Measurement of airways closing pressure in man.

In 7 healthy males, lung static and quasi-static pressure-volume (QS-PV) curves were measured simultaneously with closing volume by the single breath N2 washout test. Comparison was made of four methods for estimation of the closing pressure (CP) at the onset of phase IV. In the first method the transpulmonary pressure (PL) that directly coincided on the tracing with onset of phase IV was measured, results varied widely with a mean of -1.3 +/- 2.2 cm H2O (X +/- SD). The second method was to identify on the QS-PV curve the point of transition in the shape of the curve from concavity to convexity. This method yielded a mean CP of 2.8 +/- 1.7 cm. H2O, bu did not correlate statistically with closing capacity (r = 0.07, p greater than 0.05). The third method was to treat the whole PV volume corresponding to the onset of phase IV, and calculate PL at this point. This method yielded CP of 2.1 +/- 0.76 cm H2O. When the same method was applied to static PV curve, CP was 2.0 +/- 0.7 cm H2O. Conformity of CP values obtained using extrapolation methods to those expected from theoretical considerations and from in vitro measurements suggests that the extrapolation methods most likely yield the true PL at the onset of airway closure.