Terbutaline does not improve lung function in preterm rabbits.

OBJECTIVE: We used the premature rabbit model of surfactant deficiency to test the hypothesis that perinatal administration of terbutaline would lead to increased secretion of surfactant into the alveolar space and increase lung compliance during mechanical ventilation. STUDY DESIGN: Fetuses underwent delivery at a gestational age of 28 days (term 31 days) followed by mechanical ventilation. Fetuses were subdivided into four treatment protocols: control, fetuses given terbutaline at birth, fetuses of mothers given terbutaline 1 hour before delivery, and fetuses of mothers given terbutaline intramuscularly 12 hours before delivery. Dynamic compliance was determined. After this, alveolar lavage fluid was obtained for phosphatidylcholine content determination. Some fetuses were killed at birth and their alveolar lavage phosphatidylcholine was determined. RESULTS: Among the fetuses undergoing mechanical ventilation, perinatal terbutaline exposure did not alter either dynamic compliance or alveolar lavage phosphatidylcholine. Mechanical ventilation was associated with large increases in alveolar lavage phosphatidylcholine content. CONCLUSION: Perinatal beta-adrenergic agonist exposure does not alter in vivo lung function following preterm delivery.

Effect of mean airway pressure on gas exchange during high-frequency oscillatory ventilation.

We studied the effect of mean airway pressure (Paw) on gas exchange during high-frequency oscillatory ventilation in 14 adult rabbits before and after pulmonary saline lavage. Sinusoidal volume changes were delivered through a tracheostomy at 16 Hz, a tidal volume of 1 or 2 ml/kg, and inspired O2 fraction of 0.5. Arterial PO2 and PCO2 (PaO2, PaCO2), lung volume change, and venous admixture were measured at Paw from 5 to 25 cmH2O after either deflation from total lung capacity or inflation from relaxation volume (Vr). The rabbits were lavaged with saline until PaO2 was less than 70 Torr, and all measurements were repeated. Lung volume change was measured in a pressure plethysmograph. Raising Paw from 5 to 25 cmH2O increased lung volume by 48-50 ml above Vr in both healthy and lavaged rabbits. Before lavage, PaO2 was relatively insensitive to changes in Paw, but after lavage PaO2 increased with Paw from 42.8 +/- 7.8 to 137.3 +/- 18.3 (SE) Torr (P less than 0.001). PaCO2 was insensitive to Paw change before and after lavage. At each Paw after lavage, lung volume was larger, venous admixture smaller, and PaO2 higher after deflation from total lung capacity than after inflation from Vr. This study shows that the effect of increased Paw on PaO2 is mediated through an increase in lung volume. In saline-lavaged lungs, equal distending pressures do not necessarily imply equal lung volumes and thus do not imply equal PaO2.

Frequency, tidal volume, and mean airway pressure combinations that provide adequate gas exchange and low alveolar pressure during high frequency oscillatory ventilation in rabbits.

We studied healthy and saline lavaged rabbits during high frequency oscillatory ventilation to determine what combination of frequency (f), tidal volume (Vt), and mean airway pressure (Paw) produced the lowest peak-to-peak alveolar pressure amplitude (Palv) and physiologic blood gas tensions. Sinusoidal volume changes were delivered through a tracheostomy by a piston pump driven by a linear motor. Tracheal pressure amplitude (Ptr) was measured through a tracheal catheter and alveolar pressure amplitude was measured in a capsule glued to the right lower lobe. PaO2, PaCO2, Ptr, and Palv were measured at the following settings: FiO2 = 0.5, frequency 2-28 Hz, Vt 1-3 mL/kg (50 150% dead space) and Paw 5-15 cm H2O. Many combinations of frequency and Vt resulted in the same PaO2 and PaCO2. Paw had a large effect on Palv and minimal effect on blood gas tensions. In lavaged rabbits, the composite variable f x Vt2 described the trends in Palv and blood gas tensions. As the product of f x Vt2 increased, PaO2 initially increased and then decreased, whereas PaCO2 decreased and Palv increased. No single combination of frequency, Vt and Paw simultaneously provided the lowest Palv and physiologic blood gas tensions. Adequate blood gas tensions and low Palv were obtained at frequencies less than 12 Hz, a Vt of 2 mL/kg and a Paw of 10 cm H2O. In healthy and lavaged rabbits PaO2 increased and PaCO2 decreased as frequency increased at lower Vt.PaO2 decreased as frequency increased at higher Vt in lavaged rabbits only. Palv tended to be greater in lavaged rabbits.