Regional distribution of blood volume within the preterm infant thorax during synchronised mechanical ventilation

Purpose: Perfusion in healthy adults is gravity-dependent. Little is known about lung perfusion in the preterm infant. The aim of this study was to describe the regional distribution of blood volume within the thorax in preterm infants receiving synchronised volume-targeted mechanical ventilation (SIPPV + TTV) and to compare this to regional distribution of tidal ventilation using electrical impedance tomography (EIT). Methods: Stable supine ventilated preterm infants (<32-week gestation) were studied. Three sets of artefact-free 30-s EIT recordings of the right hemithorax were filtered in the cardiac and respiratory frequency domains to differentiate impedance change due to blood (ΔZ_c) and gas volume (ΔZ_v). The distribution of ΔZ_c and ΔZ_v in the anterior-to-posterior regions of the right chest were compared. Infants were subdivided by age (≤7, >7 days) and oxygen requirement. Results: A total of 5,471 beats were analysed from 26 infants (78 recordings); mean (standard deviation (SD)) gestational age was 26 (2) weeks and mean (SD) postnatal age was 9 (10) days. The median (interquartile range) ΔZ_c in the anterior half of the hemithorax was 1.41-fold (0.88-2.11) greater than that in the posterior half. The geometric centre of ΔZ_c was located at 46.7% of the anterior-posterior thoracic distance, compared to a more centrally located ΔZ_v (49.6%; p < 0.0001). The ΔZ_v/ ΔZ_c ratio was 1.7 in the anterior third of the chest and 2.2 in the posterior (p < 0.0001). The area under the curve (AUC) analysis showed that ΔZ_c was more evenly distributed in infants >7 days of age and not influenced by oxygen requirement. Conclusions: There are gravity dependent differences in the distribution of blood volume and ventilation in the ventilated preterm chest.