Heart Rate Behavior During 24h Cycle Ergometer Exercise Is Dependent On Total Body Water

PURPOSE: The aim of this case study was to examine changes in hemodynamic variables in a single trained male cyclist during a 24h cycle ergometer exercise test. METHODS: The subject was monitored throughout 24h at a constant workload at the first lactate turn Point (200 Watt). Heart rate (HR) and respiratory gas exchange measures were analyzed continuously [oxygen consumption (VO2), carbon dioxide production (CO2)]. Every hour systolic blood pressure (SBP), cardiac output (CO) and stroke volume (SV) were determined non-invasively (Innocor) as well as 2-D echocardiograms over the left parasternal area. Using VINGMED's Anatomical M-Mode it was possible to extract M-mode sweeps from stored 2-D-loops and perform the M-mode measurement [left atrial (LAD), ventricular end-diastolic (EDD) and end-systolic (ESD) diameters]. In addition, we also measured N-terminal pro-brain natriuretic peptide (NT-proBNP). Venous blood sample collection and body weight were obtained every 6 h. RESULTS: After an initial increase after 1 h, VO2 was (41 - 43 ml.min.-1.kg-1) and CO (19 - 18.7 l.min-1) were unchanged during 24h. HR increased at the end of the 5th h from 58-143 bpm and decreased constantly during the following 19 h (125 bpm). From the 5th h on SV (109 ml) increased steadily throughout the 24th h exercise test up to 150 ml. ESD was unchanged (32 mm) but EDD increased from 47 to 51 ml. LAD increased from 37 mm to 45 mm and body weight increased from 71 kg to 74 kg. Hematocrit decreased from 44% to 41%. NT-proBNP increased from 23 at rest to 306 pg.ml-1 at the end of exercise. CONCLUSIONS: Our study suggests that the decrease in HR during a 24 h aerobic exercise was due to hypervolemia and the associated ventricular workload that resulted in SV increase as a consequence of increases in left ventricular diastolic dimensions.