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Extremely low gestational age newborns (ELGANs) i.e. infants born before 28 weeks of gestational age (GA), require respiratory support during their hospital stay, and a majority needs mechanical ventilation (MV) at some time point.  The duration of mechanical ventilation is related to morbidity and mortality, and respiratory support needs to be approached with the aim of minimizing both short and long term effects.

In this thesis the overall aim was to explore lung mechanics and the effect of different ventilatory settings and modes during MV in ELGANs.

In Paper I and II, bedside forced oscillation technique (FOT) was combined with a positive end-expiratory pressure (PEEP)-titration trial. In Paper I, FOT measurements on day 1 showed dependence of reactance (Xrs) on PEEP. FOT-parameters correlated with days on MV, and together with radiography and GA predicted respiratory outcomes. 

In Paper II, FOT was studied on day 1, 3 and 7, and Xrs was used to find the optimal lung recruiting PEEP. The optimal PEEP was lower than the clinically set PEEP, and especially during the first day of life. Xrs indicated that the lung was easily over extended with small increases in PEEP.

In Paper III, we compared retrospectively early application of volume targeted ventilation (VTV) to pressure limited ventilation (PLV) in infants born at 22⁺⁰-25⁺⁶ weeks GA. Infants that received VTV had lower peak inflation pressures, less frequent hypocapnia and were earlier extubated to CPAP compared to infants receiving PLV.

In Paper IV, we investigated the respiratory activity in animals by measurements of phrenic nerve activity (PNA), and in infants by measuring electrical activity of the diaphragm (EAdi) during the transition from PLV to proportional assist ventilation (PAV), a proportionally adjusted ventilatory mode. PNA and EAdi increased during PAV as compared to PLV with similar tidal volumes, thus promoting increased respiratory activity and weaning. 

In conclusion, we found that FOT was feasible bedside in ELGANs and FOT-parameters were correlated to PEEP and respiratory outcomes; VTV was safely applied during the first day of life; increased breathing activity during PAV suggests this to be a suitable mode in weaning from MV to non-invasive ventilation, when optimized support is needed during this transition.