Relationships among single-breath tests of mechanics and ventilatory function and morphology in excised human lungs

To relate lung mechanical function and structure to features of the terminal portion of the single breath washout (SBW) curve, we have simultaneously measured transpulmonary pressure (P(TP), lung volune (V), flow and nitrogen concentration at the airway opening during SBW tests on postmortem human lungs suspended in air. Several expiratory flow patterns were used: slow, constant flow (0.2L/sec); faster, constant flow (0.4L/sec) and passive deflation through a resistor. Static PTP-V curves were also obtained. Lung structure was quantitated by the membranous airway diameter and alveolar surface-to-volume ratio in the upper, middle, and lower thirds of the lungs. Two points in the phase IV region of the SBW curve were identified: V₁, the volume at which an inflection point occurs and V₂, the volume at which a linear fit to the very end deviates from the remainder of the curve. V₁ occurs at the same volume independent of the expiratory flow pattern and is associated with a shape change in the inspiratory limb of the P(TP)-V curve. Thus, V₁ may indicate 'closure' of airway-alveolar units, but not absolute collapse since no inflection point occurs on the expiratory limb of the static P(TP)-V curve. V₂ and the expiratory P(TP)-V curve inflection point are both flow dependent and are associated with the volume of flow limitation. V₂ is not found in the passive deflation washout curves.