To test the hypothesis that hyaline membrane disease (HMD) has a multifactorial etiology in which barotrauma plays a major role, we compared the immediate institution of high-frequency oscillatory ventilation (HFOV; 15 Hz, n = 5) with positive-pressure ventilation with positive end-expiratory pressure (PPV; n = 7) in premature baboons (140-days gestation) with HMD. Measurements of ventilation settings and physiological parameters were obtained and arterial-to-alveolar O2 (PaO2-to-PAO2) ratio and oxygenation index [(PaO2/PAO2)-to-mean airway pressure ratio (IO2)] were calculated. At death (24 h), static pressure-volume (PV) curves were performed, and phospholipids (PL) and platelet-activating factor (PAF) were measured in lung lavage fluid. Morphological inflation patterns were analyzed using a panel of standards. By design, mean airway pressure was initially higher (19 vs. 13 cmH2O) in the HFOV animals. PaO2-to-PAO2 ratio and IO2 progressively deteriorated in the PPV animals and then stabilized at significantly lower levels than with HFOV. PV curves from HFOV animals had significant increases in lung volume at maximum distending pressure, deflation volume at 10 cmH2O, and hysteresis area compared with PPV, which showed no hysteresis. Seven of seven PPV and only one of five HFOV animals had morphological findings of HMD. PL amount and composition in both groups were consistent with immaturity, even though the quantity was significantly greater in the PPV group. PAF was present (greater than or equal to 0.10 pmol) in six of seven PPV and in the only HFOV animal with HMD. We conclude that HFOV protected PL-deficient premature baboons from changes in gas exchange, lung mechanics, and morphology typical of HMD in this model.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathophysiologic, morphometric, and biochemical (surfactant and coagulation-fibrinolytic parameters) features were studied in premature baboons with and without bronchopulmonary dysplasia (BPD). A total of 22 baboons were delivered by hysterotomy at 75% of gestation and randomized into two groups. Group 1 (PRN) animals were ventilated with high-frequency oscillation for 48 to 72 h and then changed to positive-pressure ventilation (PPV) while maintained on clinically appropriate oxygen for the 21-day experimental period. Group 2 (BPD) animals were ventilated with PPV and 1.0 FlO2 for 7 days followed by 0.8 FlO2 for 14 days. Group 3 (control) animals were delivered and immediately killed at 140 days gestation. Group 1 animals showed no significant airway or saccular lesions, and alveolarization of the saccules was present. Group 2 animals showed metaplastic or hyperplastic epithelial lesions in airways and an alternating pattern of atelectatic but more normal appearing saccules and alveoli interposed between foci of thickened overexpanded saccular walls with no alveolarization. Differences within and between the three study groups were analyzed morphometrically. When numerical densities were examined by comparing control, PRN, BPD-atelectatic areas, and BPD-overexpanded areas. Type II cells were significantly increased in the BPD-overexpanded sites above those of control and PRN values. The interstitial cells were significantly more numerous in the BPD-atelectatic blocks compared with control and BPD-overexpanded blocks. Endothelial cell numerical densities were significantly decreased in the overexpanded sites of the BPD animals compared with the control, PRN, and BPD-atelectatic values. Volume density data showed that the interstitial compartment of the BPD group was significantly larger than those of the control and PRN groups. This was seen as significant increases in the cellular, noncellular, and connective tissue fiber components. Vascular endothelium or lumen volume densities were not different between the BPD and PRN animals, but did differ from those of the 140-day gestation controls. Comparable levels of lavage plasminogen-dependent fibrinolytic activity, were detectable at the 21-day study interval. The phospholipid composition of pulmonary surfactant, including disaturated PC and total PG, was similar between BPD and PRN groups at 21 days. The pathologic, morphometric, and biochemical patterns in this study probably represent those seen in human neonates with mild to moderate clinical BPD who survive. At this time, it is not known if the destructive endothelial lesion and the lack of alveolarization in the overexpanded and fibrotic lesions will resolve over time in long-term BPD survivors.

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Surfactant proteins A and D (SP-A and SP-D) are believed to participate in the pulmonary host defense and the response to lung injury. In order to understand the effects of prematurity and lung injury on these proteins, we measured the amounts of SP-A and SP-D and their mRNAs in three groups of animals: (1) nonventilated premature baboon fetuses; (2) neonatal baboons delivered prematurely at 140 d gestation age (ga) and ventilated with PRN O(2); (3) animals of the same age ventilated with 100% O(2) to induce chronic lung injury. In nonventilated fetuses, tissue and lavage SP-A were barely detectable in baboons of 125 and 140 d ga, but they equaled or exceeded adult SP-A concentrations (g/g lung dry wt) at 175 d (term gestation, 185 d). In contrast, SP-D was readily detectable in tissue and lavage at 125 and 140 d ga. When the baboons of 140 d ga were ventilated for 10 d with 100% oxygen to produce chronic lung injury, the tissue concentration of SP-A was five times greater than that of normal adults; SP-D 16-times greater. Despite the sizable tissue pools of SP-A and SP-D, however, lavage SP-A was only 7% of that of normal adults and lavage SP-D just equaled the amount in normal adults. Nevertheless, because SP-D is normally in much lower concentration than is SP-A, their total comprised less than 12% of the SP-A and SP-D found in the lavage of a healthy adult. The results indicate that in chronic lung injury, SP-A is significantly reduced in the alveolar space. SP-D concentration in lavage is about equal to that in normal adults, possibly because of the 16-fold excess in tissue, but the total collectin pool in lavage is still significantly reduced. Because these collectins may bind and opsonize bacteria and viruses, decrements in their amounts may present additional risk to those premature infants who require prolonged periods of ventilatory support.

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