Association of metabolic and hemodynamic variables during exercise in children.

Abstract

Three investigations were conducted using pulsed Doppler echocardiography (PDE) and oxygen consumption to non-invasively determine the relationship between hemodynamic and metabolic variables from rest through submaximal and maximal exercise in early adolescent males. In the first study, interinvestigator and day-to-day variability of cardiac output measurements at rest and during exercise determined by PDE were analyzed in six junior high school age boys. Four Doppler-derived variables (cardiac output, cardiac index, stroke volume, and stroke index) were not different when calculated by independent investigators and showed the same interinvestigator variability from rest through submaximal and maximal exercise as has been reported at rest in previous validation studies. There was a slight but statistically significant increase in the absolute values of the above hemodynamic variables and simultaneously collected metabolic variables during a second identical test conducted approximately nine days later. A faster adjustment to increasing workrates in the second test could explain this latter finding since supine cycle ergometry is not as familiar an activity as more frequently used modes of exercise testing. It is concluded that hemodynamic measurements using PDE exhibit the same acceptable variability throughout exercise as has been previously demonstrated at rest. In the second study, PDE was used to determine rapid serial measurements of cardiac output during a rapid loading supine cycle ergometer exercise test in twenty-two junior high school age boys. These measurements were compared to simultaneoulsy determined measurements of oxygen consumption. Cardiac output adjusted faster to each new workrate than oxygen consumption. Further analysis of these responses revealed that increases in heart rate (as opposed to stroke volume) were responsible for this rapid adjustment. It is concluded that there is an uncoupling and recoupling of these normally closely related hemodynamic and metabolic variables during the transitional periods between increasing levels of steady-state supine exercise. In the final study, gradual loading and rapid loading supine cycle ergometer protocols were compared in fifteen junior high school age boys. Maximal metabolic measurements and heart rate from each test were not different. Maximal PDE-derived measurements of cardiac output, cardiac index, stroke volume, and stroke index were slightly, but significantly, higher in the gradual loading protocol. It is concluded that a more complete adjustment of cardiac output (reflected by stroke volume) to maximal supine exercise occurs in a more gradual loading protocol than in a rapid loading protocol.