Assessment of induced bronchoconstriction in anesthetized cats by the end-inflation occlusion method

Airway occlusion during constant flow inflation allows rapid determination of frequency-dependence of pulmonary resistance by estimating its extreme values: R_L,max (zero frequency) and R_L,min (high frequency). R_L,max represents the maximum resistance value that can be obtained with the prevailing time constant inequalities and stress relaxation, while R_L,min represents the resistance that would be obtained in the absence of time constant inequalities and stress relaxation. In 5 anesthetized, tracheostomized, paralyzed, and artificially ventilated cats, R_L,min, R_L,max, and static pulmonary elastance (E_L,st) have been measured following airway occlusion at the end of constant flow tidal inflations. Measurements were made before and during continuous infusion of increasing doses of serotonin (10–100 µg/kg/min IV). The development of intrinsic positive end-expiratory pressure (PEEPi) was also assessed. Cats varied greatly in their responsiveness to serotonin, but R_L,min, R_L,max, and E_L,st increased and PEEPi developed in all cats. Increases in R_L,max did not always parallel increases in R_L,min but were similar to those in E_L,st, suggesting that altered viscoelastic properties of the lung contributed to the increases in R_L,max. We conclude that time-constant inequalities, changes in the lung periphery, and hyperinflation probably all contribute to the observed increases in R_L,max and will influence conventional methods of measuring R_L. Measuring R_L,min potentially provides a better method for assessing the reduction in caliber of the conducting airways in isolation.