Frequency dependence of regional lung clearance of ¹³³Xe in normal men

We measured the one-half time of ¹³³Xe washout from the whole lung and from 9 horizontal lung regions in 6 normal men during normal breathing and at 50 breaths/min. The diaphragm was located with a contour map prior to the selection of the 9 horizontal regions. Artifacts caused by the tissue solubility of ¹³³Xe were reduced by expressing regional clearance relative to whole lung clearance. During normal breathing there was a progressive increase in the relative clearance of ¹³³Xe from the lung apex to the base when each region was compared to the lung as a whole. At 50 breaths/min, relative clearance increased from the lung apex only for about 18.5 to 20 cm. At that point, the relative clearance rate of the horizontal regions began to fall and at the lung base was slower than that of the whole lung. These results are consistent with the concept that dependent lung regions have longer time constants than regions at the top of the lung.