A small constant external negative pressure improves lung aeration at birth in rabbit kittens with a diaphragmatic hernia

Infants with a congenital diaphragmatic hernia often require high peak inflation pressures to aerate and ventilate their stiff, hypoplastic lungs at birth. However, although high peak inflation pressures are associated with ventilator-induced lung injury, lower peak inflation pressures (considered to be gentle) may not sufficiently aerate the lungs. The objective of the study was to investigate whether a small constant external negative pressure can improve lung aeration at birth in rabbit kittens with a diaphragmatic hernia. A diaphragmatic hernia was surgically induced in 2–3 rabbit fetuses per doe (n = 20) at 24 d gestation. At 30 d, diaphragmatic hernia kittens (n = 21) and control littermates (n = 19) were delivered by caesarean section, intubated, and placed in a water-filled plethysmograph. Kittens were ventilated (volume targeted: 8 mL/kg for control, 4 mL/kg for diaphragmatic hernia kittens) with either an external negative pressure (-6 cmH₂O) without a positive end-expiratory pressure (PEEP; Negative/0PEEP) or at atmospheric pressure (0 cmH₂O) with or without PEEP (Atmos/5PEEP and Atmos/0PEEP). Lung aeration was measured using phase-contrast X-ray imaging and plethysmography. Data (means ± SD) were analyzed using a repeated-measures linear mixed model (P < 0.05). Diaphragmatic hernia kittens ventilated in Negative/0PEEP required lower peak inflation pressures to achieve the target tidal volume than Atmos/5PEEP (10.9 ± 2.6 vs. 15.4 ± 2.6 cmH₂O; P = 0.004) and Atmos/0PEEP kittens (14.2 ± 2.6 cmH₂O; P = 0.052). Negative/0PEEP diaphragmatic hernia kittens also had greater functional residual capacities compared with Atmos/5PEEP (19.9 ± 3.9 vs. 13.8 ± 3.9 mL/kg; P = 0.015) and Atmos/0PEEP kittens (6.4 ± 3.9 mL/kg; P < 0.001). A small constant external negative pressure increased lung aeration with lower peak inflation pressures in mechanically ventilated diaphragmatic hernia rabbit kittens. NEW & NOTEWORTHY This study demonstrates that a small constant external negative pressure applied during conventional mechanical ventilation improves lung aeration and achieves set tidal volumes with significantly lower peak inflation pressures in rabbit kittens with a diaphragmatic hernia and severe lung hypoplasia. The negative extrathoracic pressure improved dynamic lung compliance and stabilized the chest wall. Our findings indicate that negative extrathoracic pressures do more than simply act to replace a positive end-expiratory pressure to enhance respiratory function.