limb ischemia. Despite these similarities, our understanding of the epidemiology of acute limb ischemia and the impact of evolving systems of care and new interventional techniques on outcomes lags far behind those of STEMI. Why the lack of attention to an emergent clinical event associated with significant morbidity and mortality? First, vascular disease, both acute and chronic forms, has traditionally suffered from a lack of recognition by providers and patients. Second, the large, comprehensive cardiovascular registries that have made detailed clinical and epidemiological characterization of STEMI possible do not exist for acute lower-extremity arterial disease. Third, administrative claims databases that are often used for foundational descriptive analyses are limited in their ability to characterize nonprocedural arterial disease, and the validity of available diagnostic codes to characterize arterial disease has not been well established. It is in this setting that Korabathina et al examined 20-year trends in hospitalizations and mortality for lower-extremity arterial thromboembolism. After assessing the sensitivity, specificity, and predictive value of International Classification of Diseases, Ninth Revision, Clinical Modification codes for acute limb ischemia and chronic limb ischemia, the authors used the National Hospital Discharge Survey to characterize admissions for and inpatient mortality associated with acute and chronic lower-extremity ischemia. The authors report a decrease in the rate of admissions for acute and chronic limb ischemia from 42.4 per 100 000 people between 1988 and 1997 to 23.3 per 100 000 people between 1998 and 2007. During the same periods, in-hospital mortality decreased from 8.28% to 6.34% and was associated with decreasing use of surgical bypass and amputation and increasing use of catheterbased thrombolysis. The internal validation of the International Classification of Diseases, Ninth Revision, Clinical Modification codes used to define lower-extremity thromboembolism is commendable and appears to be the first of its kind among hospitalized patients with limb-threatening arterial disease. The results, however, are not encouraging. Although the 3 codes reliably identified lower-extremity arterial thromboembolism, the specificity (72.2%) and positive predictive value (46.6%) of the codes for diagnosing acute limb ischemia were suboptimal. Moreover, the applicability of their single-institution validation strategy to a nationwide sample spanning 20 years is debatable. Given the lack of clinical specificity in diagnosis codes for arterial disease, institutional heterogeneity in coding practices seems likely. Thus, in using these codes to define the study population, the authors have defined a cohort of limb ischemia inclusive of both hospitalized progressive chronic limb ischemia and true acute limb ischemia. In this mixed population, the authors observe a notable drop in the age-adjusted rate of inpatient limb ischemia, from 42.4 per 100 000 individuals (1988–1997) to 23.3 cases per 100 000 individuals (1998–2007). Although this may reflect a true decrease in limb ischemia, these results should be interpreted with caution. First, what the authors refer to as disease incidence is in fact the total number of hospitalizations because the National Hospital Discharge Survey does not include unique patient identifiers. Indeed, although readmission rates for patients with acute limb ischemia have not been reported, a single-center analysis of surgically treated patients with chronic limb ischemia suggested a 24% readmission rate The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association. From the Department of Medicine (S.V., L.H.C.), and the Duke Clinical Research Institute (L.H.C.), Duke University Medical Center, Durham, NC. Correspondence to Sreekanth Vemulapalli, MD, Duke University Medical Center, Box 3126, Durham, NC 27710. E-mail sreekanth. vemulapalli@dm.duke.edu Putting the Vascular Back Into Cardiovascular Research ST-Segment–Elevation Myocardial Infarction as a Blueprint for Improving Care in Patients With Acute Limb Ischemia