ACOUSTIC-IMPEDANCE OF THE MATERNAL ABDOMEN

Contact sensors to monitor fetal heart, breathing, and movement sounds with increased sensitivity and bandwidth are under development. To un derstand the inherent acoustical properties of the maternal abdomen an d its interaction with these sensors, the driving-point impedance Z(jo mega)) was measured in nine women during their last trimester of pregn ancy. An electromechanical shaker with a contact area of 2.85 cm2 prod uced abdominal vibrations between 10 and 500 Hz, and the resulting for ce and acceleration were measured. After digitally integrating the acc eleration signal to obtain the velocity and removing the massive effec ts of the coupling between the shaker and the abdomen, Z(jomega) was e stimated using spectral techniques. The imaginary part of Z(jomega)) d epicted a dominant compliance effect at low frequencies, a resonance a t a frequency of 28 +/- 16 Hz (mean +/- s.d.), and a dominant mass eff ect at higher frequencies. The real part of Z(jomega) increased steadi ly with frequency. A series resistance-mass-compliance (R-M-C) circuit modeled these characteristics of Z(jomega) well when R was allowed to exactly mimic the frequency dependence of the real part of Z(jomega). Estimated element values of 6.6 +/- 2.2 X 10(-3) kg for M, 2.1 +/- 1. 4 X 10(-3) m/N for C, and roughly 10 Ns/m for R (at resonance) were si milar to those estimated for other body tissues such as the thigh but quite different from that of the often-studied chest wall.