HIGH-FREQUENCY RESPIRATORY INPUT IMPEDANCE MEASUREMENTS IN INFANTS ASSESSED BY THE HIGH-SPEED INTERRUPTER TECHNIQUE

High-frequency input impedance (Z(f)) measurements, including antireso nances, provide useful noninvasive information on airway geometry and especially airway wall mechanics in the canine and human adult respira tory system, A knowledge of airway wall mechanics would be particularl y important in understanding how limitation phenomena in infants. High -frequency Z(f) has not been measured in infants above 256 Hz, because the high impedance of the infantile respiratory system would be expec ted to result in low amplitudes of oscillatory how at higher frequenci es. The aim of this study was to develop a technique to measure high-f requency: Z(f) in infants and to elucidate the nature of the antireson ance phenomena in the Z(f) spectrum in infants. Z(f) was measured from 32-900 Hz during rapid airflow interruption by the highspeed interrup ter technique (NIT) in Is infants (aged 24-149 weeks) with wheezing di sorders. The HIT enables the excitement of higher how amplitudes at hi gh frequencies using a pseudostep forcing function, In all infants Z(f ) showed a mean (su) first antiresonance (far,1) of 172 (35) Hz (real part of Z(f) at far,1 (Z(f)re(far,1)): 4.9 (1.1) kPa . L-1 . s) and in five infants a second antiresonance (far,2) of 564 (51) Hz (Z(f)re(fa r,2): 2.0 (0.7) kPa . L-1 . s). The antiresonances were found to be re lated to wave propagation in the airways (acoustic antiresonances), be cause they increased by a factor of similar to 2 when He-O-2 was inhal ed. This implies that far,1 and its harmonics are a function of ail-wa y wall compliance, In conclusion, the first and second antiresonances may be helpful in understanding flow limitation in wheezing disorders in infants, because how limitation is related not only to airway diame ter but also to airway wall compliance.