Cardiopulmonary exercise capacity in CHD

The majority of patients with CHD today reach adulthood, a development mainly achieved by improvements in congenital heart surgery and long-term follow-up in paediatric cardiology. ^{Reference Warnes, Liberthson and Danielson2–Reference Müller, Amberger and Berg4} With higher life expectancy, focus shifts towards patients lifestyle and quality of life, with patient’s physical fitness taking a crucial role. ^{Reference Hager and Hess5,Reference Amedro, Picot and Moniotte6} Cardiopulmonary exercise testing plays an important part in routine checkups in patients with CHD. ^{Reference Kempny, Dimopoulos and Uebing3,Reference Nishimura, Otto and Bonow21} Peak oxygen uptake is regarded as the best indicator of cardiopulmonary function in children and adults. ^{Reference Takken, Blank and Hulzebos27}

In our group of 382 patients, cardiopulmonary exercise capacity represented by peak oxygen uptake was reduced at 25.5 ± 7.9 ml/minute/kg correspondent to 75.1 ± 18.8% of age and sex-specific reference values. Reduced peak oxygen uptake in patients with CHD has previously been described by various studies. ^{Reference Kempny, Dimopoulos and Uebing3,Reference Rhodes, Curran and Camil7–Reference Ten Harkel and Takken11}

We could see that in patients with complex cardiac defects, peak oxygen uptake as percentage of reference values was significantly lower than in the group of non-complex defects. There were also differences between the groups separated by diagnosis, in which peak oxygen uptake showed a similar distribution as in the study by Kempny et al, with lowest values seen in patients with Fontan circulation. ^{Reference Kempny, Dimopoulos and Uebing3}

Self-estimated exercise capacity

Comparable to existing literature, exercise capacity determined by peak oxygen uptake is reduced in our group of patients with CHD. Our next step was to examine if this reduction of objectively determined exercise capacity could also reliably be estimated by patient’s subjective impression of their own physical fitness.

After division of our study population into groups in regard to their subjective rating, we observed significant differences in peak oxygen uptake between the different groups. Peak oxygen uptake tended to be higher on average in groups with higher subjective rating, as we would also expect in a normal population. This tendency was also seen in regard to peak oxygen uptake as percentage of age- and sex-specific reference values and can therefore not solely be traced back to a higher age level in groups of lower self-rated exercise capacity. Interpreting the association between subjective and objective evaluation of exercise capacity, it should be noted that peak oxygen uptake was reduced, even amongst patients perceiving their exercise capacity as good or very good. These findings correspond to results from other studies that showed a tendency towards overestimation of own exercise capacity in patients with CHD. ^{Reference Hager and Hess5,Reference Chen, Su and Wang22–Reference Gratz, Hess and Hager24} A study by Chen et al showed higher self-perceived physical condition in patients with CHD, even when compared to a healthy control group. ^{Reference Chen, Chen, Beckstead, Kennel and Evans23}

This might have a negative impact on patient’s willingness to exercise, as patients being accustomed to their reduced exercise capacity might not see any call for action.

Accuracy of self-estimated exercise capacity in regard to complexity of CHD

Most of the studies that show a tendency of overestimation of physical fitness refer to the population of patients with CHD as a whole. The main aim of this study was to examine a possible impact of severity of cardiac defects as well of patient’s gender on the accuracy of self-estimated exercise capacity.

While we could see a significant correlation between self-estimated and objectively assessed exercise capacity in patients with non-complex cardiac defects, no correlation was found in patients with complex cardiac defects. No difference was found in female versus male patients.

One possible explanation for the observed overestimation could be that limitations and linked precautions have often followed through the patient’s whole life. Therefore, physical capacity meets the patient’s own standards and is not perceived as reduced. ^{Reference Hager and Hess5} As reduction of exercise capacity is highest in patients with complex cardiac defects, ^{Reference Kempny, Dimopoulos and Uebing3} these patients might also be especially accustomed to their circumstances, including to their level of physical fitness. This would suggest that due to a high level of adaption, the mismatch between self-estimated and objectively assessed exercise capacity is most prominent in patients with complex CHD.

Another explanation might result from the assumption that a sedentary lifestyle seen in patients with CHD would be more dominant in patients with complex defects. Based on this assumption, it could be argued that patients with mild defects tend to be more physically active and might therefore be more familiar with their physical abilities and limitations, while patients with complex defects might have less experience in being physically active to judge from. However, this assumed difference of physical activity in regard to the complexity of underlying cardiac defects could not be found in existing literature. ^{Reference Muller, Hess and Hager33,Reference Voss, Duncombe, Dean, de Souza and Harris34}

Another aspect contributing to the mismatch between self-estimated and objectively assessed exercise capacity might become apparent from a psychological perspective. Patients with CHD show high prevalence of depressive symptoms and lower self-esteem, both affected by disease severity. ^{Reference So, Li and Ho35,Reference O’Donovan, Painter, Lowe, Robinson and Broadbent36} This might also partly account for the misperception of physical abilities amongst these patients, since depressive symptoms have been associated with an altered self-concept. ^{Reference O’Donovan, Painter, Lowe, Robinson and Broadbent36} Another interesting finding in this context is that sport participation was found to be independently associated with reduced depressive symptoms in patients with CHD. ^{Reference Ko, White and Kovacs37} The study by Ko et al concluded that sport programs uniquely designed for patients with CHD may improve depression status in these patients. ^{Reference Ko, White and Kovacs37} These findings suggest that the high prevalence of depressive symptoms in patients with CHD might be connected to the inaccurate self-estimation of exercise capacity in these patients. Thus, specialised sport programs could improve both self-perception in terms of physical abilities as well as depressive symptoms. Moreover, such programs could provide a possibility to get motivated and accustomed to physical activity in a safe and controlled environment. Another benefit is feedback experiencing the body’s response to training as well as from comparison with peers that, with a weekly training interval, is way more frequent than cardiopulmonary exercise testing performed in routine checkups usually every 6 to 12 months. A training program specialised in patients with CHD has already been introduced at German Heart Centres in Berlin and other locations. ^{Reference Dirks38,Reference Hager39}

Considering an increasing prevalence of severe CHD as seen in a large demographic study by Pfitzer et al, ^{Reference Pfitzer, Helm and Ferentzi40} capacities for cardiopulmonary exercise testing might have to be adapted in the future.

Conclusions

The mismatch between self-estimated and objectively assessed exercise capacity is most prominent in patients with complex CHD.

Subjective self-evaluation can contribute to but not replace objective assessments, as many patients tend to overestimate their own exercise capacity. It can still provide valuable information, as declines of perceived exercise capacity can be noticed during their everyday lives, directly and independently of checkup intervals and can speed up necessary diagnostics.

Our findings support the key role of cardiopulmonary exercise testing in evaluating physical fitness, especially in patients with complex cardiac defects.

Limitations

One difficulty resulted from our inhomogeneous study population, especially present in evaluating exercise capacity. In order to make peak oxygen consumption comparable between patients of different characteristics, it is commonly not only stated in millilitre per minute and kilogram, but also as a percentage of age- and sex-specific reference values. The formulas available for calculating these reference values stem from relatively small studies performed on healthy subjects ^{Reference Cooper and Storer28,Reference Cooper and Weiler-Ravell29,Reference Jones, Makrides, Hitchcock, Chypchar and McCartney41,Reference Bruce, Kusumi and Hosmer42} Most studies included only very few subjects in middle childhood or approaching retirement age, if any. Therefore, anthropometric margins were not represented and could only be extrapolated from existing data. ^{Reference Dubowy, Baden, Bernitzki and Peters43} Due to various age groups, reference values had to be calculated using multiple formulas, which might strictly speaking still not fully account for the wide range of age (8–68 years) in this study. Reference values derived from one large study including all age groups and both sexes are needed. A study of this kind was performed by Dubowy et al, however, only for treadmill testing, not covering tests performed on also commonly used bicycle ergometers. ^{Reference Dubowy, Baden, Bernitzki and Peters43}

Further research on the subject, including the impact of sports intervention programs on patient’s subjective experience of physical fitness, is necessary.