Cardiac Response During Exercise in Adolescents with Chronic Fatigue

Abstract

PURPOSE: To describe the cardiac response to exercise in order to elucidate pathophysiology of adolescent chronic fatigue. METHODS: We reviewed records of adolescents with minimum 6-month history (in accordance with 2015 Institute of Medicine diagnostic criteria for Systemic Exertion Intolerance Disease) of chronic fatigue who underwent maximal cardiopulmonary exercise between June 2012 and December 2014. Patients in whom fatigue was not the primary problem or those with another medical diagnosis were excluded. All patients performed an incremental exercise (1-minute steps) test on a cycle ergometer to voluntary exhaustion with breath-breath measurement of gas exchange to determine peak oxygen uptake (VO2). Cardiac output (Q) was measured by acetylene rebreathing at rest plus at 1-3 levels of light-moderate exercise, and regressed against VO2. Stroke volume was normalized for body surface area to give stroke volume index (SVI). Patients were classified as unfit if their predicted peak VO2 was <90% (Cooper et al., 1984), which cutoff eliminated height/weight differences between groups. RESULTS: Our study population comprised 312 subjects with mean (±SD) age 15.6±1.7 years (77% female); Hgb level 13.5±1.2, with lowest 10 g/dL; unfit 75% of males and 80% of females. Overall Q rose normally with a mean slope 6.06±2.29 L-min-1 per L-min-1 VO2. SVI of fit males (59 mL) was significantly higher than unfit males (47 mL) at 30-60 W (p= 0.018, t-test). There were no differences in Q–VO2 relationship or HR between fit and unfit males. In contrast, Q of an unfit female rose an average of 5.96 L-min-1, significantly higher than 5.26 L-min-1 in a fit female, per L-min-1 VO2 (p= 0.012). For a 1L-min-1 rise in VO2, HR of an unfit female rose an average of 70 bpm vs 61 bpm for a fit female (p= 0.002, RM-ANOVA). There were no significant differences in SVI between fit and unfit females. CONCLUSION: Most males and females with chronic fatigue are deconditioned and show the classical cardiovascular features thereof. However, a subset of adolescent males achieves SVI in light exercise similar to that reported in healthy subjects. Findings suggest there may be muscle alterations or perceptual differences in chronic fatigue. Sex differences also exist in the cardiovascular response to exercise, perhaps related to Hgb levels or O2 extraction.