| Abstrakt: |
Qs/Qt is usually determined by the "oxygen" method. The standard equation for calculation of percentage shunts is therefore: (see article) In the case for an FiO2 of I and when PaO2 is greater than 150 mm Hg, the equation can be expressed in terms of the difference in partial pressures of oxygen between the alveolus and the artery: (see article) The determination of Qs/Qt then necessitates, apart from calculation of PaO2 measurement of the O2 content of mixed venous blood, taken from the pulmonary artery. When an indwelling catheter for the sampling of mixed venous blood is not available, samples of superior vena cava blood can be used instead. The error thereby introduced into the calculation of Qs/Qt is debatable. Strictly, only the sampling of mixed venous blood permits precise determination of Qs/Qt. As long as the variations in Qs/Qt, more than its real value at a give time, are worth supervising, superior vena cava blood gives a rather satisfactory approximation. Various graphs relating Qs/Qt to PaO2 or to the alveolo-arterial difference have been proposed and are discussed. The other methods of determining Qs/Qt are also looked at. The value of determination of the shunt during non-hemodynamic edema, and especially in the patient under artificial ventilation with P.E.E.P., is emphasised. |
