Inspiration regulates the rate and temporal pattern of lung liquid clearance and lung aeration at birth

1 Department of Physiology, 2 School of Physics, 3 Monash Centre for Synchrotron Science, and 4 Department of Biological Engineering, Monash University, Melbourne; 5 The Division of Newborn Services, Royal Women's Hospital, Carlton, Victoria, Australia; and 6 SPring-8, Japan Synchrotron Radiation Research Institute, Sayo, Hyogo, Japan Submitted 24 November 2008 ; accepted in final form 31 March 2009 At birth, the initiation of pulmonary gas exchange is dependent on air entry into the lungs, and recent evidence indicates that pressures generated by inspiration may be involved. We have used simultaneous plethysmography and phase-contrast X-ray imaging to investigate the contribution of inspiration and expiratory braking maneuvers (EBMs) to lung aeration and the formation of a functional residual capacity (FRC) after birth. Near-term rabbit pups ( n = 26) were delivered by cesarean section, placed in a water plethysmograph, and imaged during the initiation of spontaneous breathing. Breath-by-breath changes in lung gas volumes were measured using plethysmography and visualized using phase-contrast X-ray imaging. Pups rapidly (1–5 breaths) generate a FRC (16.2 ± 1.2 ml/kg) by inhaling a greater volume than they expire (by 2.9 ± 0.4 ml·kg –1 ·breath –1 over the first 5 breaths). As a result, 94.8 ± 1.4% of lung aeration occurred during inspiration over multiple breaths. The incidence of EBMs was rare early during lung aeration, with most (>80%) occurring after >80% of max FRC was achieved. Although EBMs were associated with an overall increase in FRC, 34.8 ± 5.3% of EBMs were associated with a decrease in FRC. We conclude that lung aeration is predominantly achieved by inspiratory efforts and that EBMs help to maintain FRC following its formation. fetus; expiratory braking maneuvers Address for reprint requests and other correspondence: S. B. Hooper, Dept. of Physiology, Monash Univ., Melbourne, Victoria 3800, Australia (e-mail: stuart.hooper{at}med.monash.edu.au )