Flow and volume dependence of pulmonary mechanics in anesthetized cats

The effects of inspiratory flow rate and inflation volume on pulmonary mechanics were investigated in six anesthetized-paralyzed cats ventilated by constant-flow inflation. Pulmonary mechanics were assessed using the technique of rapid airway occlusion during constant-flow inflation which allows measurement of the intrinsic pulmonary resistance (RLmin) and of the overall "pulmonary flow resistance" (RLmax), which includes the additional pulmonary pressure losses due to time constant inequalities within the lung and/or stress adaptation. We observed that, at fixed inflation volume, 1) RLmin fitted Rohrer's equation, 2) RLmax was higher at low than intermediate flows, and 3) RLmax-RLmin decreased progressively with increasing flow. At fixed flow, RLmax increased, whereas RLmin decreased with increasing volume. We conclude that during eupneic breathing in cats, the pulmonary flow resistance as conventionally measured includes a significant component reflecting stress adaptation.