Sloping alveolar plateaus of He and SF 6 measured in excised cat lungs ventilated at constant volume by pressure changes

Abstract

Single-breath washout experiments with He and SF 6 were performed in excised cat lungs placed in a closed, liquid-filled reservoir, where lung volume was clamped by the surrounding liquid and breathing was accomplished by hyperbaric pressure changes ( pressure breathing) produced by a piston pump. Under these conditions the flow into each lung unit was assumed to be proportional to its volume, and sequential filling and emptying of lung units by convection probably did not occur. Thus, implicitly, gravity-dependent patterns of sequential filling and emptying of lung regions were also excluded. Different lung volumes ( V l = 50%, 75%, 100% TLC , where TLC is total lung capacity), tidal volumes ( V t = 21%, 34%, 47% TLC and durations of post-inspiratory apnea ( t a = 0, 1, 2, 4, 8 sec ) were applied. The expirograms showed that the slopes of the alveolar plateau (S) were significantly positive for both He and SF 6. For t a = 0 sec S He ranged from 8.7 to 62.8% and S SF6 ranged from 24.4 to 87.8% (S is expressed in % per unit V e/TLC, where V e is expired volume). The ratio S SF6/S He was larger than unity for each combination of V l and V t. Further, for t a = 0 sec both S He and S SF6 showed a tendency to decrease with increasing V l and with increasing V t. For t a = 8 sec both S He and S SF6 were close to zero. Additional single-breath washout experiments were performed with the same cat lungs by applying normal breathing where lung volume was not clamped and asynchronous unequal ventilation might have occured. For comparable values of V l and V t, there were no clear differences between the slopes obtained at normal breathing and those obtained at pressure breathing. We conclude that asynchronous unequal ventilation plays only a minor role in the sloping alveolar plateau during normal breathing, and that the mechanism underlying the sloping alveolar plateau is diffusion dependent.