Effect of lung volume on ventilation distribution.

To examine the effect of preinspiratory lung volume (PILV) on ventilation distribution, we performed multiple-breath N2 washouts (MBNW) in seven normal subjects breathing 1-liter tidal volumes over a wide range of PILV above closing capacity. We measured the following two independent indexes of ventilation distribution from the MBNW: 1) the normalized phase III slope of the final breaths of the washout (Snf) and 2) the alveolar mixing efficiency during that portion of the washout where 80-90% of the lung N2 had been cleared. Three of the subjects also performed single-breath N2 washouts (SBNW) by inspiring 1-liter breaths and expiring to residual volume at PILV = functional residual capacity (FRC), FRC + 1.0, and FRC - 0.5, respectively. From the SBNW we measured the phase III slope over the expired volume ranges of 0.75-1.0, 1.0-1.6, and 1.6-2.2 liters (S0.75, S1.0, and S1.6, respectively). Between a PILV of 0.92 +/- 0.09 (SE) liter above FRC and a PILV of 1.17 +/- 0.43 liter below FRC, Snf decreased by 61% (P less than 0.001) and alveolar mixing efficiency increased from 80 to 85% (P = 0.05). In addition, Snf and alveolar mixing efficiency were negatively correlated (r = 0.74). In contrast, over a similar volume range, S1.0 and S1.6 were greater at lower PILV. We conclude that, during tidal breathing in normal subjects, ventilation distribution becomes progressively more inhomogeneous at higher lung volumes over a range of volumes above closing capacity.(ABSTRACT TRUNCATED AT 250 WORDS)