Residential altitude and change in six-minute walk distance during cardiac rehabilitation

Abstract Background The six-minute walk test is a guideline aligned tool for submaximal capacity in cardiac rehabilitation. In cardiac populations, shorter six-minute walk distance (6MWD) predicts higher risk of death and major events, whereas improvement in 6MWD relates to better outcomes. With increasing elevation above sea level, lower inspired oxygen may reduce walking performance. However, findings from healthy residents are mixed, creating uncertainty about the practical impact of residential altitude in patients entering rehabilitation. Purpose This study examines whether residential altitude is associated with the change in 6MWD from admission to discharge during exercise-based cardiac rehabilitation (EBCR) at 1’600 meters above sea level. A secondary aim was to assess whether differences in baseline characteristics between residential altitude groups are associated with changes in 6MWD. Methods Cardiac patients referred for inpatient EBCR between December 2022 and June 2024 were included in this retrospective single-center study. Residential altitude served as the exposure and the primary outcome was the change in 6MWD from admission to discharge. A linear regression was performed estimating the association between residential altitude and change in 6MWD, adjusted for age, sex, oxygen use during testing, and ICD 10 diagnoses. Effect sizes are expressed per 100 meters of altitude. Residential altitude was also categorized into four predefined groups (0–499, 500–999, 1000–1499, 1500–2000 meters), and group-specific 6MWD changes were presented using medians with quantiles Q1-Q3. Results A total of 799 patients were analysed, completing a median of 20 days (IQR 20–27) of EBCR. Overall, 175 patients (21.9%) were women, with similar proportions across altitude groups. Patients residing at 1000–2000 m (n = 47) were older (medians 72 vs 65 years, p = 0.009) and more of them were diagnosed with heart failure (47% vs 25%, p = 0.002) and chronic kidney disease (36% vs 15%, p 0.001) compared with patients residing below 1000 m (n = 752). Median 6MWD increased from 377 m at admission to 525 m at discharge, with a median change of 140 m. Higher residential altitude is not associated with a change in 6MWD. This was further confirmed after adjusting for age and sex, the estimate was −3.2 m per 100 m altitude increase (95% CI −6.6 to 0.3; p = 0.070; n = 791). In a descriptive analysis by altitude groups, median changes in 6MWT were 141 m at 0–499 m (n = 462), 144 m at 500–999 m (n = 289), 126 m at 1000–1499 m (n = 35), and 63 m at 1500–2000 m (n = 13). Conclusion Patients living at higher residential altitudes had slightly smaller, non-significant improvements in 6MWD. These smaller gains seem to be explained primarily by older age and more comorbidity in this group, rather than by altitude itself.For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text.