Effect of electronic compensation on plethysmographic airway resistance measurements

OBJECTIVES: To compare the performance of a plethysmograph which incorporated electronic compensation (Jaeger) to one which incorporated a heated humidified breathing system (Hammersmith plethysmograph). WORKING HYPOTHESIS: The performance of a plethysmograph which incorporated electronic compensation would be impaired compared to that which incorporated a heated humidified system. STUDY DESIGN: In vitro and in vivo comparison. PATIENT SELECTION: Eleven children, median postnatal age 13 (range 5-15) months. METHODS: In vitro, the plethysmographs were assessed using known resistances (1.94, 4.85, and 6.80 kPa, equivalent to 20, 50, and 70 cm H(2)O/L/sec, respectively). In vivo, comparison was made of the results of children studied in both plethysmographs. RESULTS: In vitro, the resistance results of the two plethysmographs were similar to each other and to the known resistances. In vivo, the median "effective" airways resistance result of the Jaeger (4.15 kPa/L/sec) was significantly higher than the inspiratory resistance of the Hammersmith plethysmograph (3.0 kPa/L/sec), but the median inspiratory resistances of the Jaeger were significantly lower than those of the Hammersmith plethysmograph (2.8 kPa/L/sec vs. 3.0 kPa/L/sec). The mean within patient coefficient of variability for inspiratory resistance of the Jaeger plethysmograph (16.7%) was significantly higher than that of the Hammersmith plethysmograph (11.6%) (P = 0.014). CONCLUSION: These results suggest plethysmographs which incorporate electronic compensation may be inappropriate for use in infants and very young children.