Introduction
The life expectancy for children and adolescents born with a congenital heart defect (CHD) has radically improved the last decades [
1,
2]. This development has formed a relatively new patient group. Prior research in the general population has established the positive effects of physical activity (PA) on cardiovascular health, psychosocial health, self-esteem, learning, motor-skill development, and quality of life [
3], with high-intensity PA appearing as particularly beneficial to health [
4]. Physical limitations of the cardiovascular system [
5,
6], restrictions from parents and caregivers [
7,
8], and low self-efficacy [
7] have been emphasized by prior research as factors that prevent PA participation in children and adolescents with CHD. Consequently, it may be assumed that they are less physically active than their healthy peers.
Congenital valvular aortic stenosis (VAS) occurs in 3–5% of children with CHD. The severity of the defect varies from life-threatening condition at birth (critical aortic stenosis in the neonate) to a condition with need for intervention later in life. Treatment during childhood aims to preserve the native aortic valve with either surgical valvotomy or balloon valvotomy, but often with a need of aortic valve replacement at a later stage. Apart from the life-threatening situation in the neonate with critical valvular aortic stenosis, a mean Doppler gradient of 50 mmHg or more is the main indication for treatment. In Sweden, the primary treatment is surgical valvotomy. As VAS is a complex and lifelong disease where re-interventions and further surgery is common [
9], patients often show residual or acquired abnormalities of the left heart structure which might affect cardiovascular capacity [
10]. Even if exercise function is preserved in most patients treated for VAS in infancy, there may still be reduced peak VO
2 in some individuals, probably reflecting inability to increase stroke volume [
6].
As the positive effects of PA is well acknowledged, it is important to establish the habitual PA levels in children and adolescents with VAS and other CHDs as part of the clinical assessment and to provide individually adapted prescriptions [
11‐
13]. Exercise restrictions have traditionally been recommended for some patients with VAS because of a perceived increased risk for sudden death. In the current era, sudden unexpected death during exercise in this patient group is uncommon [
14,
15], probably due to a more active approach to treatment than in the earlier era of pediatric cardiology. Still, many patients with VAS might be advised to avoid strenuous PA [
11‐
13].
Uncertainties exist regarding the PA levels of children and adolescents with CHD, as prior studies show contradicting results and in general have pronounced methodological limitations and diversities [
16]. The quantification of PA relies primarily on objective methods, with the accelerometer being the most frequently used one [
17], as subjective methods typically possess poor validity and reliability in children. However, great irregularities have been observed in data collection, value calibration, and data processing in accelerometer-based studies, making comparisons of results difficult [
17]. Moreover, the accelerometer method commonly used in previous research in CHD, the ActiGraph counts (AG), shows difficulties in capturing intermittent and high-intensity PA [
4,
18,
19], which have restricted correct classification of PA [
20]. These measurement errors are more prominent in children due to their movement patterns compared to in older individuals. Recent methodological developments have improved assessment of PA [
4,
18‐
21] and the association with cardiometabolic health [
22]. These improvements may help to clarify the uncertainties regarding the PA level of children and adolescents with CHD.
The aim of the present study was to compare PA patterns between children and adolescents treated for VAS and healthy controls using new improved accelerometer assessment with the Frequency Extended Method (FEM) [
18‐
21] and a detailed spectrum of PA intensities [
22].
Discussion
The main result of the present study is that the application of the improved FEM with the detailed PA intensity spectrum revealed differences in PA patterns that were less evident using the traditional accelerometer method with its measurement errors. The children treated for VAS had a pattern of less PA at the highest PA intensity spectra and more SED than their controls, while the adolescents treated for VAS tended to have less PA of higher intensities overall and more SED.
The PA performed at the highest intensity spectra by children is considered to include intermittent bursts, typically sustained in the inborn movement pattern of children when doing different sports activities [
4]. The pattern with lower PA in children treated for VAS compared to their healthy peers in the present study may indicate that this type of activity is restrained in this patient group. The less frequent sports participation in children with VAS compared to their controls further supports the findings from the accelerometer assessment using the FEM. A previous study of Swedish children with CHD also reports less sports participation than in their healthy peers [
25]. The reason why this accelerometer PA pattern was not visible with the crude classification is because the total volume of time spent at the highest PA levels is relatively small, mainly containing seconds rather than minutes in children. One may speculate on how this reduced intermittent moment pattern affects further on participation in activities and sports. The tendency of less PA of higher intensity overall in the adolescents treated for VAS may indicate a major behavior change, in addition to what is observed in the general population [
26]. A relevant question is whether the PA pattern in adolescents treated for VAS is a consequence of the behavioral pattern observed in the children treated for VAS. To confirm this transfer of behavior, a longitudinal study design is required.
Previous research using accelerometers for the assessment of PA in children and adolescents with CHD has shown contradicting results, possessing great variances in sample size and characteristics of the studied CHD, in data collection and processing settings, using the AG method [
16]. Consequently, the varying results may be caused by numerous possible measurement errors, as the results presented are highly reliable on the methodology used even if displayed similarly [
17]. As an illustration, epoch-lengths ranging from 3 to 60 s (or not-reported) have been observed in these studies. Different epoch-lengths lead to significant variations in time spent in the different PA intensities [
4]. Shorter epochs have been recommended, especially when studying children whose movement pattern mainly consists of intermittent, sporadic bursts [
4].
Further, misclassification of PA with the traditional AG method has been reported as being low at the lighter intensities (1–2%) and large at the higher intensities (> 90%) when compared to wider filters [
20]. Therefore, there was little difference between the FEM and the AG method in the assessment of SED in the present study. However, with a classification agreement of only approximately < 10% at VPA and VVPA level [
20], classification of PA in the higher intensities when using the AG method is more or less sorted by random chance. The misclassification is assumed to be present within this study as well, making interpretation of the PA results from the AG method questionable. For example, Fig.
1d in the present study showed that the largest relative difference between adolescent patients and controls was found at higher PA intensities using the AG method compared to using the FEM shown in Fig.
1c. The AG method contributes to an inconsistent overestimation of PA by the AG method at higher PA intensities [
20], which is assumed to be more in the controls than in the patients.
In addition, the larger inter-individual variation in PA observed with the FEM is explained by that its wider filter range captures more of the variation in PA [
19]. Similarly, the switch in difference between adolescent patients and controls at the highest PA intensity spectra with the FEM (Fig.
1c) is explained by the marked decrease in the proportion of individuals with recorded data (Fig.
2), which would lead to skewed distribution. The influence would be more pronounced with a frequency filter allowing more of the acceleration signal to be recorded. This reduction in the proportion of individuals having data at higher PA intensities, with larger reduction among the patients, provides information about the behavior pattern variation we seek to detect. At the same time, it makes it a challenge to perform statistical analyses across the whole PA intensity spectrum. As an example of the impact, it has previously been shown that the association with health weakened at the PA intensity level where 50% or less of the individuals provided data [
22].
Physical activity is crucial for children’s normal development and health [
3]. Most patients and controls in the two age-groups tended to meet the recommended amount of PA in the present study. However, conclusions from PA data collected with accelerometers in relation to the PA recommendations need to be drawn with caution, as they are based on different, not directly comparable methods. Further, the choice of cut-point for MVPA (lower cut-point, more MVPA) and the epoch-length of accelerometer data (shorter epoch-length, more MVPA) affect the proportion reaching the recommended amount of PA [
4,
17]. For example, Voss et al. used 15-s epochs and a higher cut-point for MVPA than in our study when investigating children and adolescents 8–18 years with CHD [
24]. We used 3-s epochs and a lower cut-point for MVPA was achieved by using the VO
2net calibration in order to reach a reference measure of metabolic effort equivalent by age. Consequently, only 8% of the participants in their study reached the recommended amount of PA compared to 54–93% in our study. In addition, adherence to the PA recommendation also depends on the interpretation of the recommendation as ≥ 60 min daily on average (less strict) or on most days (stricter). Voss et al. applied the stricter interpretation (6 out of 7 days), which we also included in our study. Interestingly, the proportion reaching recommended PA (using the stricter criterium) was lower in the adolescents compared to the children, which is in line with the observed global trend of decreasing PA level across childhood into adolescence [
26].
An amount of > 14.2 min day
−1 less MVPA in the adolescent patient group was observed with the FEM, with approximately 5.4 min day
−1 within the VPA + VVPA spectra where the greatest cardiometabolic health effects appear to be found [
4,
22]. Although not statistically significant, these differences might still be substantial. A study by Ekelund et al. in healthy children and adolescents showed that 10 min difference in MVPA per day was associated with a 0.5 cm difference in waist circumference and a 1 pmol L
−1 difference in fasting insulin [
27]. Still, there are contradictory results regarding the cardiometabolic risk in patients with CHD. While Dean et al. reported that the metabolic syndrome was more common in adults with CHD [
28], Zaquot et al. found that children with CHD did not have an increased metabolic risk [
29].
Due to the relatively small sample and the novelty of the methods and findings, it is too early to draw definitive conclusions concerning the clinical implications of the present study. Children and adolescents treated for VAS may have specific limitations of performing PA [
6,
9,
10]. Therefore, it is recommended that individually adapted PA determined from the clinical assessment may be prescribed (e.g., Physical Activity on Prescription), targeting also other barriers that may occur (e.g., low self-efficacy, overprotection, and restrictions from parents) [
11‐
13]. The prescription may promote the natural and spontaneous PA pattern in children and maintenance of a physically active lifestyle including sports in adolescents. Assessment of PA may be an important part of the clinical assessment and follow-up, to provide adequate guidance, support, and feedback. New advances in the accelerometer methodology may improve PA assessment and its utility in clinical settings further.
Limitations and Future Research
Certain limitations of the present study should be acknowledged. A larger sample would be desirable to establish the different PA patterns in children and adolescents treated for VAS. Even though all eligible patients treated for VAS were identified in the Swedish registers, there was a considerable loss of participants during recruitment. It is possible that those with less restraints and more physically active chose to participate. The loss of participants in the control group was also large. Another limitation is that gender analysis could not be performed due to the low participant number. There were also some differences in the proportion of females between the groups. These limitations may have affected the results in various ways and the identification of statistically significant differences between the groups. To be able to determine how the PA pattern in children is related to the PA later in the adolescence, a longitudinal study design would be required. Finally, with the PA being highly variable in all individuals, and probably even more inconsistent in children, the inclusion of more days than the set minimum of 4 days should be considered.
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