USE OF THE FREQUENCY-TRACKING LOCUS IN ESTIMATING THE DEGREE OF RESPIRATORY ENTRAINMENT IN PRETERM INFANTS

In order to define the complex interactions between external stimuli a nd non-linear physiological systems, a technique (the frequency-tracki ng locus, FTL) was devised that describes the cycle-by-cycle changes i n phase angle and amplitude between two signals. Qualitative assessmen t of the nature of interactions between the signals can be made by exa mining the FTL. Quantitation of the extent of entrainment of the spont aneous physiological rhythm is possible after deriving a numerical ind ex (the path-length index, PLI) describing the departure of the system from a fully entrained state. The FTL was applied to the study of int eractions between spontaneous respiratory effort and mechanical inflat ion in preterm newborn babies undergoing mechanical ventilation. Stabl e and unstable states of 1:1 interaction were noted while integer-rati o relationships were seen at low rates of mechanical ventilation. Stab le states of entrainment corresponded to a PLI value near unity, and t he value of PLI increased rapidly as interactions became unstable. The FTL may be used to describe complex interactions in physiological sys tems, and may be used as a guide to baby-ventilator matching during me chanical ventilation of the newborn.