Fractal scaling properties of heart rate dynamics in persons with Down syndrome

It has been shown that the fractal scaling properties of heart rate dynamics, in healthy aging, differ from that seen in heart disease and this favors the use of fluctuation measures as diagnostic tools. The purpose of this study was to evaluate the fractal heart rate dynamics in adults with Down syndrome (DS) under different physiological conditions (rest, exercise and post-exercise recovery) and compare their responses with those of nondisabled individuals. Fourteen participants (10 males; and 4 females) with DS and 13 nondisabled (9 males, and 4 females) controls performed maximal and submaximal treadmill tests with metabolic and heart rate measurements. Detrended fluctuation analysis was performed on the last 256 consecutive R-R intervals obtained under the following conditions: (1) standing rest, (2) submaximal treadmill exercise and (3) standing post-exercise recovery. Participants with DS presented lower chronotropic response than controls to peak and submaximal exercise (p<0.05). The short-term scaling exponent (α₁) was greater in the DS group at rest (1.29±0.06 vs 1.07±0.08), during exercise (1.24±0.09 vs 0.99±0.08) and recovery (1.31±0.06 vs 1.21±0.06) (p<0.05). Furthermore, the fractal scaling distance score (I1-α₁I) of participants with DS was also greater than that of controls under each physiological condition (p<0.05). This confirmed their greater fractal distance from the healthy value of 1.0. In conclusion, adults with DS show a breakdown of scale-invariant organization in heart rate dynamics towards Brownian noise and this is similar to that described in healthy aging.