Arterial to end-tidal carbon dioxide tension difference in children with congenital heart disease.

In children with congenital cyanotic heart disease, right-to-left intracardiac shunting causes an obligatory difference between arterial and end-tidal carbon dioxide tension (PaCO2-PE'CO2) as venous blood, rich in carbon dioxide, is added to the arterial circulation. This obligatory PaCO2-PE'CO2 difference, which can be predicted from knowledge of oxygen saturation, haemoglobin concentration and PaCO2, increases as oxygen saturation decreases, most markedly when the haemoglobin concentration is high. A second possible cause of the PaCO2-PE'CO2 difference is the effect of pulmonary hypoperfusion caused by the shunt. We studied 60 children undergoing cardiac surgery and compared the predicted the PaCO2-PE'CO2 difference with measured values to investigate the extent to which additional factors influence the clinically observed PaCO2-PE'CO2. In many children, observed values were much greater than predicted, which is compatible with some degree of pulmonary hypoperfusion. However, this was not felt to represent the complete picture in all patients. Another cause of ventilation-perfusion mismatch was suspected in those children who showed a considerable improvement in oxygen saturation during ventilation with an increased FIO2. We believe that pulmonary congestion caused by large left-to-right shunts may further increase the PaCO2-PE'CO2 difference.