Comparison of resistance measured by the interrupter technique and by passive mechanics in sedated infants

Airways resistance measured by the interrupter technique (Rint) requires li ttle patient cooperation and has been successfully used in young children, but little studied in infants. The authors aimed to evaluate the measuremen t of Rint in infants, using a commercially available device (the MicroRint) , by comparing it with an established technique to measure respiratory resi stance: the single breath occlusion technique (SBT); and a measure of airfl ow obstruction during forced expiration. Infants < 18 months old with a history of wheeze, sedated with triclofos fo r pulmonary function testing, had measurements taken and compared to Rint ( using the MicroRint), respiratory system resistance (Rrs) by SBT, and to ma ximal flow at functional residual capacity (V' maxFRC). Paired data from 25 of 37 infants studied was obtained. There was a signifi cant difference between Rint (mean 2.94 +/-0.68) and Rrs (4.02 +/-0.87), bu t the two measures were strongly correlated (r=0.7). Rint was negatively co rrelated with V' maxFRC (r=-0.63). Smaller infants failed to trigger the Mi croRint. Interrupter resistance values in infants are significantly lower than value s of respiratory system resistance obtained by passive mechanics. However, there is a strong correlation between the two measurements, as well as betw een resistance measured using the interrupter technique and maximal flow at functional residual capacity, which indicates that resistance measured usi ng the interrupter technique may be a useful marker of airway obstruction i n infants. There remain a number of theoretical and technical problems whic h require further exploration.