Clinical implications of variation in total venoarterial shunt fraction calculated by different methods during severe acute respiratory failure

Ventilation-perfusion imbalance is the major physiologic disturbance that produces hypoxemia in acute respiratory failure, and total venoarterial shunt fraction is frequently used as a measure of its severity. Ninety-one total venoarterial shunt fractions were calculated from 29 patients with severe acute respiratory failure. Four different methods were used for each estimation, only two of which considered the influence of cardiac output and tissue oxygen uptake. The differences among the results were statistically significant and rendered invalid those that were calculated independently of mixed venous oxygen values. Lack of uniformity of the methods that have been used for calculating shunts in respiratory failure makes it difficult to compare individual patients or groups of them from previous reports. Use of a standard method is desirable so that statistical evaluation of severity and response to treatment can be undertaken. Older data on which therapeutic decisions may be based can have misleading variability from those derived from currently accepted techniques and could appreciably influence patient care.