Calibration of respiratory induction plethysmography (Respitrace®) in infants

To determine whether the recently increased sensitivity of the variable frequency oscillator and the use of separate rib cage and abdominal transducers made calibration of the Respitrace® system easier, we performed 106 different calibration procedures against a pneumotachygraph in 36 normal infants, 41 using 2 separate periods of quiet sleep, 49 using quiet and REM sleep, and 16 using 2 separate periods of REM sleep. When the calibration was done using 2 separate periods of quiet sleep, or using periods of quiet and REM sleep, a change of at least 50% in the amplitudes of both the abdominal and rib cage signals between the 2 sleep periods, gave accurate calibration factors in 92%, compared with only 30% when the amplitude of either signal changed by less than 50%. Calculation of the calibration factors can be done either by the least squares method or by solving simultaneous equations with no significant difference between the results.