Altitude and All-Cause Mortality in Incident Dialysis Patients

patients with end-stage renal disease (ESRD) living at higher altitude achieved greater hemoglobin concentrations while receiving lower doses of erythropoietin. 1 This study raised the possibility that certain biological factors that are induced by hypoxia may be responsible for this finding.Hypoxia-induced factors, whose transcriptional activities may be increased at higher altitude in patients with ESRD, have been shown to improve iron availability, which may lead to more efficient erythropoiesis. 2,3Hypoxia-induced factors also regulate many enzymes that could affect cardiovascular risk, such as vascular endothelial growth factor, heme oxygenase-1, inducible nitric oxide synthase, and cyclooxygenase 2. 4,5 From these theoretical effects of hypoxia-induced factor activation at higher altitude, we hypothesized that increased altitude would be associated with reduced mortality risk among patients initiating chronic dialysis in the United States, and that this association would be more pronounced than in the US general population due to the blunted erythropoietin response to hypoxia-induced factors activation in patients with ESRD. METHODS Data SourcesThis study used data from the United States Renal Data System (USRDS) and the US Geological Survey (USGS).The USRDS contains detailed data on all pa-tients in Medicare's ESRD program.The Medical Evidence Form contains demographic data; the likely cause of ESRD; some clinical baseline data such as weight and height; and certain laboratory measurements, such as serum albumin, creatinine, and hematocrit concentrations.In addition, the USRDS contains all Medicare Part A and B claims that include information on diagnoses and procedures recorded for all hospitalizations and outpatient visits.As described previously, we used data from the USGS and each patient's resi-dential zip code to define the altitude of each study patient's residence. 1he Brigham and Women's Hospital Institutional Review Board approved this research.