Understanding the meaning of the shunt fraction calculation.

The pulmonary shunt fraction (Qs/Qt) is frequently calculated in critically ill patients to monitor the effectiveness of pulmonary oxygenation. The breathing of pure oxygen often results in higher calculated Qs/Qt values that have been attributed to the development of atelectasis, ventilation-perfusion imbalance, or both. To interpret properly the changes in calculated Qs/Qt that occur when the inspired oxygen fraction is altered, the changes produced in all the variables affecting Qs/Qt must be known. This tutorial presents an in-depth analysis of the four variables affecting the calculation of Qs/Qt: VO2 (oxygen uptake), Qt (cardiac output), Cc'O2 (oxygen content in pulmonary end capillaries), and CvO2 (oxygen content in mixed venous blood). These variables are related according to the following equation, which is derived by combining the Fick and the classic shunt equations: Qs/Qt = 1 - [(VO2/Qt)/(Cc'O2 - CvO2)]. Three-dimensional surface representations relating these variables are also presented to help the reader understand the effects of these variables on the calculated Qs/Qt.