However, others have described stable values for VEmax per kilogram across the childhood years. In the study of Robinson,2 the 6- and 15-year-olds demonstrated a mean value of 1.59 and 1.60 L/kg/ min, respectively. Morse et al could find no relationship of maximal Ve per kilogram to age in boys whose ages ranged from 10 to 17 years. The longitudinal data from Rutenfranz et al describe a linear relation of VEmax to stature in female subjects until a height of 160 cm was reached. Above this height, values decreased. In male subjects, the relationship between height and maximal ventilation remained linear throughout the study. As with submaximal exercise, Rutenfranz et al were unable to demonstrate a fall in Ve/Vo2 at maximal effort as their subjects aged. This is a contradiction to findings in the present study as well as those of Astrand and Andersen et al which indicate a decline in maximal Ve/Vo2 with increasing age in children. canadian neighbor pharmacy
The explanation for these patterns—an increasing reliance on Vt for maintenance of exercise ventilation and the improvements in ventilatory “efficiency” (decline in Ve/Vo2)—in the growing child is unclear. There is some evidence that younger children possess a greater central ventilatory neural drive and a lower carbon dioxide set point, resulting in greater ventilatory rates for a given metabolic demand. Others have considered such age-dependent patterns to reflect variations in ventilatory mechanics. Work of ventilation, for instance, is strongly influenced by the balance of lung compliance and airway resistance. Since these factors may not develop in parallel during childhood, a change in their relationship might alter the ratio of Vt and respiratory rate that would provide the most “economical” means of accomplishing Ve during exercise.
Level of sexual development was not assessed in this study. As indicated by the brief parent-completed questionnaire, many subjects were certainly in the early stages of puberty before the completion of the study. However, no growth spurt was observed in either height or weight. Also, with the exception of a late acceleration of Vt in the boys, no clear-cut alterations in patterns of changes in ventilation, Vt, fn and ventilatory equivalent were observed in the latter years of the study. This suggests that as far as the children had progressed in sexual development, puberty had no significant impact on ventilatory responses to exercise.
As ventilation plays a key role in the oxygen delivery chain, it is of interest to compare the patterns observed during growth in this longitudinal study with those of Vo2max and cardiac output. At submaximal treadmill exercise, the energy demands relative to body size progressively decrease with age in children. This decline in Vo2 per kilogram mimics that of Ve per kilogram, although, as noted above, Ve/Vo2 during submaximal exercise declines with age during childhood. At maximal exercise, Vo2 per kilogram remains relatively stable during childhood in boys but gradually declines in girls. This pattern was duplicated in the current study.
Cardiac responses to exercise, particularly at high intensities, are often difficult to study. It is of interest to compare such changes with those of ventilation, however, since both are characterized by a minute volume (cardiac output, Ve) that is determined by a size-dependent volume (stroke volume, Vt) and a size-independent rate (heart rate, fR). At a given submaximal exercise, the responses are similar: as the child ages, the volume variable increases in parallel to body size, while the rate variable decreases. As a consequence, the minute volume variable increases in absolute terms but declines relative to body mass. At maximal exercise, however, ventilatory and cardiac patterns may differ, since maximal heart rate remains constant across ages during childhood, while the maximal fR decreases.
In summary, this 5-year longitudinal descriptive study provides a picture of ventilatory responses to exercise in children that generally supports that created by previous cross-sectional studies. Vt is closely linked to body size during growth, while fR gradually declines. This results in a fall in Ve per kilogram at submaximal exercise. Such a decline has also been described by others at maximal exercise, but in the present study, the decrease in Ve per kilogram at maximal exercise was not statistically significant. Children demonstrate a greater ventilatory efficiency during exercise as they grow, with a progressive decline in submaximal and—at least in boys—maximal Ve/Vo2.