HEART-RATE RESPONSE IN INDIVIDUAL HUMAN FETUSES TO STIMULATION WITH ALOW-INTENSITY SPEECH SOUND

Objective: The heart rate (HR) responses to high and low intensity sou nds reflect different central nervous system processes. The HR acceler ation evoked by high intensity stimuli is part of a defensive reaction to limit sensory intake, and the HR deceleration elicited by low inte nsity stimuli is a specific physiological correlate of cognitive proce ssing. The fetal defensive reaction to vibroacoustic stimulation has b een studied extensively, but little is known regarding the fetal respo nse to low intensity sounds, This study was undertaken to evaluate the HR response to a low intensity speech sound in 58 human fetuses at 36 -40 weeks gestation. Methods: The fetal cardiac electrical signal was sampled transabdominally at a resolution of 1 ms, and fetal R-waves we re extracted using adaptive signal processing. A 30-s speech sound was delivered at an intensity of 83 dB while fetuses were in quiet sleep. For each fetus, the HR response was analyzed in consecutive 10-s peri ods (P-1, P-2, P-3) using very conservative statistical criteria; HR c hanges during stimulation were then compared with spontaneous changes in HR in a 10-s control period. The HR response was examined in terms of maternal (abdominal wall thickness, placental location, and amnioti c fluid volume) and fetal (gestational age, prestimulus HR and HR vari ability, and prestimulus respiratory pattern) variables. Results: Comp ared with the control period, a HR deceleration occurred more often (3 8 (65.5%) versus 11 (19.0%), P < 0.001) and was of greater amplitude ( -5.8 +/- 3.2 bpm versus -2.6 +/- 1.0 bpm, P = 0.003) during stimulatio n period P-1. More fetuses exhibited a HR deceleration in P-1 compared with either P-2 (P = 0.011) or P-3 (P = 0.017), although the frequenc y of a HR deceleration in these latter two periods was still significa ntly greater than in the control period (P-2 versus control: P = 0.038 ; P-3 versus control: P = 0.022). These results were independent of ma ternal characteristics. However, fetuses with increased prestimulus HR variability exhibited a greater decrease in HR than fetuses with rela tively lower base-line variability (r = 0.591, P < 0.001>. Conclusions : Approximately two-thirds of low risk human fetuses exhibited a HR de celeration during the first 10 s following the onset of an 83-dB speec h sound. The frequency and magnitude of the HR deceleration were indep endent of maternal factors, raising the possibility that the HR decele ratory response may provide a physiological measure of fetal cognition .