External Dead Space Explains Sex-Differences In The Exercise Ventilatory Response In Obese And Nonobese Children

PURPOSE: Increases in external dead space augment the exercise ventilatory response independent of the potential respiratory effects of sex and obesity. Therefore, the dead space imposed by the mouthpiece and breathing valve (VDM) should be accounted for when comparing the exercise ventilatory response, particularly in prepubescent children who have smaller lungs. We evaluated the impact of VDM on the exercise ventilatory response, defined as the slope of the relation between minute ventilation (V̇E) and carbon dioxide (V̇CO2), in obese and nonobese prepubescent boys and girls. METHODS: 27 nonobese (age: 10 ± 1 y; height: 146 ± 8 cm; weight: 39 ± 7 kg; BMI percentile: 57.2 ± 21.3) and 46 obese (age: 10 ± 1 y; height: 147 ± 8 cm; weight: 62 ± 16 kg; BMI percentile: 98.0 ± 1.3) children were studied. Subjects were divided into groups by sex (nonobese: 13 girls and 14 boys; obese: 17 girls and 29 boys). All subjects performed a 6-minute constant load cycling test at a fixed intensity (40W). To correct the V̇E/V̇CO2 slope for the effects of VDM, we subtracted VDM (0.225 L) from V̇E to derive a slope that was absent from the effects of VDM. A two-way group (obese vs nonobese) by sex (girls vs boys) analysis of variance was conducted. RESULTS: When V̇E was not corrected for VDM, there was no group by sex interaction in the V̇E/V̇CO2 slope (p = 0.76). V̇E/V̇CO2 slope was not different (p = 0.48) between obese (32.7 ± 4.3) and nonobese children (32.2 ± 6.1) however, there was a main effect for sex (p = 0.03) where the V̇E/V̇CO2 slope was higher in girls (35.4 ± 5.6) compared with boys (32.6 ± 4.9). When V̇E was corrected for VDM, the V̇E/V̇CO2 slope remained similar (p = 0.31) between obese and nonobese children; however, the main effect for sex was eliminated (p = 0.12). There was no main effect for group (obese vs nonobese) or sex (girls vs boys) in the end-tidal partial pressure of carbon dioxide at rest (p > 0.05) or during exercise (p > 0.05). CONCLUSIONS: Accounting for the external dead space imposed by the valve and mouthpiece eliminates the difference in the exercise ventilatory response detected between pre-pubescent boys and girls. Considering that the breathing apparatus comprises a large portion of both resting and exercise tidal volume in children, it should be accounted for before evaluating the V̇E/V̇CO2 slope, especially in prepubescent children.