Abstract
Background: Ventilated preterm infant lungs are vulnerable to overdistension and underinflation. The optimal ventilator-delivered tidal volume (V<sub>T</sub>) in these infants is unknown and may depend on the extent of alveolarisation at birth. Objectives: We aimed to calculate respiratory dead space (V<sub>D</sub>) from the molar mass (MM) signal of an ultrasonic flowmeter (V<sub>D,MM</sub>) in very preterm infants on volume-targeted ventilation (V<sub>T</sub> target, 4-5 ml/kg) and to study the association between gestational age (GA) and V<sub>D,MM</sub>-to-V<sub>T</sub> ratio (V<sub>D,MM</sub>/V<sub>T</sub>), alveolar tidal volume (V<sub>A</sub>) and alveolar minute volume (AMV). Methods: This was a single-centre, prospective, observational, cohort study in a neonatal intensive care unit. Tidal breathing analysis was performed in ventilated very preterm infants (GA range 23-32 weeks) on day 1 of life. Results: Valid measurements were obtained in 43/51 (87%) infants. Tidal breathing variables were analysed using multivariable linear regression. V<sub>D,MM</sub>/V<sub>T</sub> was negatively associated with GA after adjusting for birth weight Z score (p < 0.001, R<sup>2</sup> = 0.26). This association was primarily influenced by the appliance dead space. Despite similar V<sub>T</sub>/kg and V<sub>A</sub>/kg across all studied infants, respiratory rate and AMV/kg increased with GA. Conclusions:V<sub>D,app</sub> rather than anatomical V<sub>D</sub> is the major factor influencing increased V<sub>D,MM</sub>/V<sub>T</sub> at a younger GA. A volume guarantee setting of 4-5 ml/kg in the Dräger Babylog® 8000 plus ventilator may be inappropriate as a universal target across the GA range of 23-32 weeks. Differences between measured and set V<sub>T</sub> and the dependence of this difference on GA require further investigation.
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