Cardiac output can be measured from the rate of absorption of soluble inert gas across the lung using an electric model of human body suggested by W. Mapleson. Step response of the model becomes the sum of exponentials. Cardiac output is estimated from time constants and amplitude coefficients of the response. System for the measurement of the cardiac output which is based on this theory will be introduced below.

It is known that psychotropic compounds give change to evoked response of human. A system for the Auditory Brain Stem Response (ABR) was experimentally developed, and ABR of subjects in general anesthesia were measured for the purpose of assessing the depth of anesthesia.