Monitoring of lung volume recruitment and derecruitment using oscillatory mechanics during high-frequency oscillatory ventilation in the preterm lamb

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Abstract
To determine whether the low-frequency forced oscillation technique (LFOT) can track changes in lung mechanics resulting from prophylactic intratracheal surfactant and a volume recruitment-derecruitment maneuver during high-frequency oscillatory ventilation (HFOV) and how this relates to the damping of the tracheal pressure waveform (Ptr). Interventional laboratory study. Rural animal research facility in Western Australia. Sedated newborn preterm lambs. Two separate studies were performed. Study 1 involved a volume recruitment-derecruitment maneuver during HFOV using stepwise changes (4 cm H2O) in mean airway opening pressure (Pao, n = 5). Study 2 involved instillation of 4 mL/kg fetal lung fluid (n = 5) or exogenous surfactant (n = 8) at birth and subsequent intermittent mandatory ventilation for 40 mins. Arterial blood gases were recorded every 10 mins and ventilation was appropriately adjusted to achieve moderate hypercarbia (Paco2, 50–60 mm Hg). Lung mechanics were measured using LFOT 10 mins following each adjustment in Pao in parallel with measurements of Ptr and tidal volume (study 1) or after 40 mins (study 2). Both the recruitment-derecruitment maneuver and prophylactic surfactant administration achieved similar improvements (decrease) in tissue impedance (Zti). The coefficient of tissue resistance (G) decreased more than the coefficient of tissue elastance (H), consistent with improved homogeneity of the lung tissue compartment as lung volume was recruited. Minimum Zti coincided with minimum tracheal pressure amplitude (ΔPtr) on the deflation limb of the volume recruitment-derecruitment maneuver during HFOV. There was a linear relationship between ΔPtr and H. In the preterm lamb, the LFOT can successfully detect changes in lung mechanics resulting from volume recruitment maneuvers during both HFOV and ventilation at conventional rates and may provide information on ventilation inhomogeneity. Minimization of Zti is crucial to damping of Ptr and may limit potential barotrauma to proximal alveolar units during establishment of HFOV.

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