Study population

From April 2009 to February 2014, all outpatients with CHD seen for regular visits in our CHD unit were assessed once for potential contraindications and, if none, patients were asked to see a specially trained physical therapist connected to the unit. Patients who came for revisits were not assessed or included more than once.

Exclusion criteria were severe arrhythmias, patients waiting to undergo surgery, advanced heart failure, severe cerebral lesions, and intellectual disabilities resulting in difficulties in performing the submaximal ergometer bicycle test, or if previously included in the study.

This study was a single-centre cross-sectional study. Patients were given oral and written information about the study, and each patient signed a letter of informed consent. Patients of the CHD unit agreed to have their data registered in the national Swedish Registry of CHD, which made it possible to analyse data for patients who opted not to participate in this study.

One thousand three hundred and ten outpatients seen at a specialised CHD unit were assessed. The demographics of this population are given in the Supplementary Table S1. Two hundred and twenty-three (17%) patients did not perform the tests due to one of the following reasons: musculoskeletal disorders (n = 9), intellectual/cerebral disability (n = 31), psychiatric disorders (n = 7), unstable or severe cardiac condition (n = 23), pregnancy (n = 31), discontinuation of planned care (i.e., moving outside catchment area) (n = 17), or other or unknown reasons (n = 105). Three hundred and forty (26%) patients declined to participate. No significant differences were seen between included and not-included patients (Supplementary Table S1).

Reference group

A description of the specific reference values used for comparison with the results of adults with CHD is provided along with an explanation of each test or questionnaire used. If patients were older or younger than the age range for the available reference values, these patients were grouped together and calculated with the nearest reference value available for levels of exercise capacity, physical activity level, and health-related quality of life.

Exercise capacity

A submaximal ergometer bicycle test (Monark 828 E, Varberg, Sweden) was used to assess exercise capacity. The protocol used was designed according to the World Health Organization¹⁵ and is a stepwise test with increasing loads with an almost steady circulatory state at each level.^{Reference Sjöstrand16,Reference Wahlund17} Resting heart rate and blood pressure were recorded before the test with the patient in a supine position. The initial load was 25 or 50 watts in a few patients. Speed was set to 60 revolutions per minute. The load increased every 4.5 or 5 minutes by 25 watts. Time was set for 4.5 or 5 minutes to reach steady state.^{Reference Arena, Myers and Williams18} Blood pressure was recorded manually with a sphygmomanometer every 2 minutes (H.E AB, Bandhagen, Sweden). Heart rate was measured via a pulse strap sending impulses to the bicycle (Polar T31; Polar, Bromma, Sweden). Heart rate and blood pressure were registered up until 5 minutes after ending the test. The exercise test was ended when patients reached 15–17 on Borg’s rating scale of perceived exertion.^{Reference Borg19} Other reasons for ending the submaximal test before reaching an perceived exertion of 15–17 were decreased blood pressure, chest pain, or if the patient’s cardiologist had indicated that the patient should only be permitted to reach a lower level of exertion.

Due to some patients ending the submaximal ergometer bicycle test before the level of watt was finished, that is, 4.5 or 5 minutes, the results of the submaximal bicycle test were calculated according to Strandell.^{Reference Strandell20} Thereafter, calculations were made to compare the results of patients included in the study with maximal reference values.^{Reference Brudin, Jorfeldt and Pahlm21} All calculations are presented in the Supplementary material.

Physical activity

The patient’s level of physical activity was assessed with the International Physical Activity Questionnaire – Short Form.^{Reference Craig, Marshall and Sjostrom22} Each patient’s self-reported physical activity is counted in Metabolic Equivalent of Tasks and divided into one of following three subgroups: low, moderate, or high. Descriptions of each group and calculations can be found in the Supplementary material. The questionnaire has been tested for validity^{Reference Ekelund, Sepp and Brage23,Reference Lee, Macfarlane, Lam and Stewart24} and reliability.^{Reference Craig, Marshall and Sjostrom22}

Health-related quality of life

Self-reported health-related quality of life was assessed with the 36-item Short Form-36^{Reference Ware and Sherbourne27} consisting of eight scales and has good validity and reliability.^{Reference Ware and Sherbourne27}

Classification of CHDs

To form larger groups, CHD categories were combined into the following three groups: Less complicated, Corrected, and Complex diagnoses. The Less complicated diagnosis consisted of simple shunts, aortic valve malformations, aortic anomalies, mitral valve lesions, pulmonary valve lesions, and tricuspid valve lesions. The Corrected diagnosis consisted of right ventricular/tetralogy of Fallot and transposition of the great arteries. The Complex diagnosis consisted of truncus arteriosus, univentricular repair, and others. This arbitrary but prospective division was made to investigate whether differences in exercise capacity could be related to the degree and severity of malformations and to facilitate statistical analysis. The functional class of all patients was determined according to the classification system of the NYHA.^{Reference Bennett, Riegel, Bittner and Nichols30}

Spearman’s rank correlation coefficient was used to calculate correlations for the Short Form-36, and Pearson’s correlation coefficient was used to calculate correlations between exercise capacity and physical activity. Correlations were interpreted according to Munro as follows: 0.00–0.25 Little if any, 0.26–0.49 Low, 0.50–0.69 Moderate, 0.70–0.89 High, or 0.90–1.00 Very high.^{Reference Munro31}

The type of multiple linear regression used was Enter, which is a model that allows the authors to include variables thought to explain the dependent variables. It was used to assess the associations between exercise capacity (watt), International Physical Activity Questionnaire (total metabolic equivalent of tasks minutes/week), Short Form-36, sex, age, different diagnosis groups, and NYHA classification which was dichotomised into two groups, NYHA I and NYHA II–IV. Confidence interval was 95%. R ² is the coefficient of determination and the degree of explanation of each variable.

The independent-samples t-test was used for comparisons of exercise capacity between adults with CHD and reference values. The Mann–Whitney U test was used for calculations with non-parametric data (International Physical Activity Questionnaire and Short Form-36). Tests were two-sided and the significant p-value was set to <0.05.