A nomogram to obtain pulmonary shuntflow (Qs/Qt)

A nomogram to simplify the determination of pulmonary shunt is presented. It is based on Leigh's et al. shunt-equation (1969). The nomogram aggregates the factors which contribute most to the pulmonary shunt in acute respiratory failure. Arterial and venous oxygen saturation, Hb content, arterial oxygen tension and barometric pressure do influence strongly the result of calculated shuntflow. For the factors with little importance in the calculation (vapor pressure, mixed venous oxygen tension, alveolar-CO2 tension), international mean values are used. The shunt determined with the nomogram deviates less than 1% from the calculated values, if the range of PCO2 is between 32 and 52 mm Hg and the error does not exceed 1,5% with a PCO2 up to 82 mm Hg unless it is combined with a very low Hb and a high mixed venous saturation. By the nomogram the effect of the different parameters on the shunt can be very well illustrated. Especially mistakes in determination of the mixed venous saturation and of arterial O2 tension result in errors. Too high saturation values in mixed venous blood occur in distally located catheters due to pulmonary capillary admixtures. This contamination can be easily detected if the relationship between PVO2 and PACO2 is examined. The other source of error lies in the technical problems of PO2 determination in high ranges and is caused by escape of O2 into the PO2-electrode and O2-consumption of the blood. The end effect which the factors of O2 escape have on the accuracy of PO2 determination in high ranges is demonstrated with blood samples which have been equilibrated with different gas mixtures at different temperatures and had their PO2 measured at regular intervals. The error in PaO2 measurement can be reduced if blood-gas analyses are performed immediately or on cooled samples.