Utilization of deadspace, capnometry, lung mechanics, blood gas analysis and radiography to determine the efficacy of a peptide-containing synthetic lung surfactant in a rabbit surfactant-depleted model.

JM Van Zyl1, J Smith2, A Hawtrey1. 1Stellenbosch University, Division of Pharmacology, Department of Medicine, faculty of Health Sciences, Tygerberg 7505, South Africa, 2Stellenbosch University, Department of Pediatrics, Tygerberg Childrens Hospital, Faculty of Health Sciences, Tygerberg 7505, South Africa

Pulmonary surfactant which is essential for breathing is a thin lipid-protein coating of the alveolar epithelium at the air interface of the mammalian lung. It consists of a complex mixture of phospholipids and apoproteins that reduces surface tension at the alveolar surface. Although surfactant replacement therapy (SRT) is an established treatment modality for surfactant deficiency states such as respiratory distress syndrome (RDS), preparations are of animal origin, very expensive and pose a risk of transmitting infectious material. For the purpose of our study we developed a synthetic surfactant (Synsurf) consisting of phospholipids combined with poly-L-lysine and poly-L-glutamic acid.

The objective of the study was to investigate whether SRT, following bronchoalveolar lavage-induced acute lung injury resulting in surfactant deficiency, hypoxemia and increased alveolar and physiological dead space could restore the lung to its pre-lavage condition. Synsurf (S), “Exosurf Neonatal” (Ex), (previously used) and Ca2+(5 mM) supplemented “Exosurf” (ExCa) was used for SRT. The design was a prospective randomized controlled trial, animal study.

Synsurf was prepared by mixing dipalmitoyl-L-α-phosphatidylcholine (DPPC), cetyl alcohol and 1,2-Dipalmitoyl-L-α-phosphatidylglycerol (PG) in chloroform and combining the dried phospholipid with a poly-L-lysine poly-L-glutamate complex in a NaCl solution. The generic “Exosurf Neonatal” surfactant was prepared as described (Jolanta et al., 1999). Surfactant dose was 100 mg/kg.

Animal model: Animal care and experimental procedures were performed under approval from the Faculty of Health Sciences Research Committee of Stellenbosch University. Twelve adult New Zealand White (NZW) rabbits (2.5–3.75 kg) were studied (S 3.13±0.56kg, n=4; Ex 2.95±0.19kg, n=4; ExCa 3.44±0.24kg, n=4). Anaesthesia was induced with intramuscular ketamine hydrochloride 1% (50 mg/kg) and maintained with infusion of sodium pentobarbitone (6 mg/kg/h). Animals were ventilated using a pressure-control mode (Julian Anaesthetic Workstation, Drager, Germany). After paralysis with pancuronium bromide (intravenous, 0.1 mg/kg/h), animals were subjected to repeated saline lavage (20ml/kg, 37°C) via an endotracheal tube (Lachmann et al., 1980). Pulmonary function was measured with a CO2SMO Plus respiratory profile monitor (Novametrix Medical Systems Inc.). Five minutes after lavage, animals were randomized into three groups. Surfactant preparations were administered via the endotracheal tube. Antero-posterior chest radiographs were taken. Rectal temperature was kept between 38ºC and 40ºC and the study lasted 5 hours. Statistical analyses were done by one-way ANOVA (mean ± SD) and linear mixed effects modelling (Pinheiro et al., 2011; Maritz et al., 1998).

Bronchoalveolar lavage led to ventilator damage indicated by an increasing arterial PCO2 (kPa) (S 5.50±0.65; Ex 5.41±0.59; ExCa 5.30±0.69) and a simultaneous increase of alveolar deadspace/tidal volume ratio (S 1.25±0.08; Ex 1.14±0.09; ExCa 1.30±0.16) and physiological deadspace/tidal volume ratio (S 2.30±0.10; Ex 2.14±0.13; ExCa 2.40±0.17) with no inter-group differences. Arterial-end-tidal PCO2 and deadspace/tidal volume ratio correlated in all three groups(S r2 0.49, p 0.017; Ex r2 0.51, p 0.013; ExCa r2 0.72, p 0.001). Nevertheless, a significant and sustained improvement in systemic oxygenation occurred from timepoint 180 min onward in animals treated with Synsurf compared to the other two groups (global test mixed effects model χ2 = 58.81, p<0.001). A statistical significant decrease in pulmonary shunt (S 31.49±10.68 vs Ex 41.13±1.63, p 0.01 and vs ExCa 37.36±5.29, p 0.01) was found for the Synsurf treated group of animals as well as radiographic improvement in 3 out of 4 animals in that group. In general, SRT in the animal did not fully restore the lung to its pre-lavage condition. However, our data shows that the formulated surfactant Synsurf improves oxygenation by lowering pulmonary shunting.

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