Longitudinal dispersion of gases measured in a model of the bronchial airways.

We investigated the longitudinal dispersion of helium (He), methane, acetylene, butane, sulfur hexafluoride, and octafluorocyclobutane (C4F8) in a clear plastic model of the human bronchial airways. The dimensions of the seven airway generations (0-6) of the model were chosen to be equal to those given by Weibel's model A (Morphometry of the Human Lung; New York: Academic, 1963). Total volume of the model amounts to 52 ml. A small bolus of each of the tracer gases was injected within 1 ms into a constant airflow at the inlet of the model, and the bolus dispersion curve was measured at the outlet by means of a mass spectrometer. Both inspirations and expirations were simulated; the four selected values of the flow rate (V) for each simulation were 0.25, 0.5, 1, and 2 1/s, respectively. The measured bolus dispersion curves were corrected for the response characteristics of the mass spectrometer. In this way, the basal shape of the dispersion curve could be shown to be Gaussian. No significant changes in the standard deviation (sigma v) (Ultman et al., J. Appl. Physiol. 44: 297-303, 1978) of each tracer gas are found for the V values applied. sigma v increases significantly (approximately 50%), however, between the lightest (He) and the heaviest (C4F8) tracer gas used. The mean sigma v values of the 24 results (4 V values and 6 tracer gases) obtained for inspirations and expirations are 15.3 and 13.6 ml, respectively.