Altitude and all-cause mortality in incident dialysis patients.
BACKGROUND
Patients undergoing dialysis at higher altitude receive lower erythropoietin doses, yet achieve higher hemoglobin concentrations. Increased iron availability caused by activation of hypoxia-induced factors at higher altitude may explain this finding. Hypoxia-induced factors are also involved in other pathways that may affect morbidity and mortality.


OBJECTIVE
To study whether mortality differed by altitude in patients initiating dialysis.


METHODS
Retrospective cohort of patients initiating dialysis in the United States between 1995 and 2004. Patients were stratified by the average elevation of their residential zip code. Covariates included age, sex, race, Medicaid coverage, dialysis modality, comorbidities, and reported laboratory measurements. We constructed proportional hazards models of all-cause mortality, stratifying by year, and censoring patients at 5 years from first dialysis, at the end of the database (December 31, 2004), or loss to follow-up. We also compared age- and sex-adjusted standardized mortality rates of US patients receiving dialysis with the general population.


METHODS
Mortality from any cause.


RESULTS
A total of 804 812 patients initiated dialysis and were followed up for a median of 1.78 years. Crude mortality rates per 1000 person-years were 220.1 at an altitude lower than 76 m (<250 ft), 221.2 from 76 through 609 m (250-1999 ft), 214.6 from 610 through 1218 m (2000-3999 ft), 184.9 from 1219 through 1828 m (4000 to 5999 ft), and 177.2 at an altitude higher than 1828 m (>6000 ft). After multivariable adjustment, compared with patients living at an altitude of lower than 76 m, the relative mortality rates were 0.97 (95% confidence interval [CI], 0.96-0.98) for those living from 76 through 609 m; 0.93 (95% CI, 0.91-0.95), from 610 through 1218 m; 0.88 (95% CI, 0.84-0.91), from 1219 through 1828 m, and 0.85 (95% CI, 0.79-0.92) higher than 1828 m. Age- and sex-standardized mortality decreased more with altitude in patients receiving dialysis than in the general population.


CONCLUSIONS
Altitude was inversely associated with all-cause mortality among US patients receiving dialysis.
Winkelmayer
Wolfgang C
WC
Division of Pharmacoepidemiology and Pharmacoeconomics, Brigham and Women's Hospital and Harvard Medical School, 1620 Tremont St, Ste 3-030, Boston, MA 02120, USA. wwinkelmayer@partners.org
Liu
Jun
J
Brookhart
M Alan
MA
D000368:Aged; D000531:Altitude; D002423:Cause of Death; D015331:Cohort Studies; D004921:Erythropoietin; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000401:mortality* / Q000628:therapy*; D008297:Male; D008875:Middle Aged; D016016:Proportional Hazards Models; D006435:Renal Dialysis / Q000401:mortality*; D012189:Retrospective Studies; D016019:Survival Analysis; D014481:United States / Q000453:epidemiology
D004921:Erythropoietin