Cardiorespiratory Coordination During Exercise in Adults With Down Syndrome

Abstract

Introduction: Down syndrome (DS) is a chromosomal disorder affecting simultaneously cardiovascular and respiratory systems. There is no research studying the coupling between these systems during cardiorespiratory exercise testing in a population with DS. Cardiorespiratory coordination (CRC), evaluated through principal component analysis (PCA), measures the covariation of cardiorespiratory variables during exercise. Objective: To investigate and compare CRC in adults with and without DS during maximal cardiorespiratory exercise testing. Methods: Fifteen adults with DS and 15 adults without disabilities performed a maximal cardiorespiratory exercise test on a treadmill. First, the slope, and afterward the velocity was increased regularly until participants reached exhaustion. The time series of six selected cardiorespiratory variables [ventilation per minute, an expired fraction of O₂, the expired fraction of CO₂, heart rate, systolic blood pressure (SBP), and diastolic blood pressure (DBP)] were extracted for the analysis. The number of principal components (PCs), the first PC eigenvalues (PC₁), and the information entropy were computed for each group (non-DS and DS) and compared using a t-test or a Mann-Whitney U test. Results: Two PCs in the non-DS group and three PCs in the DS group captured the variance of the studied cardiorespiratory variables. The formation of an additional PC in the DS group was the result of the shift of SBP and DBP from the PC₁ cluster of variables. Eigenvalues of PC₁ were higher in the non-DS (U = 30; p = 0.02; d = 1.47) than in the DS group, and the entropy measure was higher in the DS compared with the non-DS group (U = 37.5; p = 0.008; d = 0.70). Conclusion: Adults with Down syndrome showed higher CRC dimensionality and a higher entropy measure than participants without disabilities. Both findings point toward a lower efficiency of the cardiorespiratory function during exercise in participants with DS. CRC appears as an alternative measure to investigate the cardiorespiratory function and its response to exercise in the DS population.

Keywords: Down syndrome; blood pressure; cardiorespiratory fitness; information entropy; network physiology of exercise; principal component analysis.

Figure 1

Example of a participant performing cardiopulmonary exercise test on the treadmill. In addition, it can be seen the placement of the finger cuff around the middle phalanx of the middle finger of the left hand.

Figure 2

A typical example of the formation of time series of coordination variables, expressed by means of principal components from cardiorespiratory variables in non-DS and DS groups. This figure represents the higher number of PCs in the DS compared with the non-DS group (three PCs vs. two PCs, respectively), reflecting a lower degree of covariation of physiological variables, which can be linked to lower adaptive properties of the physiological network in response to exercise workloads. (A) Time series of the six selected cardiorespiratory variables during the cardiopulmonary exercise test. (B) Time series of the PC scores, with standardized z-values in the space spanned by PCs. The average trend was computed by the weighted least squares method. The variance of the six cardiorespiratory variables in the non-DS group was captured by two PCs: five variables (VE, FeCO₂, HR, SBP, and DBP) were implicated in the formation of PC₁, whereas PC₂ was composed by a single variable (FeO₂). However, the six selected cardiorespiratory variables could only be reduced to three PCs in the DS group: PC₁ was formed by VE, FeCO₂, and HR; PC₂ by SBP and DBP, and PC₃ was mainly saturated by FeO₂. Note that the shift of SBP and DBP from the PC₁ cluster of variables provoked the formation of an additional PC in the DS group. PC, principal component; VE, minute ventilation; HR, heart rate; SBP, systolic blood pressure; DBP, diastolic blood pressure; FeO₂, expired fraction of oxygen; FeCO₂, expired fraction of carbon dioxide; DS, Down syndrome.