The Transmission Characteristics of Large and Small Pressure Waves in the Abdominal vena Cava

The mechanical behavior of the abdominal venae cavae of anesthetized dogs has been studied by measuring the speed, attenuation, and changes in waveform of various kinds of artificially induced pressure signals. The propagation of large amplitude pressure waves is shown to be affected by reflection interference and pronounced nonlinear phenomena. For pressure signals exceeding a few mmHg the speed increases with amplitude and the wave front steepens during propagation as in the early phases of the formation of a shock wave. By inducing distension waves in the form of finite trains of sine waves with amplitudes less than 20 mmH2O the dispersion and attenuation were determined without requiring Fourier transform computations. At transmural pressures between 50 and 300 mmH2O and frequencies between 20 and 100 Hz the vena cava was found to be only mildly dispersive and the logarithmic decrement appears to be independent of frequency. Irrespective of the amplitude and shape of the pressure signals, their speeds varied along the vena cava, and also with respiration. In addition, the speeds generally increased under the influence of the chemical and electrical stimuli applied, and with rising transmural pressure.