Assessment of respiratory system viscoelasticity in spontaneously breathing rabbits

Airflow, volume (V), inspiratory time (TI), tracheal pressure (Ptr), abdomen (Dab) and rib cage diameters (Drc), peak diaphragm (Adi) and parasternal muscle activity (Aic) were measured in thirteen anaesthetized and vagotomized rabbits and in six vagotomized rabbits with cordotomy at T1 during unimpeded inspirations followed by rapid end-inspiratory airway occlusion, relaxation against closed airways, and inspiratory effort. To modify the inspiratory flow pattern, such sequences were performed at different volume, levels of chemical drive, and body temperatures (BT). Under all conditions, Adi, Aic, TI, Drc and Dab at iso-volume were the same for unimpeded and occluded inspirations; end-inspiratory Ptr was lower for occluded than for unimpeded inspirations, the difference (Pdiff) being larger the lower the volume at which occlusions were performed and the higher the chemical drive and BT. After paralysis, the viscoelastic constants of the respiratory system, modelled as a Kelvin body, were assessed according to the rapid airway occlusion method and used together with the inspiratory flow waveform to predict the end-inspiratory viscoelastic pressure (Pvisc) of unimpeded inspirations. Since the slope of the Pdiff vs. Pvisc relationship never differed from unity, Pdiff under the specified conditions should represent the effective Pvisc of unimpeded inspirations.