Systemic oxygen transport derived by using continuous measured oxygen consumption after the Norwood procedure-an interim review

The balance between systemic O-2 consumption (VO2) and O-2 delivery (DO2) is impaired in children after cardiopulmonary bypass surgery, with decreased DO2 and increased VO2. The major goal, and the major challenge, of postoperative management has been to match DO2 to VO2 in order to sustain cellular metabolism, particularly in neonates after the Norwood procedure. While much effort has been put into augmenting cardiac output and DO2, VO2 remains largely ignored. Respiratory mass spectrometry allows the precise and continuous measurement of VO2. Measured VO2, using the direct Fick principle, allows for the calculation of each element of systemic O-2 transport in the complex Norwood circulation. The actual measurements of O-2 transport have allowed us, in the past five years or so, to extensively investigate the Norwood physiology in terms of the VO2-DO2 relationship and the factors affecting it in clinical treatments. Therefore, the first objective of this article is to introduce the technique of respiratory mass spectrometry and its adaption to measure VO2 across paediatric ventilators with continuous flow. The second objective is to give an interim review of the main findings in our studies on systemic O-2 transport in 17 neonates in the first 72 h after the Norwood procedure. These findings include the profiles of systemic O-2 transport, the important contribution of VO2 to the impaired balance of O-2 transport and the complex effects of some routine clinical treatments on the VO2-DO2 relationship (including catecholamines, PaCO2, Mg2+ and hyperglycaemia, as well as patient-specific anatomical variations). The influence of systemic O-2 transport on cerebral oxygenation is also introduced. This information may help us to refine postoperative management in neonates after the Norwood procedure. Our initial studies mark the end of the beginning, but much is yet explored. Ultimately, the resultant improved systemic and regional O-2 transport in the early postoperative period may have an important impact on long-term outcomes, thereby improving the quality of life for these vulnerable children.