Presented at the Neonatal Society 2002 Spring Meeting.

Pillow JJ, Hantos Z, Sly PD (introduced by Professor J. Stocks)

Centre for Child Health Research, PO Box 855, Perth, Australia 6872 and Department of Medical Informatics, University of Szeged, Hungary. 

The low-frequency forced oscillation technique (LF-FOT) was recently used to show that low compliance (high elastance) is associated with increased intrapulmonary pressure transmission during high-frequency oscillatory ventilation (HFOV). We hypothesised that 1) optimal lung mechanics measured using the LF-FOT will coincide with the point of maximal curvature on the deflation limb of the pressure volume curve and 2) tracheal pressure (Ptr) transmission during HFOV is minimised at optimal lung mechanics.

Methods: Five sedated tracheostomised preterm lambs were commenced on high-frequency oscillatory ventilation (HFOV) immediately following birth using initial mean airway pressure (MAwP) of 1.2 kPa, amplitude 4.5 kPa, percent inspiratory time 33% and frequency of 15 Hz. A volume recruitment/de-recruitment protocol was commenced at 20 minutes postnatal age using 0.4 kPa stepwise changes in MAwP. Following the recruitment/derecruitment protocol, 3 lambs were returned to a MAwP of 2.4 kPa. Flow was monitored at the airway opening (Florian, Acutronics) and Ptr was measured in the distal trachea (Codman Microtransducer, Johnson & Johnson). Lower respiratory impedance (Zlrs) was measured at each MAwP using the LF-FOT (0.5-20 Hz, 0.3 kPa). An empirical model fit to the data allowed estimation of airway resistance (Raw), airway inertance (Iaw) and the co-efficients of tissue damping (Glrs) and tissue elastance (Hlrs). Hysteresivity (h) was determined as Glrs/ Hlrs.

Results: There was a reduction in both Glrs and Hlrs following volume recruitment (Fig 1A). Hysteresivity (h) and Iaw also decreased however there was no significant change in Raw. Hlrs and pressure transmission to the tracheal compartment (Ptr) were minimised at the point of maximal curvature on the deflation limb of the pressure volume curve (Fig. 1B).

Conclusions: Volume recruitment is critical to minimisation of pressure transmission during HFOV and can be tracked in the sedated preterm lamb using sophisticated non-invasive measurements of lung mechanics.

Acknowledgements: The authors acknowledge the generous loan of equipment for this study from SensorMedics Corporation, USA and Anaequip, Australia.