If the above procedure is followed, the rate of in- crease of CO2 tension (dp/dt) in the last three min- utes of the test is usually between 3 and 5 mm Hg/min. In the first minute, dp/dt changes from zero (the plateau) to this wnstant rate, and the criteria for validation are not satisfied. Strachova and Plum measured instantaneous ventilation and the corresponding CO2 tension during this period and incorporated the data in their calculation of Sc02. Since ventilation is usually increased only slightly in the first minute, it is not surprising that their mean and range is lower than the means and ranges pre- viously reported,!, 7, 13 For the reasons outlined; we believe that their values for Sc02 are not as precise an index of the response of the medullary chemore- ceptors to CO2- The source of variability of Sco2 in repeated tests on the same subject remains an unresolved issue. We believe that standardization of the technique is re- quired, both in the conduct …