Computer analysis of heart rate variation and breathing movements in fetal lambs.

A quantitative method for studying the frequency-specific relationships between heart rate (HR) and fetal breathing movements (FBM) was developed. The reactivity of periodic HR variation in relation to FBM was investigated by means of power spectral analysis. Seven fetal lambs were studied during the third trimester of gestation using a chronic animal model. HR variability increased at the rate of FBM, as shown by an increase of spectral density at > 0.35 Hz in the HR autospectrum and in the cross-spectrum of HR and respirogram, as well as by an increase in the short-term variability index CVS. FBM were associated with the increased HR variation in all but the lowest frequency bands (0.07-1.0 Hz). Although respiratory sinus arrhythmia was found, only 10 per cent of the total HR variability and 25 per cent of the joint-density of HR and respirogram appeared at > 0.35 Hz during FBM. The greatest variation in both the HR and respirogram spectra appeared at < 0.07 Hz. Although the low-frequency variability of HR and respirogram was simultaneous, it was on the whole not synchronised. The existence of multiple control systems that simultaneously link the cardiac and respiratory control mechanisms to each other in the fetal lamb is postulated.