Effects of Sports, Exercise Training, and Physical Activity in Children with Congenital Heart Disease—A Review of the Published Evidence

Children and adolescents with congenital heart disease (CHD) should be encouraged to adopt a physically active lifestyle, ideally by participating in sports activities at school and sports clubs. Children with complex CHD or other risk factors (for example, pacemakers, cardioverter-defibrillators, channelopathies) may, however, need specific individualized training programs. This review article summarizes the current knowledge regarding the clinical effects of sports and exercise training on CHD and its pathophysiologic mechanisms. An evidence-based approach based on a literature search, using PubMed, Medline, CINHAL, Embase, and the Cochrane Library was conducted, last completed on 30 December 2021. In studies with 3256 CHD patients in total, including 10 randomized controlled trials, 14 prospective interventional trials, 9 observational trials, and 2 surveys, exercise training has been shown to improve exercise capacity and physical activity, motoric skills, muscular function, and quality of life. Sports and exercise training appears to be effective and safe in CHD patients. Despite being cost-efficient, training programs are currently scarcely reimbursed; therefore, support from healthcare institutions, commissioners of healthcare, and research-funding institutions is desirable. There is a strong need to establish specialized rehabilitation programs for complex CHD patients to enhance these patients’ access to this treatment intervention. Further studies may be desirable to confirm these data to investigate the impact on risk profiles and to identify the most advantageous training methodology and underlying pathophysiological mechanisms.


Background
During recent decades, there has been a huge change in medicine and health prevention in general to a more early and active approach towards health prevention.
It is well known that sedentary behaviors and lifestyles are associated with an increase in morbidities such as obesity, metabolic disease, and cardiorespiratory risk factors during adulthood. A more active lifestyle and health education programs could reduce some of these health problems. Nevertheless, the basis for a long and healthy life is laid in early childhood [1]. Especially in children with congenital heart defects, any acquired cardiac disease will complicate the outcome.
Current international guidelines for healthy children recommend participating in a daily minimum of 60 min of moderate-to-vigorous-intensity physical activity [2][3][4][5][6]. Physical activity includes every active movement which increases basal metabolism. Nevertheless, only a tiny number of 12% of healthy children reach this goal [7]. Recommendations for children in Germany even suggest 90 min or at least 12,000 steps per day [8]. These Regarding the large number of positive aspects related to an active lifestyle with almost no adverse effects, parents and children with CHD should be even more motivated to participate in regular sports and exercise. This will help them to become a bit more integrated into their peer groups, confident, and independent with a rising quality of life. Physical activity starts with the way to school. Children who walked or used their bicycles to go to school tended to have a higher level of physical activity in general [31]. There are also several trials focusing on ways in which healthy children could be motivated to be more active [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. A positive effect had free play times during school, colorful playgrounds, and a playful approach in general [32][33][34][35][36][37]49,50]. In Germany, there was a trial evaluating the effect of a trampoline on physical activity (PA) levels [51]. Trials focusing on the effect of church leisure events or summer camps did not show a lasting effect regarding PA levels [52].
The different activity levels are defined by Takken et al. They also defined the static and dynamic impact of sports and activity categories, giving a hint of which category is more useful for CHD patients, according to the type of heart defect and the severity of their illness [27].

Pathophysiologic Mechanisms of Sports and Training in CHD
The exercise limitation in CHD patients might be multifactorial. It can be caused by ventricular dysfunction, chronotropic incompetence, ventilatory abnormalities, and skeletal muscle dysfunction. The mechanisms of exercise intolerance are complex, likely including respiratory muscle weakness, dynamic hyperinflation and mechanical constraints, poor skeletal muscle and cerebral oxygenation, hyperventilation, and enhanced sympathetic drive. Likewise, exercise training improves the function of different body organs such as the heart, lungs, and skeletal muscles.
The mechanisms of improved hemodynamics and exercise capacity by exercise training in individuals with CHD remain incompletely understood [ Figure 1]. Increased cardiac output at rest and at maximum exercise may be explained either by a decreased afterload of the ventricles, either a muscular or molecular effect on the blood vessels with a reduction in afterload and/or pulse wave velocity, or a direct myocardial training effect. In addition, improved exercise capacity is at least in part to be explained by improved skeletal muscle function, but there are, however, no reported direct measurements of improved diffusional muscle oxygen uptake by exercise training.
Furthermore, a combined effect on different molecular pathways and organs is likely to be the pathophysiological underpinning of the improvement associated with exercise training in individuals with CHD. Further research is needed to elucidate the relevance of each of these mechanisms. It is also of great interest whether exercise training leads to epigenetic changes.

Objectives
The aim of this review is to give a broad overview of the effects of higher PA levels, participation in sports, and training/rehabilitation programs with a focus on children and adolescents with CHD. It also evaluates if there is a trend in which training programs or approaches work better. Furthermore, a combined effect on different molecular pathways and organs is likely to be the pathophysiological underpinning of the improvement associated with exercise training in individuals with CHD. Further research is needed to elucidate the relevance of each of these mechanisms. It is also of great interest whether exercise training leads to epigenetic changes.

Objectives
The aim of this review is to give a broad overview of the effects of higher PA levels, participation in sports, and training/rehabilitation programs with a focus on children and adolescents with CHD. It also evaluates if there is a trend in which training programs or approaches work better.

Search Strategy
An internet-based literature search for relevant, original research articles on the effects of sports, fitness, and physical activity on the effect of cardiorespiratory fitness, reductions in morbidities, muscle function, and quality of life in children with CHD was conducted (last completed 30 December 2021). We searched PubMed, Medline, CINHAL, Embase, and the Cochrane Library using the search terms: Furthermore, a combined effect on different molecular pathways and organs is likely to be the pathophysiological underpinning of the improvement associated with exercise training in individuals with CHD. Further research is needed to elucidate the relevance of each of these mechanisms. It is also of great interest whether exercise training leads to epigenetic changes.

Objectives
The aim of this review is to give a broad overview of the effects of higher PA levels, participation in sports, and training/rehabilitation programs with a focus on children and adolescents with CHD. It also evaluates if there is a trend in which training programs or approaches work better.

Search Strategy
An internet-based literature search for relevant, original research articles on the effects of sports, fitness, and physical activity on the effect of cardiorespiratory fitness, reductions in morbidities, muscle function, and quality of life in children with CHD was conducted (last completed 30 December 2021). We searched PubMed, Medline, CINHAL, Embase, and the Cochrane Library using the search terms: Furthermore, a combined effect on different molecular pathways and organs is likely to be the pathophysiological underpinning of the improvement associated with exercise training in individuals with CHD. Further research is needed to elucidate the relevance of each of these mechanisms. It is also of great interest whether exercise training leads to epigenetic changes.

Objectives
The aim of this review is to give a broad overview of the effects of higher PA levels, participation in sports, and training/rehabilitation programs with a focus on children and adolescents with CHD. It also evaluates if there is a trend in which training programs or approaches work better.

Search Strategy
An internet-based literature search for relevant, original research articles on the effects of sports, fitness, and physical activity on the effect of cardiorespiratory fitness, reductions in morbidities, muscle function, and quality of life in children with CHD was conducted (last completed 30 December 2021). We searched PubMed, Medline, CINHAL, Embase, and the Cochrane Library using the search terms: : reduction; FEV1: Forced expiratory volume.

Search Strategy
An internet-based literature search for relevant, original research articles on the effects of sports, fitness, and physical activity on the effect of cardiorespiratory fitness, reductions in morbidities, muscle function, and quality of life in children with CHD was conducted (last completed 30 December 2021). We searched PubMed, Medline, CINHAL, Embase, and the Cochrane Library using the search terms: ((((((((((((congenital heart defect AND sport AND children)) AND (Congenital heart disease AND sport AND children)) AND (congenital heart defect AND fitness AND children)) AND (Congenital heart disease AND fitness AND children)))) AND (congenital heart defect AND physical activity AND child)) AND (congenital heart disease AND physical activity AND child)) AND (congenital heart defect AND physical activity AND cardiorespiratory fitness AND children)) AND (congenital heart disease AND physical activity AND cardiorespiratory fitness AND children))) → 134 articles ((((((((congenital heart defect AND exercise training AND child)) AND (congenital heart defect AND exercise training AND children)) AND (congenital heart disease AND exercise training AND child)) AND (congenital heart disease AND exercise training AND children)) AND (congenital heart disease AND exercise training AND child AND cardiorespiratory fitness)) AND (congenital heart disease AND exercise training AND children AND cardiorespiratory fitness)) AND (congenital heart defect AND exercise training AND child AND cardiorespiratory fitness)) AND (congenital heart defect AND exercise training AND children AND cardiorespiratory fitness) →177 articles Physical activity interventions for children with congenital heart disease → 1353 articles ((((((((exercise training AND congenital heart disease AND child)) AND (exercise training AND congenital heart disease AND children)) AND (physical activity AND congenital heart disease AND child)) AND (physical activity AND congenital heart disease AND children)) AND (physical activity AND congenital heart disease AND youth)) AND (physical activity AND congenital heart disease AND adolescent)) AND (exercise training AND congenital heart disease AND youth)) AND (exercise training AND congenital heart disease AND adolescent) → 4629 articles This search produced a total of 6293 articles. The articles were thoughtfully sorted, and duplicates were removed. Afterwards, the references were searched for additional literature.

Inclusion Criteria
We included all kinds of studies evaluating the effect of either PA, sports, exercise training, or rehabilitation programs with a focus on PA levels on children and adolescents with CHD. The articles must be available as full texts.

Exclusion Criteria
We excluded studies not relevant to the pediatric population, letters to the editor, review articles or commentaries, and articles only available as abstracts. Studies that were not yet completed were also excluded, as well as reviews and meta-analyses.

Data Extraction
A total of 40 studies matched our inclusion criteria [ Figure 2]. Those were further grouped into 29 studies examining any kind of intervention with its effects, and 11 studies describing the effects according to questionnaires.

Level of Evidence
Clinical trials were graded according to the standardized Level of Evidence (LOE) I-

Level of Evidence
Clinical trials were graded according to the standardized Level of Evidence (LOE) I-IV [36,37].

Results
The studies were published between 1990 and 2021 [ Tables 1 and 2]. The number of participants varied from 7 to 477. The follow-up time in the intervention group also differed, ranging from 8 weeks to 24 months. Longmuir et al. used earlier gathered data and performed a 5-year follow-up with them [53]. Additionally, the effects underwent different examinations. The age group varied between toddlers and young adults. Therefore, the transmission of the results to patients with CHD, in general, is not rational and should be considered carefully.     RCT CHD 143 12-20 The activity day was conducted as a motivational interview-style group exploring the nature of motivation towards exercise using small groups and visualization techniques. The concept of motivation was introduced. Participants completed a motivational rating sheet assessing their own feelings towards the importance of exercise, confidence, and readiness with regard to increasing their activity presession and postsession. Each participant was seen individually, and suggestions were discussed for ways to increase their activity over the next 6 months in a manner suitable for their diagnosis. They were also given a written exercise training plan to implement at home. Each participant was contacted once a month to check on progress with their exercise plan and discuss any problems.   The training program lasted for 8 months. Ten lessons were held at a local gymnasium in Padova, Italy, twice a week for the first 3 weeks and once a month for the next 4 months of the training program. The remaining part of the program was held at each child's home twice a week for 30 to 45 min under the supervision of parents. Each child was given a wristband heart rate monitor and instructed to keep their heart rate in the prescribed aerobic range. The exercise level during training was designed to range from 50% to 70% of maximal oxygen consumption. Parents were requested to write the frequency and duration of each exercise session in a log. Moreover, we contacted the families monthly by phone to inquire about the children's compliance with the training program and to encourage continued participation.

CPET
Max. oxygen pulse significantly increased training results in an improvement in an aerobic capacity. Important increase in maximal oxygen consumption as well as a decrease in the heart rate curve and an increase in the oxygen pulse curve during submaximal exercise.
Reduced heart rates and improved cardiovascular efficiency during usual daily activities. Pilot study CHD (mostly Fontan) 19 8-17 Rehabilitation sessions were conducted for 1 hour twice a week for 12 weeks at the clinic. 5 to 10 minutes of stretching exercise. 45 minutes of aerobic and light weight/resistance exercises. Activities included aerobic dance, step aerobics, calisthenics (sit-ups, crunches, jumping jacks, push-ups, etc.), kickboxing, and jumping rope. When the weather permitted, outdoor games such as capture the flag and relay races were conducted. Resistance exercises were performed with 3 and 5 lb barbells, light elastic bands, and cords. Games, rubber balls, music, and simple, age-appropriate prizes (e.g., baseball cards) were incorporated into the activities to promote enthusiasm and motivation. Attempts were also made to vary activities and to accommodate the moment-to-moment desires of the patients to optimize participation and interest. The last 5 to 10 min of each session were devoted to cooling down and stretching. Patients were also encouraged to exercise at home on at least two additional occasions per week. Heart rate was checked (manually) at the start of each session and on two or three additional occasions during each session. The patients were encouraged to exercise at an intensity sufficient to raise their heart rates to levels equal to that associated with the ventilatory anaerobic threshold.

Fredriksen et al. 2000 [79]
Prospective interventional CHD 129 10-16 Rehabilitation facility-or home-based training for 5 months, 2x/week. The subjects were introduced to swimming, football, volleyball, and also more general activities which had the purpose of facilitating strength, balance, coordination, flexibility, and stamina. Outdoor activities included downhill skiing, cross-country skiing, and hiking. The activities had one common feature; the intensity of the exercise should induce 65-80% of peak heart rate for at least half of the time spent in physical activity, using the peak heart rate assessed at the exercise test prior to training. Three-month home exercise program (directly postoperative, included endurance activities and jogging, a special program these groups used before).

Healthy children
Questionnaire of PA child and parents Exercise and scoring identical to an earlier study (1985). Reassessment after 6 months.
PA improved in those of the healthy pairs and persisted in the 5-year follow-up.   Self-efficacy scores were moderately correlated with physical activity participation (r = 0.47; p < 0.001).

Motoric Skills
It is well known in healthy children that regular exercise improves motoric skills such as climbing, balance, and coordination. Stieber et al. had the youngest examined age group of children with CHD with a group of toddlers, the youngest aged 12 months, after surgical correction or palliation. This study solely focused on the development of motoric skills, as these children, especially those after Fontan palliation, are significantly delayed in their motoric skills in comparison to their healthy peers. Those toddlers underwent a 10-week play-based, parent-delivered rehabilitation program with a follow-up examination after completing the 10-week program with daily training of 20 mins. Patients after Fontan palliation reached significant improvements in their motoric skills and reached values comparable to their age group [74].
Müller et al. showed similar results in their pilot study group of 14 CHD patients aged 4-6 years. In this trial, the impaired children significantly improved their motoric skills after 3 months of training with a 60 min session per week [73].
Ferrer-Sargues et al. conducted a study on 15 adolescents (12-16 years old), with two separate focuses. In a first approach, they examined the effect of their IMPROVE project (Initiative for Monitored Pediatric cardiac Rehabilitation Oriented by cardiopulmonary Exercise testing), designed following the American College of Sports Medicine (ACSM) on peripheral muscle function. This included 2x/week sessions of 70 min for a total of 24 sessions. The sessions included endurance and strength-resistance training. The intensity was defined by the subject's CPET results, initially aiming for a heart rate (HR) near the first ventilatory threshold (VT1) HR and displacing this target frequency progressively towards the secondary ventilatory threshold (VT2) HR or a maximal HR of 75% of their peak HR in cases where the VT2 was not available. The muscle function was tested at baseline, after completion, and 6 months after. In a second approach, they focused on respiratory muscle function. This study did not evaluate the impact on the CRF nor QoL. However, they could show an improvement regarding muscle function [55,56].
Longmuir et al. also evaluated the effect of either a parent-delivered home exercise or education program, both aiming to increase PA levels. Patients were randomized to either of these programs. Each program lasted for 12 months with a 1.5-2 h session per week. At the end of the trial, the patients showed significantly improved gross motor function and significantly increased moderate to vigorous physical activity (MVPA). These results lasted until the 2 years follow-up. This study had a relative representing a number of 61 Fontan patients, aged 6-12 years [69].
Brassard et al. studied seven Fontan patients with an 8-week training program, including an aerobic and a resistance part. They could show an effect on neuromuscular function and a strong but not significant reduction regarding the pulse wave velocity with a consecutive reduction in blood pressure [76].
Moalla et al. showed reduced muscle function in patients with CHD when performing resistance training and slower reperfusion. In another study, they randomized patients in aerobic cycling training and could afterwards show an improvement in strength and endurance. Moreover, there also was improved oxygenation and faster recovery [71,72].
In general, the included studies could show a positive effect on muscle development and motoric skills in children and adolescents with CHD, resembling the effect physical activity has on healthy individuals.

BMI
BMI and weight often correlate to physical activity levels in healthy children. Exercise participation leads to weight loss and a reduction in waist circumference.
Altamirano-Diaz et al. studied the effect of biweekly fitness and nutrition counseling delivered via smartphone over a period of 12 months in 34 obese CHD patients aged 7-17 years. The sessions lasted 30 min each. The groups were divided into CHDoperated and not-operated groups. Waist circumference significantly decreased by 2.61 cm at 6 months and by 2.25 cm at 12 months in the operated group. A significant increase in lean body mass was observed in both groups. No differences were seen regarding blood tests and CPET [62].
In a study conducted by Fredriksen et al. which included a 5-month training program in 10-16-year-old patients with CHD, compared to a control group without training. They observed weight gain in the control group without training [79].
There are also other studies that assessed BMI and PA levels with questionnaires. They could show a significant correlation between PA levels and BMI in patients with CHD. With a higher level of activity, the BMI decreases [80,84].
Other studies showed a strong correlation between reduced PA levels in CHD patients and a higher percentage of obese patients [91][92][93].
Regarding the effects on BMI and body weight, physical activity has also a similar effect on patients with CHD as it has in healthy individuals. A higher level of physical activity is associated with a lower body weight and therefore a lower BMI, as well as a reduced waist circumference.

Cardiorespiratory Fitness
Cardiorespiratory fitness is known to be associated with morbidity and mortality; therefore, this part is of special interest regarding the effects of interventions.
Sutherland et al. assessed the effect of an 8-week home vs. hospital-based training program in 17 Fontan patients aged 12-19 years of age. The program was performed according to a standardized program with 2x1 h training sessions per week. The training followed a structure of a 5-10 min warm-up, followed by an equally divided 20-30 min aerobic and resistance part, following a 5-10 min cooldown. The effect on the cardiorespiratory fitness was assessed after completing this 8-week training program with a CPET examination and a 6MWT. Both training programs showed a significant improvement regarding oxygen consumption, peak pulse as well as the distance committed during 6MWT. Additionally, the patients had to fill in a questionnaire evaluating their quality of life. This also showed a significant improvement after completing the training programs [61].
Kroll et al. recently published their results in 25 Fontan patients completing a homebased year-long cardiac rehabilitation program. This program included four in-person visits of an interdisciplinary team, including a cardiologist, a physical therapist, an occupational therapist, a psychologist, and an exercise physiologist every 3-6 months as well as wearing an activity monitor. The effects of the program on the exercise capacity were assessed using the Progressive Aerobic Cardiovascular Endurance Run (20-meter shuttle test run). Additionally, the patients had to fill in several questionnaires. The results showed a significant increase in the median completed shuttles from 5 to 10. The quality of life was not significantly improved regarding the patients' forms. Conversely, the parents stated that there was a significant improvement. Eleven of the twenty-five included patients had an additional diagnosis, for instance, attention deficit hyperactivity disorder (ADHD) [57].
Jacobsen et al. conducted a pilot study in 14 Fontan patients using a 12-week moderate/high-intensity home-based cardiac physical activity program, including three formalized in-person exercise sessions at weeks 0, 6, and 12. The exercise sessions were 45 minutes with a mixture of dynamic and static exercises. Additional data were collected with an activity monitor the children had to wear during the study time. For the evaluation, there were also CPET and Shuttle Test runs used. The objective measurements regarding endurance and cardiorespiratory fitness improved significantly. They also used questionnaires to evaluate the effects on quality of life. In this trial, the statements of the parents and children brought controversial results [64].
Moalla et al. studied 17 patients with CHD aged 12-16. Nine of them were randomized to a 12-week home-based training program with three 1 h sessions per week. The adolescents used a cycle ergometer and a pulse monitor for the duration of the study. The training program included a 10 min warm-up, followed by a 45-minute interval training session alternating with 10 min active and 5 min passive periods on the cycle ergometer. The active periods aimed to achieve an individualized target heart rate according to the HR achieved at VT in a previously conducted CPET. The other eight patients were randomized to a passive control group as well as fourteen healthy peers. The results were measured using CPET and 6MWT. After the 12 weeks of training, there could be seen a significant improvement regarding cardiorespiratory fitness in the training group [71,72].
Callaghan et al. studied 163 patients with CHD. They were randomized in an intervention group, starting with a 1-day education session followed by a 4-month training program with an individualized written exercise plan. An evaluation regarding cardiorespiratory fitness was conducted using CPET, and activity was monitored using an accelerometer. They could show a significant improvement in peak exercise capacity in the intervention group as well as a trend towards increased daily activity levels [54].
Brassard et al. studied seven Fontan patients with an 8-week exercise program either at home or in the hospital. The training program was split into an aerobic and a resistance part. The conducted CPET before and after the exercise training showed no significant benefit. Nevertheless, they showed some changes in muscle function and blood pressure reduction [76].
In the multicenter trial of Dulfer et al. and Duppen et al, there could be seen a significant improvement in cardiorespiratory fitness (CRF) with an improvement in VO 2 max and O 2 -pulse in patients with the tetralogy of Fallot, whereas this effect could not be verified in Fontan patients [67,68].
Hedlund et al. showed similar results in 30 Fontan patients after a 12-week endurance training at a submaximal level. Healthy controls increased their CRF. Fontan patients did not increase their CRF, but they increased the distance in the 6MWT and their quality of life [63].
Fredriksen et al. achieved a significant increase in VO 2 max in 129 patients who underwent a 5-month exercise training program at home or in a center [79].
Meyer et al. couldn t see any improvement after a 24-week web-based exercise training program. The program was divided into 3 × 20 mins per week. The exercise sessions included child-friendly video instructions and demonstrations of the different exercises. It also included a virtual training partner, and the exercise was performed simultaneously with the video. Participation was assessed with tracking, and regular reminders were sent. Afterwards, the HRPF was assessed in accordance with the FITNESSGRAM, which is a standardized test to assess HRPF with exercises such as curl-ups, trunk lifts, and push-ups. There was no improvement found after the completion of the program [58].
Opocher et al. studied 10 Fontan patients and could measure under submaximal endurance an increased O 2 pulse and an increase in VO 2 [78].
In summary, different interventional programs seem to have a positive effect with an improvement in VO 2 max and therefore a measurable increase in cardiorespiratory fitness in almost all patients with CHD. Nevertheless, this seems not to be achievable in Fontan patients. In these patients, there seems to be no increase at the maximum exercise test, but Opocher et al. could show an improvement at a submaximal level [78], as well as Brassard et al., which could show a reduction in pulse wave velocity and therefore blood pressure [76]. Therefore, in Fontan patients, the benefits seem to be found on another level.

Physical Activity
Physical activity is the crucial point in everyday activity. It decreases with age in healthy individuals.
Longmuir et al. assessed the effect of a 3-month home exercise program on the level of physical activity using a questionnaire in patients with CHD. The program was not described in detail but included endurance activities such as jogging. A control group of healthy peers was questioned. They showed a significant improvement in PA levels, even after a 5-year follow-up [53].
Dulfer et al. and Duppen et al. examined 93 TOF/Fallot patients aged 10-25. The intervention group was randomized to a 12-week standardized exercise training program with 1 h sessions three times a week. The training started with a 10 min warm-up, then 40 min aerobic dynamic cardiovascular training, and ended with a 10 min cooldown. The results were assessed via questionnaires and accelerator data. Exercise training decreased passive, sedentary leisure-time spending; active leisure time-spending was not reduced. The patients seemed to take part in their regular active activities. Nevertheless, there could not be seen an increase in PA in general [66][67][68].
Fredrikson et al. measured PA in 129 CHD patients and a control group before and after either training or no intervention. They showed a significant increase in PA 1-2 weeks after the completion of the training [79]. Klausen et al. used an eHealth application for a period of 1 year to assess the effects of PA and CRF. The app reminded the participants regularly to move and was set up in a playful way with achievements, but nevertheless, the app was hardly used, and they could not achieve any significant changes. They concluded that this approach was not useful [65].
Lopez et al. randomly measured physical activity in children with CHD without any intervention program. They showed a correlation between PA and lower pulse wave velocity. They concluded that physical activity seems to have an impact on vascular function and stated that a higher level of physical activity should be engaged in these children [60].
Morrison et al. included a sports psychological part to improve the long-lasting benefit of the training program and succeeded with an improvement in physical activity [70].
In summary, most of the conducted training programs and interventions had a positive effect on overall physical activity, even beyond the study period.

Quality of Life and Other Questionnaires
Another important point, which is known to be lower in patients with CHD, especially with aging, is quality of life. Therefore, it is also of importance to evaluate the effect of sports and physical activity on quality of life, as this might lead to better well-being and a reduction in psychological illnesses such as depression.
Dulfer et al. also assessed a quality-of-life questionnaire, which showed, especially for the ages of 10-15 years, a significant improvement in overall quality of life. Furthermore, they reported a significant cognitive improvement. Parents also reported an improvement in social interaction, whereas they could not find an improvement regarding emotional and behavioral problems [66][67][68].
Hedlund et al. showed in Fontan patients an increase in quality of life after a training program, even if they did not increase their CRF, whereas healthy individuals did increase their CRF but did not subjectively increase their quality of life [63].
Blais et al. studied 11 children with CHD aged 7-10 years who participated in a 10-week program, including once-weekly multi-sports programs. Each lesson focused on a different ball sport. The participating children were asked to fill in questionnaires and attend group sessions. The results showed that enjoyment of physical activity is a primary source of motivation. Therefore, intrinsic motivation seems to be crucial [59].
Fredriksen et al. also showed a significant reduction in withdrawal and somatic complaints in the exercise intervention group [79].
Moons et al. could show an improvement and a lot of benefits during the attendance of the camp but not afterwards [77]. Kroll  Jacobsen et al. studied 14 Fontan patients and could show an objective improvement in CRF after the intervention. Nevertheless, there was no improvement in quality of life according to the child's questionnaire, whereas parents stated a significant improvement [64].
As stated above, there are different results regarding perceptions of the influence the stated exercise programs had on the patients' quality of life. There are controversial reports in the available literature regarding the patients' own judgement for the improvement of their quality of life as well as in comparison to their parents' reports. In the abovementioned studies, often, parents stated a significant improvement regarding quality of life and psychosocial and behavioral issues, which was not recognized the same way by the patients themselves.
This is discussed to be a matter of self-awareness, which seems to be a difficult matter in children and adolescents, as this self-awareness has to be learned.
According to this subjective awareness, other studies comparing the results of questionnaires or accelerometer data of CHD patients with healthy pears also showed differing results. As reported by those studies, children and adolescents with CHD can have similar physical activity levels, but they also can vary sharply as well. Maybe these confusing results are due to the different severities of the examined CHD population as well as their environment.
In conclusion, quality of life seems to be improved, and psychosocial and behavioral issues seem to be reduced by a higher level of physical activity and participation in sports. Even if patients are not immediately aware of this overall huge positive effect, their surrounding peers and family recognize early positive effects.

Discussion
The reviewed studies suggest clearly that participating in sports and exercise/rehabilitation programs, or simply the increase in physical activity, are not only safe but also beneficial for patients with CHD.
The level of evidence nevertheless is moderate as there is only a limited number of clinical trials with a small sample size of patients, focusing on children and young adults with CHD, which leads to controversial and confusing results. Regarding the widely different conditions gathered under the term "congenital heart disease", general transferability is almost impossible.
In summary, it can nevertheless be stated that an increase in a physically active lifestyle results in a number of positive adaptions of the body which may lead to a reduction in morbidities and even influence the mindset of the individual positively.
Regarding the relatively small number of patients with CHD living in the same area and having a comparable range of age, a home-based training and rehabilitation program seems to be the most convenient approach to start and retain families. Those programs can use modern media such as online video conferences or app-supported training, even if the trials gathered controversial results using this approach.
Nevertheless, safety aspects are still important issues regarding, for instance, cardiac and orthopedic conditions. Therefore, a thoughtful examination, including the measurement of blood pressure, ECG, clinical examinations, and a detailed past medical and family history should be conducted. In addition, echocardiography, CPET, and Holter-ECG are useful to ensure safety and exercise tolerance. Furthermore, the trainer should supervise the correct exercise performance.
The used exercise programs also differed widely with some using a combination of aerobic and resistance exercises and others focusing on either or.
Some trials seemed to be more training-orientated and performed a CPET at the beginning to measure thresholds and use them to control training with target heart rates in the intended range [30,55,56,61,66,[76][77][78][79]. As in athletes, they trained their patients to focus on their heart rates measured by, for instance, an accelerator or a smartwatch. This seems a good approach to increase the independence of patients with congenital heart defects, which correlates with normalization and an increase in quality of life.
Maybe we should rather aim to increase this independence even more, inspired by the abovementioned approach. After supervised home-based or institutional training to assure patients and their families, healthcare providers should aim to include those children in regular sports clubs, matching their interests. This might be an approach to increase intrinsic motivation and create peer groups, which has been shown to be crucial to increase PA in general.
Another approach that was used in some of the trials, which is also used by athletes, is the Borg Scale, which assesses subjective exhausting levels and can help to assume which training intensity is equivalent to different levels in CPET.
Furthermore, it should be noted that questionnaire results between parents and children differed largely in some studies. This may be due to the gathered security of parents achieved by knowing their children participated in a safe program and gained a step towards normality. Insecurities in parents often result in the overprotection of these children. Healthcare professionals are also insecure due to lacking guidelines and recommendations for this patient group; they often also tend towards overprotection. Therefore, it is crucial to inform especially healthcare professionals to encourage participation in sports. Healthcare prescriptions also have shown a beneficial effect. The recent guideline of the DGPK can therefore be an additional help for healthcare professionals [29].
For children in general, it is important to be led at an early age to a healthy, physically active lifestyle, and the latest data on adults with congenital heart defects showed a huge dropout rate during transmission from pediatricians to adult cardiology [16]. Patients with CHD showed a reduced quality of life decreasing with age [92].
Therefore, the maintenance of the interest in gathered routines should be one of the most challenging aspects for further research.

Limitations
This review has limitations. There is only a limited number of trials, focusing on children with congenital heart disease. As abovementioned, beyond that, the reviewed studies have limitations, such as small sample sizes, varying levels of evidence, and different approaches regarding interventions and measuring results.
Moreover, the search was conducted by only one researcher; therefore, the false exclusion of articles cannot be fully dismissed. Nevertheless, the methods and results were approved by the other researchers.
Furthermore, academic disciplines and medical science are rapidly developing fields. Therefore, there might be other studies completed in the meantime, as the underlying search was last completed at the end of 2021.

Conclusions
In summary, PA, participation in sports, and exercise training seem to have a wide range of positive impacts, starting with improving motoric skills and muscle function, bringing health benefits, and, finally, having a huge impact on quality of life.
PA has been shown to be influenced by several factors such as psychosocial aspects with motivation due to a peer group or family members, as well as prescriptions of healthcare professionals and others with a high impact on intrinsic motivation.
Sports, exercise training, and physical activity have an across-organ positive effect on the whole body. Even in patients with CHD, this effect is outreaching, with an improvement in motor development and muscle function, a reduction in body weight and metabolic diseases, as well as an improvement in the quality of life in children and their parents.
The heterogeneity of CHD underlines the need for individual training programs and adapted intensities.
Nevertheless, there is a low but appreciable risk of sports-related complications; however, sports in general do not increase the overall risk of mortality; this may be relevant. Most adverse events happen with participation in competitive sports and not just during participation in leisure sports.
In general, there seems to be only a small group of patients with heart diseases which should be excluded, or the risks should at least be discussed openly with parents and patients as in life-threatening arrhythmias (long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia (CPVT), or other channelopathies), aortic dissection, or severe pulmonary hypertension.
A considerable approach might be a detailed past medical and family history, as well as exercise tolerance in a daily routine, and a clinical examination measuring blood pressure and ECG. For more complex or insecure patients, additional CPET or Holter-ECG might be useful.
In order to increase the security of families and medical professionals, it might be useful to start training in a supervised home-based or institutional setting after adequate and individual pre-exercise assessment. Afterwards, individual interests and conditions should be taken into consideration when choosing a sport, including the intensity and weighing up the dangers.
Regarding the increase in quality of life when gaining normality and independence, the approach with training, based on target heart rates, gathered during CPET, seems to be desirable. This can increase the opportunity for patients with CHD to take part in sports clubs with their peers.

Conflicts of Interest:
The authors declare no conflict of interest.