Transposition of exercise protocols: cardiovascular response to exercise in patients with transposition of the great arteries

The benefits of exercise at reducing cardiovascular and all-cause mortality have been well described, but certain cohorts of individuals exist in which the safety of exercise has not been studied and its benefits and harms are thus far unknown. During participation in sports or other forms of exercise, patients with transposition of the great arteries (TGA) with systemic right ventricles (SRVs) may have a higher risk of adverse events, including sudden cardiac death. There is little evidence regarding the exercise tolerance and safety in this cohort of patients and it is unknown if exercise will increase the stress on the SRV and result in diminished function. Shafer et al. (2015) examined the cardiovascular and metabolic responses to exercise training in the SRVs of patients with TGA in order to investigate exercise tolerance and its effect on the SRV. All subjects underwent exercise testing before and after 3 months of an intensive training programme, with a primary endpoint of change in oxygen consumption () as a surrogate marker for changes in exercise tolerance. Exercise testing was performed using treadmill testing with continuous ECG monitoring and automated blood pressure measurements every 2 min. and cardiac output () were measured standing, at rest, and during low and moderate intensity exercise. In addition, patients returned on a subsequent day to perform a 60 min prolonged submaximal endurance exercise and high-intensity interval training (HIIT) with echocardiography assessment. After preliminary testing, all subjects engaged in a 3 month progressive exercise-training programme. The programme consisted of base pace exercise (60–70% of max heart rate (HR)) for 60 min, maximal steady state exercise (70–80% of max HR), and HIIT (85–95% of max HR) with the duration and intensity of exercise increasing each month. The study examined 14 patients (50% male) with TGA and nine controls (44% male) with an average age of 34, and otherwise similar demographics. Results suggest that TGA patients with SRV are able to safely participate in a 3 month moderate to high intensity exercise-training programme. They found lower and stroke volumes (SVs) in TGA patients versus controls during submaximal and peak exercise. TGA patients also had lower SV reserve (SVR). Additionally, both groups maintained similar exercise intensities during the prolonged endurance training, indicated by similar capillary lactate levels, respiratory exchange ratios, and ratings of perceived exhaustion. The second half of the study entailed a 12 week exercise training programme, which resulted in a 1.3 ml kg−1 min−1 increase in peak , and decreases in cardiac stress, measured by cardiac troponin T. They also observed an increase in relative blood volume, which did not translate to increases in ; however, that may be influenced by the small sample size. Data suggest that an increase in aerobic fitness by 1 metabolic equivalent (MET) translates to a 10% reduction in all-cause mortality. The results of Shafer et al. (2015) reported a 0.5 MET increase in aerobic fitness, which would translate to an approximate 5% reduction in all-cause mortality if the subjects were healthy. The direct effects of an increase in aerobic fitness in TGA patients are unknown without long-term follow up; however, we expect an improvement similar to that observed in heart failure (HF). Exercise has been shown to increase aerobic fitness and improve quality of life without diminished left ventricular ejection fraction (LVEF) in patients with HF, a phenomenon that may translate to TGA patients with a SRV (Pandey et al. 2015). Although increased aerobic fitness may suggest decreases in adverse events and all-cause mortality, a similar exercise study by Winter et al. (2012) did not observe increases in quality of life (QoL), despite increased peak. One factor that was not considered in these studies were the implications for the patient's lifestyle, such as participation in sports, and how that might subsequently affect their QoL. Opic et al. (2015) illustrated increased exercise capacity and no change in adverse events with participation in light to moderate (1–5 h per week) or extensive (>5 h per week) sports. Unlike a strict exercise-training regimen, the ability to engage in sporting activities may be more likely to increase QoL. Having established the safety of training in this population, new questions arise. The training programme assigned by Shafer et al. (2015) and Winter et al. (2012) required an intensity and duration that may be difficult to sustain long-term. While this high intensity was necessary to ascertain whether or not TGA patients could tolerate exercise without negative effects on ventricular function, follow up studies should explore less extensive exercise regimens, including sports participation, to determine if similar results can be observed in programmes that are likely to have less attrition. In addition, other studies working with TGA patients show that exercise-induced changes in aerobic fitness do not persist during a 3 year follow-up. This is likely to be a result of lack of compliance after completion of the study, but follow-up work should test what quantity and intensity of exercise is necessary to maintain gains and compliance. Long term follow-up with these patients and other TGA patients involved in exercise programmes would be of value in order to determine whether the observed improvements in were sustained and to investigate any effect exercise training programmes have on all-cause mortality in TGA patients. After examining disparities between the results of TGA subjects, the authors concluded that there might exist a subset of TGA patients with underlying ventricular dysfunction who lacked the potential to increase their and thus gain no value from the experimented exercise protocol. The two predictors of this ventricular dysfunction identified were a decline in SV during the transition to maximal intensity exercises and a limited SVR. When transitioning to maximal effort patients saw a decline in SV, indicating that the heart was unable to compensate for the increased effort. Additionally, analysis showed that initial SVR was the only correlate with training response, with higher SVR predicting higher . Identifying this population before the outset of a new study could prove valuable when attempting to delineate the effectiveness of exercise programmes with TGA patients and prove useful when providing lifestyle recommendations. The limited sample size of 14 participants may be of statistical concern, but the number of participants seems adequate considering the rarity of this condition, the high level of commitment expected, and the broadness of the central question being investigated. Further investigations into this population may require larger sample sizes in order to study more subtle outcomes. In summary, Shafer et al. (2015) have demonstrated that TGA patients with systemic right ventricles are able to tolerate moderate to high intensity exercise training programmes with no detectable injury; however, the small sample size may impact the studies external validity. This opens the door for further investigations into optimal exercise protocols, predictors of efficacy and prognostic changes after exercise. None declared.

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