Understanding exercise BP in adolescence : fitness, fatness, and cardiovascular risk

Cardiovascular disease is the number one cause of morbidity and mortality worldwide, of which high blood pressure (BP) is the single greatest risk factor. Despite improvements in prevention and management strategies, more than a third of Australian adults have high BP, many more with undiagnosed or inadequately controlled high BP. Thus, there is a continued need to find alternative methods to detect high BP and its associated cardiovascular risk. Recently, the BP response to exercise has been shown useful in the identification of high BP gone undetected in the clinic at rest and is associated with increased risk of cardiovascular morbidity and mortality. Much of the research on exercise BP and its clinical relevance has been carried out in middle-to-older age or clinical population groups. However, cardiovascular risk (including raised BP) originates in early life and often tracks through to adulthood. Thus, gaining an understanding of the principal factors that contribute towards elevated exercise BP in adolescence, as well as its associated cardiovascular risk is important. The broad aims of this thesis were to gain a greater understanding of factors that contribute to exercise BP (including fitness and fatness), and the potential cardiovascular risk (high BP and cardiovascular structure) associated with exercise BP in adolescence. Study 1 (chapter 2) sought to determine whether exercise BP was associated with underlying high BP or 'masked hypertension' and cardiovascular risk in adolescence. A cross-sectional analysis of participants (n=657, mean age 17.7 ¬¨¬± 0.3) from the Avon longitudinal study of parents and children (ALSPAC) was completed. Exercise BP was measured pre-, post- and in recovery from a submaximal exercise step-test, with the clinic and 24‚ÄövÑv™hour ambulatory BP measurement (to categorise masked hypertension) and clinical assessment of left ventricular (LV) mass index and carotid‚ÄövÑv™femoral pulse wave velocity (aortic PWV). The prevalence of masked hypertension was 7.8% in the total population, with pre-, post-, and recovery-exercise systolic BP thresholds of 126, 150, and 130 mmHg found to rule out masked hypertension with high specificity and negative predictive value (either respectively or combined). Exercise systolic BP above these thresholds was also associated with greater LV mass index and aortic PWV. These results suggested that exercise BP might be a useful tool to detect underlying high BP and related cardiovascular risk in adolescents. In study 2 (chapter 3), the influence of fitness on exercise BP and its relationship with cardiac structure was examined in a group of adolescents (aged 15.4 ¬¨¬± 0.3 years) from the ALSPAC cohort. Post-exercise BP was measured after completion of a submaximal cycle test in 4835 participants and fitness was estimated as physical work capacity 170 adjusted for lean body mass. Cardiac structure (LV mass index) was measured in a subgroup of 1457 participants by echocardiography 2.4 years later. Post-exercise systolic BP increased with fitness level, which modified its association with LV mass index. The post-exercise BP-LV mass index relationship was 'U shaped', such that both those with low and high fitness shared a similar association (˜í‚â§: 0.27, 95%CI: 0.07, 0.46 g/m\\(^{2.7}\\) /mmHg and 0.18, 95%CI: -0.02, 0.39 g/m\\(^{2.7}\\) /mmHg for those with low and high fitness respectively). These results highlighted the importance of considering fitness as a factor that influences exercise BP and its association with cardiac structure in adolescence. In study 3 (chapter 4), the association between cardiorespiratory fitness, fatness, and exercise BP was determined in a cross-sectional analysis of adolescents (n=2292, mean age 17.8 ¬¨¬± 0.4 years) again drawn from the ALSPAC cohort. All participants undertook a submaximal step test with post-exercise BP measurement. Fitness was estimated from a validated equation and fatness was quantified by dual-energy x-ray absorptiometry (DXA). Post-exercise systolic BP decreased with fitness but increased with fatness. Although both fitness and fatness were associated with post-exercise systolic BP independently of age, sex, height, and socioeconomic status (standardized ˜í‚â§: -1.80, 95%CI: -2.64, -0.95 mmHg/SD and 4.31, 95%CI: 3.49, 5.13 mmHg/SD respectively), when fitness and fatness were included in the same model, only fatness remained independently associated with post-exercise systolic BP (4.65, 95%CI: 3.69, 5.61 mmHg/SD). Overall, results indicated that fatness might be a more important factor contributing to exercise BP than fitness in adolescence. To further explore the influence of body composition on exercise BP, fatness and leanness trajectories from childhood (mean age 9.8 ¬¨¬± 0.3 years) to young adulthood (mean age 24.3 ¬¨¬± 0.6 years) were derived in 2210 participants from the ALSPAC cohort by group-based trajectory modelling in study 4 (chapter 5). All participants completed a submaximal exercise step test with post-exercise systolic BP measurement in early adulthood. Three distinctive trajectories of fatness and leanness were identified and were associated with post-exercise systolic BP (3.86, 95%CI: 2.56, 5.15 mmHg, -3.93, 95%CI: -5.19, -2.67 mmHg respectively). However, associations were attenuated after adjustment for adult measures of fatness and leanness (-0.22, 95%CI: -1.88, 1.44 mmHg, 0.69, 95%CI: -1.24, 2.62 mmHg). Overall, results indicated that modelling trajectories of fatness and leanness from childhood to adulthood do not provide additional information about exercise BP beyond a single contemporaneous measurement in adulthood. Taken altogether, this thesis provides evidence to suggest that the BP response to exercise can be useful in the screening of adolescents with underlying high BP and cardiovascular risk, which may have been missed by traditional (resting) BP measurement. Moreover, fitness appears to be an important factor contributing to exercise BP and modifies its association with cardiovascular structure in adolescence. However, whilst fitness is clearly associated with exercise BP in adolescence, fatness might be a more dominant contributor to the exercise BP response. Finally, whilst trajectories of fatness and leanness from childhood to adulthood contribute to exercise BP, they do not provide additional information about exercise BP beyond contemporaneous measures of body composition in adulthood.