Prolonged ventilation of the premature newborn rabbit after treatment with natural or apoprotein-based artificial surfactant

Dr. Tetsuro Fujiwara-My Memories from the Early Days of Dr. Fujiwara's Research

This brief commentary honors Dr. Tesuro Fujiwara, the first person to treat infants with respiratory distress syndrome by instilling surfactant into their trachea. In the 1960s, mortality from RDS, which could only be treated with oxygen, was about 50 percent. Based on the physiology Fujiwara learned that lung immaturity could be treated with doses of surfactant from animals in sheep models. He then made a surfactant from cow lungs called Servanta and treated 10 infants with RDS, who all had a large improvement in oxygenation. Other new therapies, such as continuous positive airway pressure and newer infant ventilators-in combination with surfactant therapy have decreased infant mortality to less than 1% from RDS in the most recent US infant death statistics.

Effects of surfactant replacement on alveolar overdistension and plasma cytokines in ventilator-induced lung injury

BACKGROUND

Overdistension of the lung causes ventilator-induced lung injury (VILI) accompanied by surfactant abnormalities and inflammatory changes. We investigated the effects of surfactant replacement on overdistension of the terminal airspaces and plasma cytokine levels in VILI.

METHODS

VILI was induced by high-pressure ventilation (HPV) in rats anesthetized with pentobarbital, followed by ventilation for 2 h in the maintenance mode (tidal volume=10 ml/kg, positive end-expiratory pressure = 7.5 cmH(2)O) with or without surfactant replacement. The sizes of the terminal airspaces were determined after fixing the lungs at an airway pressure of 10 cmH(2)O on deflation. Cytokine levels were assessed by enzyme-linked immunosorbent assay.

RESULTS

The mean ratio of the largest terminal airspace size class (> or = 64,000 microm(2)) was increased from 13.4% to 32.0% by HPV (P<0.05). After maintenance-mode ventilation, the ratio decreased to 16.1% with surfactant replacement (P<0.05), but increased to 44.6% without surfactant replacement (P<0.05). Mean macrophage inflammatory protein-2 (MIP-2) levels in the plasma increased from <0.02 to 6.9 ng/ml with HPV (P<0.05), and further increased to > or = 11.8 ng/ml, regardless of surfactant replacement after maintenance-mode ventilation. Similar tendencies were observed in the interleukin (IL)-6 and IL-10 levels. Tumor necrosis factor-alpha levels were almost negligible during the experiment.

CONCLUSION

In rats with VILI, surfactant replacement reversed overdistension of the terminal airspaces that may induce barotrauma, but not upregulation of MIP-2, IL-6, and IL-10 within 2 h.

Effects of surfactant replacement on irregular overdistension of meconium-injured lungs in rats

BACKGROUND

Overdistension of the lungs is a cause of ventilator-induced lung injury. In meconium aspiration syndrome, irregular overdistension of the lungs often occurs.

OBJECTIVES

We investigated whether surfactant replacement could restore the terminal airspaces in the lungs that had been distended after meconium aspiration.

METHODS

Meconium aspiration was induced by injecting meconium (50 mg x kg(-1)) into the airways of adult rats anesthetized with pentobarbital and ventilated with pressure-preset mode. The animals were further ventilated with or without surfactant replacement (100 mg x kg(-1)), and the sizes of the terminal airspaces were determined after fixing the lungs at an airway pressure of 10 cm H2O on deflation.

RESULTS

Approximately 75 min after aspiration (early analysis point), alveolar ducts were widened and the mean ratio of the largest terminal airspace size class (> or =63,000 microm(2)) was 38.7% (n = 7), which was significantly higher than that of controls (6%, n = 7). Three hours after the early analysis point, the ratio increased to 50.2% (n = 7, p < 0.05), but surfactant replacement reversed the ratio to 18.8% (n = 7, p < 0.05).

CONCLUSIONS

In rats with meconium aspiration, surfactant replacement restored the distended terminal airspaces of the lungs and kept the spaces from irregular overdistension.

Biochemical and pharmacological differences between preparations of exogenous natural surfactant used to treat Respiratory Distress Syndrome: role of the different components in an efficient pulmonary surfactant

The pharmaceutical application of exogenous natural pulmonary surfactant preparations has shown its efficiency in the therapeutical treatment of infants with Respiratory Distress Syndrome. At the same time, the use of these preparations in patients with Acute Respiratory Distress Syndrome, although not still an effective therapy, shows promising results. The analysis of composition, structure and surface activity of some of the different natural surfactant preparations available today for clinical use reveals important differences, a fact that opens horizons in the optimization of new effective formulations in the treatment of the Acute Respiratory Distress Syndrome. The purpose of this review is to carry out an updating of the current models interpreting the role of the main components of pulmonary surfactant as a reference to evaluate the biochemical composition of the preparations of exogenous natural pulmonary surfactant currently in use and their apparent pharmacological effect.

Biophysical and Physiological Properties of Porcine Surfactant Enriched with Polymyxin B

OBJECTIVE

We examined whether the biophysical and physiological properties of Curosurf were improved by the cyclic amphipathic decapeptide polymyxin B (PxB).

METHODS

Curosurf was diluted to 1-5 mg/ml with PxB added at 1, 2 or 3% (w/w). Albumin was added at 40 mg/ml. Minimum surface tension (gammamin) during surface compression was determined for each mixture with pulsating bubble. Immature newborn rabbits were treated with 2.5 ml/kg of Curosurf 80 mg/ml, or Curosurf 32 mg/ml with or without 2% PxB and ventilated for up to 5 h.

RESULTS

At surfactant concentration 2 mg/ml, gammamin was high (17 +/- 8.9 mN/m) but remained low (2.7 +/- 0.8 mN/m) when PxB was added. Albumin inactivated Curosurf at both 2 and 3.5 mg/ml; this inactivation was prevented by 2% PxB. Treatment of newborn rabbits with Curosurf 80 mg/kg + 2% PxB significantly decreased incidence of pneumothorax in comparison with controls but had no significant effect on lung-thorax compliance or alveolar expansion.

CONCLUSION

Addition of 2% PxB improves surface activity of Curosurf at low concentration, increases its resistance to inactivation by albumin, and reduces the incidence of pneumothorax in immature newborn rabbits undergoing prolonged ventilation.

Surfactant therapy for respiratory distress syndrome in premature neonates: a comparative review

Exogenous surfactant therapy has been part of the routine care of preterm neonates with respiratory distress syndrome (RDS) since the beginning of the 1990s. Discoveries that led to its development as a therapeutic agent span the whole of the 20th century but it was not until 1980 that the first successful use of exogenous surfactant therapy in a human population was reported. Since then, randomized controlled studies demonstrated that surfactant therapy was not only well tolerated but that it significantly reduced both neonatal mortality and pulmonary air leaks; importantly, those surviving neonates were not at greater risk of subsequent neurological impairment. Surfactants may be of animal or synthetic origin. Both types of surfactants have been extensively studied in animal models and in clinical trials to determine the optimum timing, dose size and frequency, route and method of administration. The advantages of one type of surfactant over another are discussed in relation to biophysical properties, animal studies and results of randomized trials in neonatal populations. Animal-derived exogenous surfactants are the treatment of choice at the present time with relatively few adverse effects related largely to changes in oxygenation and heart rate during surfactant administration. The optimum dose of surfactant is usually 100 mg/kg. The use of surfactant with high frequency oscillation and continuous positive pressure modes of respiratory support presents different problems compared with its use with conventional ventilation. The different components of surfactant have important functions that influence its effectiveness both in the primary function of the reduction of surface tension and also in secondary, but nonetheless just as important, role of lung defense. With greater understanding of the individual surfactant components, particularly the surfactant-associated proteins, development of newer synthetic surfactants has been made possible. Despite being an effective therapy for RDS, surfactant has failed to have a significant impact on the incidence of chronic lung disease in survivors. Paradoxically the cost of care has increased as surviving neonates are more immature and consume a greater proportion of neonatal intensive care resources. Despite this, surfactant is considered a cost-effective therapy for RDS compared with other therapeutic interventions in premature infants.

Aerosolized surfactant and dextran for experimental acute respiratory distress syndrome caused by acidified milk in rats

BACKGROUND

Inhibition of pulmonary surfactant by plasma-derived proteins is an important pathogenetic factor of acute respiratory distress syndrome (ARDS). Inhalation of aerosolized surfactant may be suitable for early treatment of ARDS. However, requirement of a high dose is a drawback. Because dextran reverses surfactant inhibition, we examined whether dextran improves the therapeutic effects of aerosolized surfactant in rats with experimental ARDS.

METHODS

Acidified milk (pH 1.8, 1.5 ml kg(-1)) was injected into the trachea of the rats ventilated with pure oxygen using 2.45 kPa peak inspiratory pressure and 0.74 kPa positive end-expiratory pressure. When PaO2 decreased to <13 kPa, the rats were assigned to four groups: control group (n = 8), receiving no material; D-only group (n = 6), receiving aerosolized dextran for 45 min; S-only group (n = 8), receiving aerosolized modified natural surfactant (MNS) for 30 min; and S-plus-D group (n = 9), receiving aerosolized MNS for 30 min followed by aerosolized dextran for 15 min.

RESULTS

In the control group and D-only groups, the mean PaO2 remained at <10 kPa for 180 min. In the S-only and S-plus-D groups, the PaO2 increased to 50 kPa (P < 0.01 vs. untreated). The PaO2 of the surfactant-only group gradually decreased to <17 kPa at 180 min, whereas the PaO2 of the S-plus-D group was maintained at >38 kPa for 180 min (P < 0.01 vs. S-only group).

CONCLUSION

Inhalation of aerosolized dextran potentiates the effects of aerosolized surfactant by prolonging the therapeutic response.

Aerosolized surfactant therapy for endotoxin-induced experimental acute respiratory distress syndrome in rats

We have compared the effects of inhalation of aerosolized surfactant on experimental acute respiratory distress syndrome. Escherichia coli endotoxin (55 (SD 20) mg kg(-1)) was injected into the tracheas of 36 adult rats anaesthetized and mechanically ventilated with pure oxygen. When the Pa(O(2)) decreased to 11.3 (3.3) kPa, the animals were randomly subjected to inhalation of aerosolized modified natural surfactant (MNS) for 0 min (control group), 30, 60, and 120 min. In the control group, Pa(O(2)) remained below 12 kPa for 180 min. In the groups receiving inhalation of surfactant for 30 and 60 min, Pa(O(2)) increased but decreased soon after termination of the inhalation. In contrast, Pa(O(2)) of the group receiving inhalation of surfactant for 120 min continued to increase, reaching 52.1 (12.5) kPa at 180 min (P<0.05 vs control). Thus, we conclude that improvement in gas exchange as a result of inhalation of MNS depends on the duration of inhalation.

Alveolar metabolism of natural vs. synthetic surfactants in preterm newborn rabbits

We compared the recoveries of four surfactant preparations: two natural [term fetal rabbit surfactant (FRS) and adult rabbit surfactant (ARS)] and two commercially available preparations [apoprotein-based Survanta (S) and synthetic Exosurf (E)] from 27-day gestation rabbit pups treated at birth and ventilated up to 120 min. At 5, 60, and 120 min, we measured the recovery of the heavy-aggregate, metabolically active form (H) and the light-aggregate, nonsurface active metabolic breakdown form (L) of alveolar surfactant and determined the phospholipid content and composition of the intracellularly stored lamellar body (LB) pool. Pups treated with FRS had <15% loss of H by 2 h. ARS-treated pups had a >50% loss of H by 1 h, and E- and S-treated pups had approximately 50% loss by 5 min, with a slower rate of continuing loss of up to 80% by 2 h. The major losses of H phospholipid were not explained by the L-form recovery. LB phospholipid significantly increased only in the E-treated pups and only at 2 h. FRS provides a biologically active form (H) of surfactant that appeared to remain in the airway for a significantly longer time than the other surfactant preparations. The unique properties of FRS merit further study.

Porcine-derived lung surfactant. A review of the therapeutic efficacy and clinical tolerability of a natural surfactant preparation (Curosurf) in neonatal respiratory distress syndrome

Porcine-derived lung surfactant (PLS; Curosurf) has shown efficacy in neonatal respiratory distress syndrome. PLS consists of phospholipids, mainly dipalmitoylphosphatidylcholine, the primary surface-active agent of natural lung surfactant, and pulmonary surfactant-associated proteins which facilitate spreading and adsorption of the surface-active agent at the air-alveolar interface. Intratracheal administration of a single dose of PLS 200 mg/kg significantly improves the survival rate and reduces the incidence of bronchopulmonary dysplasia at 28 days in premature infants (birthweight 700 to 2000g) with severe respiratory distress syndrome (fraction of inspired oxygen > or = 0.60). PLS also reduces the incidence of air leak events such as pulmonary interstitial emphysema and pneumothorax. The response rate may be further improved by administration of additional 100 mg/kg doses at 12-hour intervals to infants showing a poor response or relapse after a single dose. PLS prophylaxis reduces the incidence and severity of respiratory distress syndrome in premature infants at high risk of developing the disease; however, it remains unclear whether the eventual clinical outcome is similar or superior to that observed in infants who receive rescue treatment. PLS is well tolerated and does not appear to increase the incidence of complications of prematurity or respiratory distress syndrome, including patent ductus arteriosus and intraventricular haemorrhage. Although its effect on long term development require further investigation, early indications are that PLS is not associated with any long term adverse sequelae. Comparative trials are clearly warranted to determine the efficacy and tolerability of PLS relative to that of other available surfactant preparations, particularly to explore preliminary indications that a more rapid effect of natural surfactants such as PLS (compared with synthetic products) may correlate with improved clinical outcomes, and that PLS may result in fewer complications than synthetic preparations. Thus, available data show PLS to be a very effective agent for the treatment and prophylaxis of neonatal respiratory distress syndrome, and that it may have some advantages over synthetic preparations.

The critical concentration of surfactant in fetal lung liquid at birth

Various doses (0-4.8 mg) of porcine surfactant were administered into the airways of immature newborn rabbits delivered at a gestational age of 26 days and 17-23 h. When the estimated concentration of exogenous surfactant in the lung liquid was less than or equal to 0.75 mg/ml (dose 0.6 mg), an average tidal volume of no more than a 3.0 ml/kg was obtained by mechanical ventilation with a peak insufflation pressure of 25 cm H2O, but when the estimated concentration was increased to 1.5 mg/ml (dose 1.2 mg), an average tidal volume of 17.7 ml/kg was attained, and the survival rate during a 30-min period of artificial ventilation improved significantly, from 14% to 53%. Even larger average tidal volumes, about 25 ml/kg, were recorded in animals with estimated surfactant concentrations of 3 and 6 mg/ml (doses 2.4 and 4.8 mg, respectively). In vitro observations revealed that the surface adsorption time of the surfactant suspension decreased non-linearly from 20 to 1 sec when the concentration was increased from 1 to 3 mg/ml. The minimum surface tension during cyclic film compression also decreased non-linearly from greater than 15 to less than 3 mN/m with the same increments in concentration. This led us to conclude that, under the present experimental conditions, the critical concentration of surfactant in fetal lung liquid at birth (about 3 mg/ml) is close to the concentration required in vitro for rapid adsorption and optimal dynamic surface properties.

Lung lavage and surfactant replacement for hydrochloric acid aspiration in rabbits

Hydrochloric acid (0.1 N, 5.0 ml.kg-1 in total) was administered intratracheally to 28 adult rabbits anesthetized with pentobarbital and mechanically ventilated with pure oxygen. When the PaO2 decreased to 14.1 +/- 2.8 kPa (mean +/- s.d.), the PaCO2 increased to 8.9 +/- 2.5 kPa, and the minute ventilation (VE) decreased to 51 +/- 8% of the baseline value, animals were divided into 4 groups. The deteriorated values did not improve in the non-treated (control) animals, whereas the animals treated with lung lavage and surfactant replacement showed a significant increase in PaO2 to 35.1 +/- 12.2 kPa, and maintained lower PaCO2 and larger VE than the controls. These parameters showed no significant improvement with surfactant replacement alone, and deteriorated further with lung lavage alone. The minimum surface tension (gamma min) of the edema fluid that accumulated in the airways after acid administration was 22.5 +/- 1.7 mN.m-1, and was not lowered by adding surfactant preparation (10 mg.ml-1) whose original gamma min was less than 2 mN.m-1. We concluded that surfactant inhibition by edema fluid was a cause of respiratory failure, and that lung lavage followed by surfactant replacement might be of therapeutic value for acid aspiration.

Inhibition of exogenous surfactant in ventilated immature newborn rabbits

Immature newborn rabbits were treated at birth by tracheal instillation of porcine surfactant (100 microliters, phospholipid concentration 80 mg.ml-1), to which [14C]dipalmitoylphosphatidylcholine had been added as a marker. They were kept in a body plethysmograph/pneumotachygraph system at 37 degrees C. During a 120 min period of artificial ventilation with a peak insufflation pressure of 20 cm H2O, there was a gradual decrease in tidal volumes (36%.h-1). This decrease was correlated to an elevation of minimum surface tension (r = 0.81; P less than 0.01) and to a prolongation of the adsorption rate (r = 0.80; P less than 0.01) of surfactant recovered by lung lavage from the same animals. There was also correlations between duration of ventilation and minimum surface tension (r = 0.56; P less than 0.01), and between duration of ventilation and adsorption rate (r = 0.73; P less than 0.01). The surface properties of phospholipids extracted from the lavage fluid were similar to those of the original surfactant preparation. Our data suggest that, in immature newborn rabbits subjected to artificial ventilation, exogenous surfactant may become inactivated, probably due to protein leakage into the airspaces.

Comparative studies on the biophysical activities of the low-molecular-weight hydrophobic proteins purified from bovine pulmonary surfactant

Two low-molecular-weight hydrophobic proteins with nominal molecular weights Mr = 15,000 and Mr = 3,500 have been isolated from the lipid extracts of bovine pulmonary surfactant by several methods, including (a) dialysis plus silicic acid chromatography, (b) elution from Waters SEP-PAK silica cartridges with a variety of solvent mixtures, and (c) ultrafiltration. As detailed in the text, these proteins have been designated surfactant-associated protein-BC (SP-BC) (15 kDa: nonreduced), and SP-C (3.5 kDa). The biophysical activities of reconstituted surfactant containing these proteins and the phospholipids present in lung surfactant have been compared with the biophysical activities of bovine lipid extract surfactant on a pulsating bubble surfactometer using a phospholipid concentration of 10 mg/ml. At this concentration, unmodified lipid extract surfactant reduces the surface tension of the pulsating bubble to near 0 within 10 pulsations at 20 cycles per min. Similar biophysical properties were observed with modified lipid extract surfactant in which the relative concentration of hydrophobic protein had been reduced from 1 to 0.4% (W/W) of the phospholipids by addition of dipalmitoylphosphatidylcholine (DPPC) or DPPC plus phosphatidylglycerol. Reconstituted surfactants, which contained partially delipidated SP-BC (15 kDa: nonreduced) obtained by method (a) at a relative concentration of 0.1%, were also capable of reducing the surface tension to near 0 mN/m. Preparations of SP-BC (15 kDa: nonreduced) obtained by method (b), which had been subjected to very low pH levels during isolation and were extensively delipidated, exhibited full biophysical activity only at higher protein concentrations and with prolonged pulsation. Extensively delipidated samples of SP-BC obtained by method (c) exhibited impaired biophysical activities, even when prepared with neutral organic solvents. Reconstituted surfactant samples containing SP-C (3.5 kDa) obtained by any of the methods listed above were only able to reduce the surface tension at minimum bubble radius to approx. 20 mN/m. The biophysical activity of SP-C (3.5 kDa) was not significantly affected by low pH or extensive delipidation. Reconstituted samples containing mixtures of SP-BC (15 kDa: nonreduced) and SP-C (3.5 kDa) were more effective than samples containing either protein alone. Furthermore, with samples containing both hydrophobic proteins the final surface tensions at maximum bubble radius were attained within a few bubble pulsations.(ABSTRACT TRUNCATED AT 400 WORDS)

Respiratory failure in mice caused by a hybridoma making antibodies to the 15 kDa surfactant apoprotein

Hybridoma cells were obtained by fusing spleen cells from mice, immunized against the 15 kDa porcine surfactant apoprotein, with a myeloma cell line. Adult mice were inoculated intraperitoneally with this hybridoma; mice that were not inoculated or were inoculated with myeloma cells served as controls. Lung-thorax compliance was measured at various intervals after inoculation. The animals were then killed for histologic-morphometric evaluation of alveolar air expansion, inflammatory reaction in the pulmonary parenchyma, and intraalveolar edema. In the hybridoma group, the mice developed respiratory failure 9 days after inoculation, with markedly reduced lung-thorax compliance, lung congestion, alveolar collapse, hemorrhagic pulmonary edema, and hyaline membranes. Morphometric data from the same animals showed reduced volume density of alveolar air, and increased volume densities of intraalveolar "fluid" (edema) and tissue components. These lung lesions are similar to those in the adult respiratory distress syndrome.