COMPARISON OF AIRWAY-RESISTANCE AND TOTAL RESPIRATORY SYSTEM RESISTANCE IN INFANTS

Airway resistance (Raw) can be measured throughout the respiratory cyc le by whole body plethysmography. Total resistance of the respiratory system (Rrs) can be measured from the relaxed expiration that follows end inspiratory occlusion. The purpose of this study was to compare th e two methods in normal infants and in infants with airway obstruction of different types and severity. Fifteen infants with essentially nor mal lungs aged 24.6 +/- 18.0 (SD) wk, nine infants with congenital str idor aged 36.0 +/- 17.3 wk, and eleven wheezy infants aged 20.1 +/- 11 .3 wk had simultaneous measurements of Raw and Rrs. Rrs was similar to Raw both during inspiration and expiration in the normal infants, to all expiratory Raw in those with congenital stridor, and to all inspir atory and early expiratory Raw in the wheezy-infants. Raw was markedly and significantly higher than Rrs during mid and late inspiration in infants with congenital stridor and during late expiration in the whee zy infants. We conclude that Rrs is a good estimate of Raw in normal i nfants and of early expiratory Raw in all infants. In infants with air way obstruction, Rrs does not reveal the dynamic changes in Raw during tidal breathing, nor can it differentiate between infants with upper and lower airway obstruction.