Arterial to end-tidal carbon dioxide tension difference in children under halothane anaesthesia

Using blood gas determinations and capnography, the relationship between arterial and end-tidal PCO2 was investigated in 20 children under halothane anaesthesia with spontaneous respiration. A median arterial to end-tidal carbon dioxide tension difference of 0.66 kPa (5 mm Hg) was found. There was a close correlation between PaCO2 and the magnitude of the carbon dioxide difference. Our findings may largely be explained by an increase in VD/VT (presumably mainly due to a reduction of VT) causing admixture of dead space air throughout expiration. It is concluded that though end-tidal carbon dioxide does not exactly reflect PaCO2 capnography may be of value as a monitor of respiration in paediatric anaesthesia at normal or near-normal values of end-tidal carbon dioxide.