Analysis of bronchomotor tone in anesthetized guinea pigs by impedance plethysmography: a simple method for the evaluation of bronchodilator action.

The transthoracic impedance method for measurement of bronchomotor tone was studied in anesthetized guinea pigs. Impedance change (delta Z) recorded from the 2nd to the 6th rib level increased linearly in accordance with the increment of inspirated volume (delta V); the maximum ratio of delta Z to delta V with the least cardiac artifact was measured at the level of the 6th rib. Histamine (5 micrograms/kg, i.v.) caused an increase in intratracheal pressure and a decrease in delta Z in guinea pigs. The fact that the delta Z peak lagged behind that of intratracheal pressure indicated that air flow movement was disturbed at the restricted bronchioles. In anesthetized dogs, the waveform of DC-impedance was similar to that of both flow volume and intraesophageal pressure. The waveform of AC-impedance, the signals of which are differentiated from DC-impedance, was similar to that of the flow rate. Following histamine administration (10 micrograms/kg, i.v.), the flow rate amplitude increased during inspiration and decreased during expiration. In passively immunized guinea pigs, antigen challenge induced a rapid increase in intratracheal pressure and a concomitant decrease in the amplitude of AC -impedance. Pretreatment with fenoterol (5-20 micrograms/kg, i.v.) inhibited these changes dose-dependently. It was concluded that delta Z measurement is a beneficial method for investigating respiratory dynamics in guinea pigs and is especially useful for evaluating the efficacy of bronchodilating action of test compounds.