Elevated airway liquid volumes at birth: a potential cause of transient tachypnea of the newborn

Excessive liquid in airways and/or in distal lung tissue may underpin the respiratory morbidity associated with transient tachypnea of the newborn (TTN). However, its effects on lung aeration and respiratory function following birth are unknown. We investigated the effect of elevated airway liquid volumes on newborn respiratory function. Near term rabbit kittens (30 days gestation; term ~32 days) were delivered, had lung liquid drained and either had no liquid replaced (Control, n=7) or 30 mL/kg of liquid was re-added to the airways (liquid added; LA, n=7). Kittens were mechanically ventilated in a plethysmograph. Measures of chest and lung parameters, uniformity of lung aeration and airway size were analysed using phase contrast X-ray imaging. The maximum peak inflation pressure required to recruit a tidal volume of 8 mL/kg was significantly greater in LA compared to Control kittens (35.0±0.7 vs 26.8±0.4 cmH₂O; P<0.001). LA kittens required greater time to achieve lung aeration (106±14 vs. 60±6 inflations; P=0.03) and had expanded chest walls as evidenced by an increased total chest area (32±9%; P<0.0001), lung height (17±6%; P=0.02) and curvature of the diaphragm (19±8%; P=0.04). LA kittens had lower functional residual capacity during step-wise changes in positive end expiratory pressures (PEEP; 5, 3, 0, 5 cmH20). Elevated lung liquid volumes had marked adverse effects on lung structure and function in the immediate neonatal period and reduced the ability of the lung to aerate efficiently. We speculate that elevated airway liquid volumes may underlie the initial morbidity in near term babies with TTN after birth.

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