The Science Journal of the American Association for Respiratory Care

1999 OPEN FORUM Abstracts

OPTIMAL MEAN AIRWAY PRESSURE FOR HIGH-FREQUENCY-OSCILLATORY-PARTIAL-LIQUID-VENTILATION IN AN OVINE MODEL OF ADULT ARDS

Sven Goddon, MD; Muneyuki Takeuchi, MD; Jonathan Hromi, BA; Robert Kacmarek, PhD, RRT. Anaesthesia and Respiratory Care Laboratories, Massachusetts General Hospital and Harvard Medical School, Boston, MA.

Previous preclinical investigations on the effects of High-Frequency-Oscillatory-Partial-Liquid-Ventilation (HFO-PLV) on pulmonary gas-exchange have yielded varied results. One reason for that being that the mean airway pressure (PAW) was always set arbitrarily. We therefore investigated four different settings for PAW, based on the results of the static P-V curve of the respiratory system. We hypothesized that the PAW resulting in the best oxygenation (optimal PAW) can be related to and thus predicted from the static P-V curve of the respiratory system in a large animal model of ARDS.

Methods: In six sheep (31±5 kg) severe lung injury was induced by repeated saline lung lavages until PaO2 decreased to less than 120 torr at volume controlled ventilation (FiO2 1.0, VI 12 ml/kg, PEEP 5 cm H2O). The lower inflection point (LIP. 20±0.8 cm H2O) on the inspiratory limb and the point of maximal curvature (PMC, 26±0.8 cm H2O) on the expiratory limb of the static P-V curve were determined by a blinded investigator. 10 ml/kg perflubron (PFB) were slowly instilled through a sideport of the endotracheal tube during volume controlled ventilation with the PEEP increased to 13 cm H2O. 30 min after the PFB instillation another P-V curve was obtained. The sheep were then subjected to four half-hour cycles of HFOV (f 4 Hz, amplitude 40 cm H2O) at different levels of PAW (PMC-8. PMC-4, PMC, PMC+4 cm H2O), applied in random order (FiO2 1.0). The first cycle was preceded by a sustained insufflation (recruitment manoeuvre) with a CPAP of 40 cm H2O for 60 s.

Results: HFO-PLV with a PAW equal to the PMC resulted in a highly significant (p<0.01 vs. Injury) improvement of oxygenation and venous admixture (QS/QT). A higher PAW did not yield further improvement in PaO2 or QS/QT but significantly impaired cardiac output (CO) and oxygen delivery (DaO2).

Injury Postfill PMC-8 PMC-4 PMC PMC+4
PaO2 72±21 256±107 132±99 263±103? 445±102??* 430±107??*
CO 5.1±0.8 4.6±1.0 5.1±0.8 4.8±0.4 4.6±0.3 3.1±0.7??*o
DaO2 551±93 577±142 606±143 591±109 589±77 394±93??*o
QS/QT 0.35±0.04 0.27±0.07 0.35±0.08 0.25±0.03 0.15±0.05?? 0.15±0.08??
OI 18±7 9±5 20±11 10±5 6±2?? 7±2?
Table 1 mean±SD P<0.05: ? vs. Injury, ? vs. PMC-8, * vs. PMC-4, o vs. PMC

Conclusion: In this ovine model of adult ARDS the optimal PAW (least pressure with highest PaO2) in HFO-PLV can be predicted from the static P-V curve. It coincides approximately with the PMC on the expiratory limb of the P-V curve.

This study was funded by Alliance Pharmaceutical Corp., San Diego, CA

OF-99-216

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