Relationship between alveolar deadspace and arterial oxygenation in children with congenital cardiac disease

Fifty-eight children were studied during nitrous oxide in oxygen and fentanyl anaesthesia before undergoing closed or open cardiac surgery. FlO2 was 0.5. Alveolar deadspace was measured using the carbon dioxide single breath test (SBT-CO2) obtained from a computerized online system for monitoring expired CO2 and airway flow, based on the Servo ventilator, Arterial blood was sampled simultaneously for measurement of PaCO2 and PaO2. There was a marked reciprocal relationship between PaO2 and the alveolar deadspace fraction. In children with a normal pulmonary circulation and good oxygenation, alveolar deadspace fraction was approx. 0.05. Shunts which reduced PaO2 to 10 kPa produced a deadspace fraction of 0.15. When PaO2 was 3-4 kPa, alveolar deadspace fraction was approx. 0.4. In well-oxygenated children, alveolar deadspace fraction was only slightly greater than predicted by a model of the effects of pure right-to-left shunting. In severely cyanotic children, the discrepancy between predicted and observed VDalv/VTalv was greater. The mean arterial-end-tidal CO2 difference was zero in children in whom PaO2 was greater than 10 kPa, despite a measurable alveolar deadspace. In severely hypoxic children, the difference was 1-2 kPa. In a retrospective analysis of published data from anaesthetized adults without intracardiac shunting, no relationship was found between alveolar deadspace and PaO2.