Effects of impulse noise stimulation on electrocorticogram and heart rate.

It is now recognized that high-level impulse noises penetrate the uterus of pregnant sheep, elevate thresholds of fetal auditory-evoked potential and produce damage to fetal inner ear hair cells. However, little is known about functional effects of airborne impulse noise on the late-term fetus. In the present study, the effects of a series of 20 impulses on the behavioral state of 6 fetal sheep were tested. Noise impulses produced 169.3 peak sound pressure level (pSPL) in air. Peak levels recorded near the fetal head averaged 161.1 dB. Impulses delivered to the fetus during periods of NREM sleep resulted in a decrease in average fetal heart rate (FHR) from 185 +/- 22 beats/min (bpm) before stimulation to 174 +/- 23 bpm 2-5 s after stimulation (p < 0.05). During REM sleep, an FHR acceleration occurred (before stimulation: 177 +/- 24 bpm, after stimulation: 189 +/- 31 bpm; p < 0.05). Impulse exposure during NREM sleep resulted in reductions in delta-, theta- and alpha-band powers. As a consequence, total power decreased from 100 to 72 +/- 16% (p < 0.05). During REM sleep, stimulation provoked a short decrease in total band power from 100 to 73 +/- 20% and a similar decrease in the theta- and beta-band powers. The results indicated that impulse noise evoked short-term alterations in FHR and cortical activity. These changes were mediated by auditory brain stem activation that led to cortical desynchronization during both NREM and REM sleep in late-term fetal sheep.