Anatomical dead space was measured in 72 normal children aged from 5 to 16 years, using the single breath method. There was a linear increase in this measurement with height, weight, and end-inspiratory lung volume. Physiological dead space was measured in 52 normal children using the Bohr equation and substituting a rebreathing PCO2 for alveolar PCO2. There was a parallel increase in this measurement with height, weight, and end-inspiratory lung volume. The difference between the two dead space measurements constitutes the alveolar dead space and was constant over the whole age range at 45 +/- 22 ml. The ratio of physiological dead space to tidal volume was 33-6 +/-4-6% and was unaltered by age or change in lung volume. The effect of airways obstruction on the dead space volumes was studied in 36 children with asthma and 28 with cystic fibrosis. Physiological dead space increased with increasing airways obstruction. Anatomical dead space remained constant in spite of marked increases in lung volume associated with the airways obstruction.
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