Hence (a-ET)PCO 2 is an indirect estimate of V/Q mismatching of the lung. In healthy children, the (a-ET)PCO 2 gradient is smaller (-0.65-3 mm Hg) than in adults.9-14 This is due to a better V/Q matching, and hence a lower alveolar dead space in children than in the adults.9 The (a-ET)PCO 2 / PaCO 2 fraction is a measure of alveolar dead space, and changes in alveolar dead space correlate well with changes in (a-ET)PCO 2.4 An increase in (a-ET)PCO 2 suggests an increase in dead space ventilation. Under normal circumstances, the PETCO 2 (the CO 2 recorded at the end of the breath which represents PCO 2 from alveoli which empty last) is lower than PaCO 2 (average of all alveoli) by 2-5 mmHg, in adults.1-8 The (a-ET)PCO 2 gradient is due to the V/Q mismatch in the lungs (alveolar dead space) as a result of temporal, spatial, and alveolar mixing defects. (a-ET)CO 2 gradient as an index of alveolar dead space:
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