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

Heart Rate Variability in Individuals with Down Syndrome: A Scoping Review with Methodological Considerations

Individuals with Down syndrome (DS) present similar heart rate variability (HRV) parameters at rest but different responses to selected movement maneuvers in comparison to individuals without DS, which indicates reduced vagal regulation. The present study undertakes a scoping review of research on HRV in individuals with DS, with special attention paid to the compliance of the studies with standards and methodological paper guidelines for HRV assessment and interpretation. A review was performed using PubMed, Web of Science and CINAHL databases to search for English language publications from 1996 to 2020 with the MESH terms "heart rate variability" and "down syndrome", with the additional inclusion criteria of including only human participants and empirical investigations. From 74 studies, 15 were included in the review. None of the reviewed studies met the recommendations laid out by the standards and guidelines for providing the acquisition of RR intervals and necessary details on HRV analysis. Since authors publishing papers on this research topic do not adhere to the prescribed standards and guidelines when constructing the methodology, results of the research papers on the topic are not directly comparable. Authors need to design the study methodology more robustly by following the aforementioned standards, guidelines and recommendations.

Predictive equations to estimate peak aerobic capacity and peak heart rate in persons with Down syndrome

Estimations based on the available equations for predicting oxygen uptake (VO₂) from treadmill speed of locomotion are not appropriate for individuals with Down syndrome (DS). We aimed at developing prediction models for peak absolute oxygen uptake (VO_2peak) and peak heart rate (HR_peak) based on retrospective data from a healthy population with and without Down syndrome (DS). A cross-sectional analysis of VO_2peak and HR_peak was conducted in 196 and 187 persons with and without DS, respectively, aged from 16-45 years. Non-exercise data alone versus combined with HR_peak were used to develop equations predictive of absolute VO_2peak. Prediction equations for HR_peak were also developed. Two additional samples of participants (30 with, 29 without DS) enabled model cross-validation. Relative VO_2peak and HR_peak were lowest for persons with DS across all ages (~ 40% and 20 bpm, respectively). For persons with DS, VO_2peak predictions provided no differences compared with actual values. Predicted HR_peak was similar to actual values in both groups of participants. Large limits of agreement were obtained for VO_2peak (DS: 735, non-DS: 558.2 mL.min^-1) and HR_peak (DS: 24.8, non-DS: 16.6 bpm). Persons with DS exhibit low levels of VO_2peak and HR_peak in all age groups included in this study. It is possible to estimate absolute VO_2peak in persons with DS using non-exercise variables. HR_peak can be accurately estimated in groups of people with and without DS. Yet, because of large limits of agreement, caution is advised if using these equations for individual estimations of VO_2peak or HR_peak in either population.

A Review of Theoretical Perspectives in Cognitive Science on the Presence of 1/f Scaling in Coordinated Physiological and Cognitive Processes

Time series of human performances present fluctuations around a mean value. These fluctuations are typically considered as insignificant, and attributable to random noise. Over recent decades, it became clear that temporal fluctuations possess interesting properties, however, one of which the property of fractal 1/f scaling. 1/f scaling indicates that a measured process extends over a wide range of timescales, suggesting an assembly over multiple scales simultaneously. This paper reviews neurological, physiological, and cognitive studies that corroborate the claim that 1/f scaling is most clearly present in healthy, well-coordinated activities. Prominent hypotheses about the origins of 1/f scaling are confronted with these reviewed studies. It is concluded that 1/f scaling in living systems appears to reflect their genuine complex nature, rather than constituting a coincidental side-effect. The consequences of fractal dynamics extending from the small spatial and temporal scales (e.g., neurons) to the larger scales of human behavior and cognition, are vast, and impact the way in which relevant research questions may be approached. Rather than focusing on specialized isolable subsystems, using additive linear methodologies, nonlinear dynamics, more elegantly so, imply a complex systems methodology, thereby exploiting, rather than rejecting, mathematical concepts that enable describing large sets of natural phenomena.

Heart rate complexity in response to upright tilt in persons with Down syndrome

People with Down syndrome (DS) show altered autonomic response to sympatho-excitation. Cardiac autonomic modulation may be examined with heart rate (HR) complexity which is associated uniquely with cardiovascular risk. This study examined whether the response of HR complexity to passive upright tilt differs between persons with and without DS and whether potential between-group differences in this response are accounted for by differences in body mass index (BMI). The electrocardiogram of 16 persons with DS (8 women, 8 men) and 16 persons without DS (8 women, 8 men) was recorded during 10 min of supine rest and 10 min of upright tilt. For each participant, 550 continuous, steady state, and ectopy-free R-R intervals under each condition were analyzed. Dependent variables were approximate entropy, correlation dimension, StatAv, and the mean R-R interval. In response to tilt, changes in approximate entropy and correlation dimension were reduced in participants with DS (p<0.05). These differences were explained by higher BMI in participants with DS. StatAv increased in persons with DS (p < 0.05) and stayed the same in those without DS even when controlling for BMI. The response of R-R interval did not differ between groups. None of the variables differed between groups at rest. Therefore, people with DS show smaller decrease in HR complexity in response to upright tilt than people without DS partially due to their higher BMI. Resting HR complexity does not differ between persons with and without DS. These results may have implications for cardiovascular risk in people with DS.