COMPARISON OF SINGLE-BREATH AND PLETHYSMOGRAPHIC MEASUREMENTS OF RESISTANCE IN INFANCY

Single-breath technique (SET) measurements of total respiratory resist ance (Rrs) were compared with plethysmographic measurements of airway resistance (Raw) in healthy infants less than or equal to 13 wk of age (Group 1; n = 49) and > 13 wk of age (Group 2; n = 37) and in infants > 13 wk of age with prior wheeze (Group 3; n = 49). A significantly h igher percentage of Rrs (19%) than of Raw (2%) measurements were techn ically unsatisfactory, alinearity of the flow-volume curve accounting for 54% of Rrs failures. Although both Rrs and Raw were significantly higher in Group 3 infants, between-subject variability was wide in all groups. Rrs was significantly higher than initial expiratory (IE) Raw in all groups. Mean difference Rrs - IE Raw (95% CI) values were 1.98 (1.51, 2.48), 1.29 (0.96, 1.62), and 1.97 (1.56, 2.38) kPa . L(-1). s for Groups 1, 2, and 3, respectively. Significant but smaller differe nces were seen for end-expiratory (EE) Raw in Groups 1 and 2 but not i n Group 3. Mean difference Rrs - EE Raw (95% CI) values were 0.68 (0.1 1, 1.26), 0.55 (0.19, 0.92), and 0.31 (-0.06, 0.69) kPa . L(-1). s for Groups 1, 2, and 3, respectively. Despite wide between-subject variab ility in Rrs and a relatively high failure rate, the SET is simple to use, and it may be applicable to epidemiologic studies. However, clini cal applications in individual infants may be limited by failure to de tect the dynamic changes in resistance throughout the breath evident f rom plethysmographic studies.