Purpose: In adult animals, ventilation with variable tidal volume and rate improves lung mechanics, arterial oxygenation and ventilation compared to a monotonously controlled ventilation pattern. We assessed the physiological consequences of variable ventilation in the immature lung.
Methods: Lambs delivered at 129 days (term = 150 days) were euthanised (n = 9) or anaesthetised, tracheostomised and suctioned prior to prophylactic intra-tracheal surfactant instillation (Curosurf(®), 100 mg/kg) and commencement of controlled ventilation (50 breaths/min, tidal volume 7.7 ± 0.8 mL/kg). Volume history was standardised at 20 min with two sustained (3 s) inflations to 30 cmH(2)O followed immediately by measurement of baseline dynamic lung mechanics (FlexiVent, Scireq, Canada). Ventilation was continued according to prior randomisation (variable or conventional ventilation). For variable ventilation (n = 9), breath-to-breath tidal volume and respiratory rate varied but intra-breath minute volume (MV) and average tidal volume were equivalent to the conventional ventilation group with fixed tidal volume and rate (n = 7). Lung mechanics and gas exchange were measured at intervals. Lambs were euthanised at 2 h. Inflammatory cell counts and protein from bronchoalveolar lavage fluid and lung tissue cytokine mRNA were quantified.
Results: At study completion, PaCO(2) (p = 0.026) and mean airway pressure (p = 0.002) were lower and pH (p = 0.047), ventilation efficiency index (p = 0.021) and dynamic compliance were higher (p = 0.003) in lambs on variable rather than conventional ventilation. However, oxygenation indices and post-mortem static compliances were not different between groups.
Conclusion: Variable ventilation improves ventilation efficiency and in vivo lung compliance in the preterm lung, but unlike adult models, had no effect on arterial oxygenation.