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Chronotropic index mediates the relationship between lower-limb loss level and 6-minute walk test performance
Samantha J Stauffer1, Ryan T Pohlig2,3, David G Edwards4
1Department of Physical Therapy, Delaware Limb Loss Studies, University of Delaware, STAR Campus, Newark, DE, United States.
Introduction:
Lower-limb loss (LLL) significantly impacts exercise capacity. Low heart rate, i.e., chronotropic, response to exercise is known to contribute to reduced exercise capacity in other clinical populations; associations between population-specific variables (e.g., LLL level), chronotropic response to activity, and exercise capacity among adults with LLL remain unknown.
Methods:
In this cross-sectional study, 130 community-dwelling adults with unilateral below-knee (n = 95; 58.5 ± 14.9 years-old, 74.7% male) or above-knee (n = 35; 54.0 ± 13.7 years-old; 68.6% male) LLL provided demographic and past medical history, including LLL-specific data. Vital signs were assessed at rest and after completing a 6-Minute Walk Test (6MWT). Chronotropic index, defined as the percent of heart rate reserve used, was calculated using the equation [(posttest heart rate-resting heart rate)/[208 - (0.7*age) - resting heart rate]]. Between-group differences in participant characteristics and cardiovascular response to the 6MWT were evaluated. Predictors of chronotropic index were identified using linear regression modeling; mediation modeling was used to determine if chronotropic index mediated the relationship between LLL level and 6MWT distance. Participants with above-knee LLL demonstrated significantly greater chronotropic indices (31.9% vs. 21.1%, p = 0.021) and walked shorter distances during the 6MWT (328.9 ± 112.6 meters vs. 400.5 ± 126.4 meters, p = 0.004). Chronotropic index was positively associated with 6MWT distance (β = 1.674, p < 0.001); LLL level significantly predicted chronotropic index (β = -9.929, p = 0.011). Per mediation analysis, chronotropic index partially mediated the relationship between LLL level and 6MWT distance (indirect effect = -16.616, 95% confidence interval: -35.057, -2.326). The final model explained 43.5% of the variance in 6MWT distance.
Discussion:
Findings suggest chronotropic response may be a physiological contributor to reduced exercise capacity in adults with LLL. Thus, chronotropic index may be a critical target for improving exercise capacity in adults with unilateral LLL.
