PHASE-ANALYSIS IN HIGH-FREQUENCY OSCILLATION

In an oscillating system driven by a sine wave pump, the resonance fre quency of the respiratory system can be determined using phase analysi s. At resonance frequency, when elastance and inertance cancel out, fl ow becomes in-phase with resistance. In premature infants with respira tory distress syndrome, owing to surfactant deficiency, localized area s of hyperinflation and collapse develop, resulting in complex changes in overall pulmonary mechanics. We investigated the effect of measuri ng resonance frequency of the respiratory system by phase analysis at differ ent points of the respiratory cycle: end of inspiration, end of expiration, mid-inspiration and mid-expiration. Ten ventilated premat ure infants with respiratory distress syndrome were studied, gestation al age ranged from 24 to 30 weeks (mean 27.6 weeks) and birth weight r anged from 0.7 to 1.505 kg (mean 0.984 kg). Results: The resonance fre quency was consistently higher when measured at the end of inspiration compared with the end of expiration. The expected trend of phase vari ation. that is, negative below the resonance frequency and positive ab ove, was most consistently found when analysis was done at the end of inspiration. Conclusions: These findings were most likely a result of the complexity of pulmonary mechanics in the surfactant-deficient lung s, rendering the single compartment model we based our theory on inade quate. However, phase analysis performed at the end of inspiration see med to produce the most reliable and consistent results. (C) 1998 IPEM . Published by Elsevier Science Ltd. All rights reserved.