Fractal properties in fetal breathing dynamics.

The dynamic pattern of fetal breathing was studied in 17 fetal lambs with chronically implanted electromyographic electrodes in the diaphragm. The instantaneous breathing rate time series appeared similar on different time scales, with clusters of faster breathing rates interspersed with periods of relative quiescience, suggesting self-similarity. Distribution histograms of the interbreath intervals (IBIs) showed log-normal distribution for IBIs less than 1 s and inverse power-law distribution for IBIs greater than 1 s. The ratio of log-normal distribution to power-law distribution varied from approximately 2 at 102 days to approximately 30 by 130 days of gestation. Fast Fourier transform of the breathing rate time series revealed 1/f beta power spectra for all animals, with beta increasing linearly from 0.43 to 0.88 between 102 and 139 days. Studies in the newborn lamb showed further maturation in both the distribution characteristics of the IBIs, as well as in the 1/f power spectra, with beta approaching 1.0 at 2 days after birth. The inverse power-law relationship in the distribution of the IBIs, together with the 1/f beta power spectra, indicate scale invariance and suggest that fractal mechanisms are involved in the regulation of fetal breathing.