System function between breathing rate and heart rate in fetal sheep

In order to assess the modulation of fetal heart rate (FHR) by breathing movements, the authors determined the system function between heart rate and breathing rate (FBR) in the fetal lamb. The FHR was obtained from the R-R' interval of the electrocardiogram (EKG) and the FBR from the burst-to-burst interval of diaphragmatic electromyogram (EMG). The R-R' interval of the EKG and the burst-to-burst interval of the EMG were determined using a template recognition method. The data were analyzed in both the frequency and time domain. Power spectral analysis reveals a frequency component in the FHR which corresponds well with the breathing rate. The strength of the modulation of the FHR by a breathing burst is indicated by the system function. In addition, the correlation function reveals other slower frequencies common to both the FHR and FBR. >