Influence of posture on flow dependence of distribution of inhaled 133Xe boli

The second 100 ml of gas inspired at constant flow rates (V) from 0.1 to 2.5 l/s was tagged with 133Xe to determine the apicobasal bolus distribution ratio (VAlvA/VAlvB) for seven normal subjects in both the upright and supine postures. Gas was preferentially distributed to the base at low V. As V increased, redistribution to the apex occurred but was greater in the supine posture. As apicobasal time constant differences should be minimal in the supine posture, the greater flow dependence of bolus distribution suggests that the pressure swing was greater over apical zones than basal zones. Model studies indicate that the pressure differences (deltaPd) necessary to predict the measured VAlvA/VAlvB at all flow rates are less than 1.0 cmH2O. Such sensitivity of gas distribution to deltaPd at both low and high flow rates makes it appear relatively insensitive to regional compliance and resistance.