HARMONIC DISTORTION FROM NONLINEAR-SYSTEMS WITH BROAD-BAND INPUTS - APPLICATIONS TO LUNG-MECHANICS

We present a simple index, extended harmonic distortion (k(d)), to rep resent the degree of system nonlinearity under sparse pseudorandom noi se inputs (SPRN). The frequencies in a SPRN waveform are neither harmo nics nor sums or differences of the other component frequencies. Expre ssed by percentage, the k(d) is the square root of the ratio of output power at non-input frequencies to the total output power. We evoke th ree simple corrections to recover the true k(d) under imperfect SPRN i nputs. Simulations on two block-structured nonlinear models (Wiener an d Hammerstein) demonstrate the necessity and effectiveness of these co rrections especially for the Wiener-type nonlinearity. By applying k(d ) to pressure-flow data of dog lungs, we found that the nonlinear harm onic interactions from a lung arise primarily from its tissues most li kely the processes governing the tissue stiffness. This nonlinearity, assessed from k(d), is stronger at higher tidal volumes and enhanced ( but to a lesser degree) during bronchoconstriction. We conclude that s ince the k(d) approach avoids the necessity for multiple-input measure ments and lengthy data records, it may be useful for tracking the dyna mic variations in nonlinearities of a physiological system.