Dialysis patients living at high altitudes have lower cardiovascular event rates, reported a recent study published online ahead of print by Nephrology Dialysis Transplantation.
Cardiovascular risk was 20% lower for patients residing at 4,000-5,999 feet and 19% lower for patients residing at 6,000 ft or higher, compared with otherwise similar patients who lived at or near sea level, found the researchers, who were led by Wolfgang C. Winkelmayer, MD, ScD, of Stanford University School of Medicine.
This work built on the researchers' previous observation that US dialysis patients and members of the general population who resided at higher altitudes had lower all-cause mortality rates.
In the new study, rates of myocardial infarction, stroke, and cardiovascular death were 31%, 32%, and 23% lower, respectively, among patients living above 6,000 ft compared with patients residing at or near sea level.
Dr. Winkelmayer and colleagues studied all adult patients from the US Renal Data System with sufficient data who initiated hemodialysis treatment between 1995 and 2006. Of the 984,265 patients included, almost 95% resided below 2,000 ft (40.3% below 250 ft, and 54.5% at 250-1,999 ft), and 4,356 patients (0.4%) resided above 6,000 ft.
Patients who lived at higher altitudes were younger, less likely to be covered by Medicaid, and more likely to be on peritoneal dialysis at the initiation of therapy.
While rates of most comorbid conditions were similar across altitude groups, patients at higher altitudes had more diagnosed diabetes and hypertension, slightly less arteriosclerotic heart disease and heart failure, and few instances of reported inability to ambulate or transfer.
There were 574,063 deaths observed, 257,955 (44.93%) of which were reported as cardiovascular. There was no meaningful association between altitude and non-cardiovascular death.
“The fact that altitude is shown to selectively attenuate the risk of cardiovascular mortality in dialysis patients suggests that there are underlying biological mechanisms that are activated at higher altitudes to protect these patients from cardiovascular risk,” the authors wrote. “These findings need to be considered with caution, however, and residual confounding remains a possibility.”
The inverse relationship between altitude and cardiovascular outcomes was examined in multivariable models.
“This observation was only slightly attenuated even after adjusting for a large number of patient characteristics and is consistent with previous studies that have demonstrated the protective effect of HIF [hypoxia-inducible factor]-1 activation on the cardiovascular system,” the authors wrote. “We propose that HIF-1 plays an important role in conferring a protective effect on cardiovascular outcomes in dialysis patients.”