Supplementary Material for: Influence of Gestational Age on Dead Space and Alveolar Ventilation in Preterm Infants Ventilated with Volume Guarantee

<p><b><i>Background:</i></b> Ventilated preterm infant lungs are vulnerable to overdistension and underinflation. The optimal ventilator-delivered tidal volume (<i>V</i><sub>T</sub>) in these infants is unknown and may depend on the extent of alveolarisation at birth. <b><i>Objectives:</i></b> We aimed to calculate respiratory dead space (<i>V</i><sub>D</sub>) from the molar mass (MM) signal of an ultrasonic flowmeter (<i>V</i><sub>D,MM</sub>) in very preterm infants on volume-targeted ventilation (<i>V</i><sub>T</sub> target, 4-5 ml/kg) and to study the association between gestational age (GA) and <i>V</i><sub>D,MM</sub>-to-<i>V</i><sub>T</sub> ratio (<i>V</i><sub>D,MM</sub>/<i>V</i><sub>T</sub>), alveolar tidal volume (<i>V</i><sub>A</sub>) and alveolar minute volume (AMV). <b><i>Methods:</i></b> 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. <b><i>Results:</i></b> Valid measurements were obtained in 43/51 (87%) infants. Tidal breathing variables were analysed using multivariable linear regression. <i>V</i><sub>D,MM</sub>/<i>V</i><sub>T</sub> was negatively associated with GA after adjusting for birth weight Z score (p<i> <</i> 0.001, R<sup>2</sup> = 0.26). This association was primarily influenced by the appliance dead space. Despite similar <i>V</i><sub>T</sub>/kg and <i>V</i><sub>A</sub>/kg across all studied infants, respiratory rate and AMV/kg increased with GA. <b><i>Conclusions:</i></b><i>V</i><sub>D,app</sub> rather than anatomical <i>V</i><sub>D</sub> is the major factor influencing increased <i>V</i><sub>D,MM</sub>/<i>V</i><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 <i>V</i><sub>T</sub> and the dependence of this difference on GA require further investigation.</p>