Despite the promise of achieving effective low tidal volume ventilation, controlled trials of high frequency oscillatory ventilation (HFOV) in neonates have failed to consistently demonstrate a reduced incidence of chronic lung disease. We hypothesised this may be partly explained by the relationships that exist during HFOV between pressure, flow and the mechanical properties of both the endotracheal tube (ETT) and the lung. Methods: We employed three different (parallel compartment computer, in vitro and in vivo rabbit) models of the intubated neonatal respiratory system. Measurements of the mean and amplitude of the oscillatory pressure waveform measured at_the airway opening (Pao, APao) and within the alveolar compartment (PA, APA) were made over a range of inspiratory to expiratory ratio (I:E), frequency, ETT size and model lung compliance (C). The effect of heterogeneous disease on each of these parameters wasexamined with the mathematical model. Results: A reduction in I:E caused FA to fail below Pao. The magnitude of Pdiff (PA - PaO) increased with increasing frequency and ETT size and could be predicted from the difference between the mean squared inspiratory and expiratory velocities. There is greater transmission of A Pao to the alveolar compartment at lower lung compliance. Regional differences in APA are proportional to the degree of disease heterogeneity. Conclusions: These findings challenge popularly held beliefs regarding the level of protection from barotrauma that HFOV provides to the neonate with severe respiratory disease. High transmission of the oscillatory pressure waveform to the alveolar compartment in the poorly compliant lung may explain the disappointing impact of HFOV on the incidence of OLD. Current clinical recommendations to employ I:E ratios of at least 1:2 create the potential for undetected and undesirable fluctuations in lung volume.
|Issue number||SUPPL. 1|
|Publication status||Published - 1 Dec 1999|