USE OF INTRINSIC MODES IN BIOLOGY - EXAMPLES OF INDICIAL RESPONSE OF PULMONARY BLOOD-PRESSURE TO + - STEP HYPOXIA/

Recently, a new method to analyze biological nonstationary stochastic variables has been presented. The method is especially suitable to ana lyze the variation of one biological variable with respect to changes of another variable. Here, it is illustrated by the change of the pulm onary blood pressure in response to a step change of oxygen concentrat ion in the gas that an animal breathes, The pressure signal is resolve d into the sum of a set of oscillatory intrinsic mode functions, which have zero ''local mean,'' and a final nonoscillatory mode. With this device, we obtain a set of ''mean trends,'' each of which represents a ''mean'' in a definitive sense, and together they represent the mean trend systematically with different degrees of oscillatory content. Co rrespondingly, the oscillatory content of the signal about any mean tr end can be represented by a set of partial sums of intrinsic mode func tions, When the concept of ''indicial response function'' is used to d escribe the change of one variable in response to a step change of ano ther variable, we now have a set of indicial response functions of the mean trends and another set of indicial response functions to describ e the energy or intensity of oscillations about each mean trend. Each of these can be represented by an analytic function whose coefficients can be determined by a least-squares curve-fitting procedure. In this way, experimental results are stated sharply by analytic functions.