Continuous measurement of respiratory resistance in asthmatic children

In 13 children (7-13 years old) with bronchial asthma in remission, respiratory system resistance was continuously measured by the forced oscillation method at 10 Hz, using on-line digital computer analysis. Corrections were made for mouth impedance which was determined from a prior Valsalva maneuver. Functional residual capacity was also periodically determined by the body plethysmographic technique in order to allow calculation of specific respiratory system conductance (SGRS). The total observation period of 45 min consisted of quiet breathing interrupted by vital capacity maneuvers. In a single-blind design, eight children were given 0.15 mg isoproterenol sulfate from a proprietary spray delivered in the beginning of an inspiratory vital capacity maneuver while five received a placebo (freon propellent alone). A significant increase in SGrs (+53%) was observed 75 sec after isoproterenol sulfate administration. SGrs peaked after 2 min (+ 61%) and remained elevated significantly for 10 min. No significant changes occurred in the placebo group. The immediate decrease in SGrs which was observed following control vital capacity maneuvers, was prevented by isoproterenol sulfate but not by the freon propellent. Our findings suggest that although peak bronchodilation does not occur before 2 min following isoproterenol sulfate inhalation, an effect, namely prevention of bronchoconstriction induced by a deep breath, is already detectable within seconds.