Journal of Applied Physiology, Vol 41, Issue 4 517-522, Copyright © 1976 by American Physiological Society
Terminal nitrogen rise
C. J. Martin, S. Das and A. C. Young
Eighteen-breath nitrogen washouts were performed on eight subjects. Each
washout could be simulated by a four-compartment model, each compartment
with a different ventilation-to-volume ratio and a variable contribution to
expiratory flow. In large breaths initiated near residual volume, a
terminal nitrogen rise (TNR) was seen. To account for the TNR with this
model, there were relatively small changes in flow from compartments with
markedly different nitrogen concentration. Reasons are given for believing
these compartments could not be the upper and lower lung. Three of these
subjects were studied in the supine, seated, and head-down positions. The
TNR was seen at the same lung volume in all positions. At routine
bronchospirometry in a second group of subjects, sampling with small
catheters during a nitrogen washout showed a TNR in the expirate of lungs,
lobes, segments, and subsegments in the upright and supine positions.
Apparently a large vertical hydrostatic gradient is unnecessary to produce
a TNR. Finally, the TNR was shown to occur at that lung volume where
transpulmonary pressure is very small and changing rapidly with volume.
This TNR was often followed by a terminal nitrogen fall while the lung was
continuing to empty. The TNR occurs when flow from a large poorly
ventilated compartment increases relative to the flow from other
compartments. A model of lung in which the poorly ventilated compartment
develops high specific compliance at low lung volume explains these data.
