Utility of large-animal models of BPD: chronically ventilated preterm lambs

Abstract

This paper is focused on unique insights provided by the preterm lamb physiological model of bronchopulmonary dysplasia (BPD). Connections are also made to insights provided by the former preterm baboon model of BPD, as well as to rodent models of lung injury to the immature, postnatal lung. The preterm lamb and baboon models recapitulate the clinical setting of preterm birth and respiratory failure that require prolonged ventilation support for days or weeks with oxygen-rich gas. An advantage of the preterm lamb model is the large size of preterm lambs, which facilitates physiological studies for days or weeks during the evolution of neonatal chronic lung disease (CLD). To this advantage is linked an integrated array of morphological, biochemical, and molecular analyses that are identifying the role of individual genes in the pathogenesis of neonatal CLD. Results indicate that the mode of ventilation, invasive mechanical ventilation vs. less invasive high-frequency nasal ventilation, is related to outcomes. Our approach also includes pharmacological interventions that test causality of specific molecular players, such as vitamin A supplementation in the pathogenesis of neonatal CLD. The new insights that are being gained from our preterm lamb model may have important translational implications about the pathogenesis and treatment of BPD in preterm human infants.

Keywords: airway expiratory resistance; alveolar simplification; alveolarization; endothelial nitric oxide synthase; epigenetics; insulin-like growth factor-1; nasal CPAP; neonatal chronic lung disease; nitric oxide; pulmonary hypertension; vitamin A.

Fig. 1.

Collage of radiographic and histological appearance of the head, neck, and lungs of preterm lambs supported by 2 modes of ventilation for 21 days. Left: invasive mechanical ventilation. The top radiograph shows the endotracheal tube's lumen highlighted in blue in the neck. Juxtaposed in the remainder of the left half are chest radiographic and lung histological images. The chest radiograph shows opacification of the lung fields, which obscure the heart shadow (posterior-anterior view; L, left). The terminal respiratory unit (TRU) has distal air spaces that are distended, distal air space walls that are thick and cellular (arrowhead), and secondary septa that are infrequent and stunted (arrow). Right: less invasive high-frequency nasal ventilation. The top radiograph shows the uncuffed nasal tube's lumen highlighted in blue in the nose. The chest radiograph shows better aeration and larger lung volume, which make the heart shadow obvious. The TRU has distal air spaces that are more uniformly shaped, distal air space walls that are thin and less cellular (arrowhead), and secondary septa that are numerous, long, and thin (arrow). The scale bar is 100 μm in length. Adapted and modified from Ref. .

Fig. 2.

Quantitative histology shows that radial alveolar count (A), alveolar secondary septal volume density (B), air space capillary surface density (C), and extra-alveolar microvessel number (D) are greater in vitamin A-supplemented preterm lambs compared with vehicle controls (bracket, P < 0.05; means ± SD; n = 4/group). Term (T) lambs served as the gestation age-matched reference for the 3-wk ventilation studies. Although vitamin A supplementation improved structural development, the improvements were significantly less than the term lambs (P < 0.05) for 3 of the 4 quantitative histological parameters. IMV, invasive mechanical ventilation. Adapted from Ref. .

Fig. 3.

Pulmonary vascular resistance (A) decreased in term reference lambs that were not ventilated from week 1 to week 3 of postnatal life (means ± SD; n = 6–7/group). By comparison, pulmonary vascular resistance did not decrease in preterm lambs that were supported by invasive mechanical ventilation (IMV) over the same postnatal time period (*P < 0.05 compared with matched term reference). Muscularization of pulmonary arterioles (B) normally decreases postnatally in reference lambs that were not ventilated. By comparison, muscularization persisted in preterm lambs that were supported by IMV for 3 wk. 1d and 21d, 1 and 21 days, respectively. Data from Ref. .

Fig. 4.

Extra-alveolar microvessel number (A) and capillary surface density (B) increased in term reference lambs that were not ventilated (means ± SD; n = 5/group). Extra-alveolar microvessel number and capillary surface density were significantly lower in preterm lambs that were supported by invasive mechanical ventilation (IMV) for 3 wk compared with both newborn references (*P < 0.05). Adapted from Ref. .

Fig. 5.

Extravascular lung water, an indicator of pulmonary edema, is significantly greater in the lung of preterm lambs supported by invasive mechanical ventilation (IMV) compared with both groups of newborn reference lambs (*P < 0.05; means ± SD; n = 6/group). Data from Ref. .

Fig. 6.

Semiquantitative densitometry for endothelial nitric oxide synthase (eNOS) (A) and soluble guanylate cyclase (sGC) (B and C). eNOS and sGC arbitrary densitometry in homogenates of 3rd to 5th generation intrapulmonary arteries (A and B), and immunohistochemical arbitrary densitometry in pulmonary arterioles landmarked next to terminal bronchioles (C) are significantly less in the lung of preterm lambs supported by invasive mechanical ventilation (IMV) for 3 wk compared with both groups of newborn reference lambs (*P < 0.05; means ± SD; n = 3–4/group). Adapted from Refs. and .

Fig. 7.

Preterm lambs were given inhaled nitric oxide (5–15 ppm) for 1 h between weeks 1 and 2 of life (days 7–12; n = 12/group) and again toward the end of week 3 of life (days 18–20; n = 8/group). Acute inhalation of nitric oxide reduced median pulmonary vascular resistance from the initial level of pulmonary vascular resistance (days 7–12). However, median pulmonary vascular resistance was slightly higher more than a week later (days 18–20). Adapted from Ref. .

Fig. 8.

Preterm lambs were given inhaled nitric oxide (5–15 ppm) continuously for 3 wk. At the end of 3 wk of continuous inhalation of nitric oxide (iNO), pulmonary vascular resistance (A) and pulmonary arteriole smooth muscle area (B) remained the same as the respective values for preterm lambs that did not inhale nitric oxide (NO iNO) (means ± SD; n = 5/group). Adapted from Ref. with permission of the American Thoracic Society.

Fig. 9.

Preterm lambs were given inhaled nitric oxide (5–15 ppm) continuously for 3 wk. At the end of 3 wk of continuous inhalation of nitric oxide (iNO), respiratory tract resistance (A) and terminal bronchiole smooth muscle area (B) were significantly lower than the respective values for preterm lambs that did not inhale nitric oxide (NO iNO) (*P < 0.05; means ± SD; n = 5/group). Adapted from Ref. with permission of the American Thoracic Society.

Fig. 10.

Preterm lambs were given inhaled nitric oxide (5–15 ppm) continuously for 3 wk. At the end of 3 wk of continuous inhalation of nitric oxide (iNO), radial alveolar count (A) and capillary surface density (B) were significantly greater than the respective values for preterm lambs that did not inhale nitric oxide (NO iNO) (*P < 0.05; means ± SD; n = 5/group). Radial alveolar count also was significantly lower than the term reference group. Capillary surface density also was significantly greater than the term reference group. Adapted from Ref. .