Bed-side measurement of Vd/Vt has not been used clinically, because the measurement of Pico2 in using the Bohr equation is complicated and a time-consuming test. A new, simplified and time-saving, bed-side calculation of Vd/Vt by using the Radford nomogram has been developed. In order to verify the method of calculated Vd/Vt, we compared the two methods(measured versus calculated Vd/Vt) in 20 open heart patients. Two methods had direct correlationship(y=0.95X+0.04) with r=0.864, and so the authors considered that the calculating method in the measurement of Vd/Vt is advantageous in clinical use.