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

In children with congenital cyanotic heart disease, right-to-left intracard iac shunting causes an obligatory difference between arterial and end-tidal carbon dioxide tension (Pa-CO2-PE'(CO2)) as venous blood, rich in carbon d ioxide, is added to the arterial circulation. This obligatory Pa-CO2-PE'(CO 2) difference, which can be predicted from knowledge of oxygen saturation, haemoglobin concentration and Pa-CO2, increases as oxygen saturation decrea ses. most markedly when the haemoglobin concentration is high. A second pos sible cause of the Pa-CO2-PE'(CO2) difference is the effect of pulmonary hy poperfusion caused by the shunt. We studied 60 children undergoing cardiac surgery and compared the predicted the PaCO2PE'(CO2) difference with measur ed values to investigate the extent to which additional factors influence t he clinically observed Pa-CO2-PECO2. In many children, observed values were much greater than predicted, which is compatible with some degree of pulmo nary hypoperfusion. However, this was not felt to represent the complete pi cture 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 pulm onary congestion caused by large left-to-right shunts may further increase the Pa-CO2-PECO2 difference.