The artificial placenta: Continued lung development during extracorporeal support in a preterm lamb model

Abstract

Purpose: An artificial placenta (AP) utilizing extracorporeal life support (ECLS) could avoid the harm of mechanical ventilation (MV) while allowing the lungs to develop.

Methods: AP lambs (n = 5) were delivered at 118 days gestational age (GA; term = 145 days) and placed on venovenous ECLS (VV-ECLS) with jugular drainage and umbilical vein reinfusion. Lungs remained fluid-filled. After 10 days, lambs were ventilated. MV control lambs were delivered at 118 ("early MV"; n = 5) or 128 days ("late MV"; n = 5), and ventilated. Compliance and oxygenation index (OI) were calculated. After sacrifice, lungs were procured and H&E-stained slides scored for lung injury. Slides were also immunostained for PDGFR-α and α-actin; alveolar development was quantified by the area fraction of alveolar septal tips staining double-positive for both markers.

Results: Compliance of AP lambs was 2.79 ± 0.81 C_dyn compared to 0.83 ± 0.19 and 3.04 ± 0.99 for early and late MV, respectively. OI in AP lambs was lower than early MV lambs (6.20 ± 2.10 vs. 36.8 ± 16.8) and lung injury lower as well (1.8 ± 1.6 vs. 6.0 ± 1.2). Double-positive area fractions were higher in AP lambs (0.012 ± 0.003) than early (0.003 ± 0.0005) and late (0.004 ± 0.002) MV controls.

Conclusions: Lung development continues and lungs are protected from injury during AP support relative to mechanical ventilation.

Level of evidence: n/a (basic/translational science).

Keywords: Artificial placenta; Extracorporeal life support; Lung development; Prematurity.

Figure 1

Schematic of the Artificial Placenta. Blood is drained from the right jugular vein by a collapsible-tubing roller pump (M-pump, MC3: Ann Arbor, MI) and propelled to an oxygenator/heat exchanger (Medos HiLite, Xenios: Heilbronn, Germany), then returned via an umbilical vein. The second umbilical vein is accessed for IV fluid and medication administration, and an umbilical arterial line is placed for hemodynamic monitoring and blood gas sampling. The lamb is intubated and the lungs remain filled with amniotic fluid by clamping the endotracheal tube. Ao – aorta; DV – Ductus venosus; IJV – internal jugular vein; IVC – inferior vena cava; RA – right atrium; SVC – superior vena cava

Figure 2

Hemodynamic, Gas Exchange, and Laboratory Data during Artificial Placenta Support. Data pictured only include values obtained prior to initiation of mechanical ventilation for all lambs (217 hours; n=5), due to variation in timing of ventilation initiation, amount of ventilator support, and duration of support. A) Heart rate (HR) and Mean arterial pressure (MAP). B) Circuit flow (weight-normalized). C) Umbilical artery blood gas values: pCO₂, pO₂, and SaO₂. D) Arterial lactate. E) Measures of renal function: blood urea nitrogen (BUN) and creatinine. F) Measures of liver function: alanine aminotransferase (ALT) and total serum bilirubin (Tbil).

Figure 3

Lung Function Data. A) Lung compliance was calculated as ΔV/ΔP with units of C_dyn (mL/cmH₂O). After 10 days of AP support and transition to mechanical ventilation, both average and maximum compliance over the course of ventilatory support were significantly higher than in early MV lambs, and were similar to late MV lambs which were delivered at 10 days greater gestational age. B) Oxygenation Index (OI) was calculated as (mean airway pressure (MAP) * FiO₂) / P_aO₂. After 10 days of AP support and transition to mechanical ventilation, average OI was significantly higher than in early MV lambs. (Mean ± SD; *p<0.01, **p<0.001, ANOVA, Tukey)

Figure 4

Lung Injury Scoring. Representative slides of H&E staining from the left lower lobe of lungs from AP lambs (A), early MV lambs (B), and late MV lambs (C) are shown (10x magnification). Lungs from AP lambs were better inflated and contained less hemorrhage than early MV lambs. D) Lung slides were scored 0-4 for injury by a pathologist blinded to experimental group in multiple categories. Significant differences were seen between lungs from AP lambs and early MV lambs in alveolar and interstitial hemorrhage as well as necrosis. E) Total injury scores were significantly lower in AP lungs than early MV lungs, and also appeared lower, though not significantly, than late MV lungs. (Mean ± SD; *p<0.01, **p<0.001, ANOVA, Tukey)

Figure 5

Immunofluorescent staining for PDGFR-α (green) and α-actin (red) at 20x magnification. Co-localization of signals at the tips of alveolar crests represent double-positive tips, signifying active alveolarization (circled, enlarged in inset images). Double-positive tips were more evident in lungs from AP lambs (A) than early MV (B) or late MV (C) lungs. Early and late MV lungs also displayed layering of α-actin along the alveolar wall without the presence of PDGFR-α (arrows), signifying a maladaptive responsive to positive pressure ventilation. D) The ratio of double-positive tips falling on standardized grid points to total grid points represents the area fraction. Area fractions of double-positive tips in AP lungs was significantly higher than that in both early and late MV lungs. (Mean ± SD; *p<0.005 vs. AP, ANOVA, Tukey)