title: Altitude and the risk of cardiovascular events in incident US dialysis patients.
abstract: BACKGROUND
  Altitude is associated with all-cause mortality in US dialysis patients, but its association with cardiovascular outcomes has not been assessed. We hypothesized that higher altitude would be associated with lower rates of cardiovascular events due to an altered physiological response of dialysis patients to altitude induced hypoxia.
  
  
  METHODS
  We studied 984,265 patients who initiated dialysis from 1995 to 2006. Patients were stratified by the mean elevation of their residential zip codes and were followed from the start of dialysis to the occurrence of several validated cardiovascular endpoints: myocardial infarction, stroke, cardiovascular death and a composite of these end points. Incidence rate ratios across altitude strata were estimated using proportional hazards regression.
  
  
  RESULTS
  All outcomes occurred less frequently among patients living at higher altitude compared with patients living at or near sea level, and the association appeared monotonic for all outcomes except for stroke, which was most incident in the 250-1999 ft group. Compared with otherwise similar patients residing at or near sea level, patients living at ≥ 6000 ft had 31% [95% confidence interval (CI): 21-41%] lower rates of myocardial infarction, 27% (95% CI: 15-37%) lower rates of stroke and 19% (95% CI: 14-24%) lower rates of cardiovascular death. Additional adjustment for biometric information did not materially change these findings. Effect modification between race and altitude was only consistently significant for Native Americans. Altitude did not significantly alter the rates of non-cardiovascular death.
  
  
  CONCLUSIONS
  We conclude that dialysis patients at higher altitude experience lower rates of cardiovascular events compared to otherwise similar patients at lower altitude.
authors.affiliation: Division of Nephrology, Department of Medicine, Stanford University School of Medicine, Palo Alto, CA, USA. wcw1@stanford.edu
authors.forename: Wolfgang C
authors.initials: WC
authors.lastname: Winkelmayer
authors.forename: Michael P
authors.initials: MP
authors.lastname: Hurley
authors.forename: Jun
authors.initials: J
authors.lastname: Liu
authors.forename: M Alan
authors.initials: MA
authors.lastname: Brookhart
mesh_terms: D000293:Adolescent; D000328:Adult; D000531:Altitude; D002140:California / Q000453:epidemiology; D002318:Cardiovascular Diseases / Q000453:epidemiology* / Q000401:mortality*; D016022:Case-Control Studies; D002423:Cause of Death; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007676:Kidney Failure, Chronic / Q000401:mortality* / Q000628:therapy*; D008297:Male; D008875:Middle Aged; D011379:Prognosis; D006435:Renal Dialysis / Q000401:mortality*; D015996:Survival Rate; D055815:Young Adult