290 - Optimising Lung Aeration using External Negative Pressures in Near-Term Rabbit Kittens.

Publication Number: 290.5746

Cailin Diedericks, Monash University, St Helena, Victoria, Australia; Kelly J. Crossley, The Ritchie Centre, Clayton, Victoria, Australia; Dominic Jurkschat, Monash University, Clayton, Victoria, Australia; Indya M. Davies, Hudson Institute of Medical Research, Clayton, Victoria, Australia; Paige J. Riddington, Monash University, Clayton, Victoria, Australia; Arjan te Pas, Division of Neonatology, LUMC, Leiden, Zuid-Holland, Netherlands; Marcus Kitchen, Monash University, Melbourne, Victoria, Australia; Stuart B. Hooper, Monash University, Black Rock, Victoria, Australia

Background: Respiratory distress in term infants (TRD) is primarily caused by elevated liquid in the airways at birth. As liquid is cleared from the airways into lung tissue at birth, the lungs normally become oedematous. However, elevated liquid levels increase the degree of oedema which results in impaired respiratory function. As we have previously demonstrated that external negative pressures improve lung aeration in near-term rabbit kittens (Abstract ID:1975306), we hypothesize that the optimal level of external negative pressure will vary depending on the volume of airway liquid.

Objective: Determine the external negative pressure level that optimises lung aeration in near-term rabbit kittens with and without elevated airway liquid.

Design/Methods: Rabbit kittens (30/32d gestation) were randomised into Control or Elevated Liquid (EL) groups. Control kittens had lung liquid drained to simulate lung liquid volumes after vaginal delivery. EL kittens had lung liquid drained and 30 mL/kg liquid added to the airway to simulate lung liquid volumes after caesarean section. After delivery, kittens were transferred into a water-filled (39℃) plethysmograph and external pressures adjusted to 0 cmH2O (Control n=7; EL n=6), -3 cmH2O (Control n=7; EL n=8), -6 cmH2O (Control n=6; EL n=7), or -9 cmH2O (Control n=6; EL n=7) (sample size calculated on G*Power). Mechanical ventilation was initiated with a sustained inflation followed by ventilation with a tidal volume of 8 mL/kg and PEEP of 0 cmH2O. Lung aeration (functional residual capacity; FRC) was measured with phase contrast X-ray imaging. Data were analysed using a linear mixed model with Sidak-corrected multiple comparisons. Significance P≤0.05.

Results: In Control kittens, FRC levels increased with the level of external negative pressure but were not significantly different between kittens exposed to -6 and -9 cmH2O (31.6±0.9 vs 39.0±1.3 mL/kg; Figure 1A). In EL kittens, FRC increased with the levels of external negative pressure applied (0 cmH2O 7.6±2.0; -3 cmH2O 15.1±1.3; -6 cmH2O 22.1±1.6; -9 cmH2O 28.3±3.1 mL/kg at 500 s; P≤0.05; Figure 1B).

Conclusion(s): Optimal lung inflation (FRC≈30 mL/kg) was achieved with an external pressure of -6 cmH2O in Control kittens, whereas an external pressure of -9 cmH2O caused over-inflation (FRC≈40 mL/kg). In EL kittens, optimal lung aeration was achieved with an external pressure of -9 cmH2O, indicating that external negative pressures may enhance lung aeration in newborns with TRD.

Figure 1. Functional residual capacity (FRC; mL/kg) after the sustained inflation (SI) in (A) Control and (B) Elevated Liquid EL) kittens exposed to external atmospheric (0 cmH2O) and negative (-3, -6, and -9 cmH2O) pressures.
The hutch was opened at 260 seconds and imaging ceased to allow researchers to adjust the external pressures applied as drifting occurred during lung aeration. This resulted in a gap in FRC recordings between 260-340 seconds. Significant differences are indicated by: (u) 0 cmH2O vs -3 cmH2O, (v) 0 cmH2O vs -6 cmH2O, (w) 0 cmH2O vs -9 cmH2O, (x) -3 cmH2O vs -6 cmH2O, (y) -3 cmH2O vs -9 cmH2O, and (z) 6 cmH2O vs -9 cmH2O. P≤0.05. Mean±SEM.