Comparison of rSP-C surfactant with natural and synthetic surfactants after late treatment in a rat model of the acute respiratory distress syndrome

1. In a previous paper we showed that an SP-C containing surfactant preparation has similar activity as bovine-derived surfactants in a rat lung lavage model of the adult respiratory distress syndrome. In this study surfactant was given ten minutes after the last lavage (early treatment). In the present investigation we were interested how different surfactant preparations behave when they are administered 1 h after the last lavage (late treatment).
2. Four protein containing surfactants (rSP-C surfactant, bLES, Infasurf and Survanta) were compared with three protein-free surfactants (ALEC, Exosurf and the phospholipid (PL) mixture of the rSP-C surfactant termed PL surfactant) with respect to their ability to improve gas exchange in this more stringent model when surfactant is given one hour after the last lavage. For better comparison of the surfactants the doses were related to phospholipids. The surfactants were given at doses of 25, 50 and 100 mg kg⁻¹ body weight. The surfactants were compared to an untreated control group that was only ventilated for the whole experimental period.
3. Tracheotomized rats (8–12 per dose and surfactant) were pressure-controlled ventilated (Siemens Servo Ventilator 900C) with 100% oxygen at a respiratory rate of 30 breaths min⁻¹, inspiration expiration ratio of 1 : 2, peak inspiratory pressure of 28 cmH₂O at positive endexpiratory pressure (PEEP) of 8 cmH₂O. Animals were ventilated for one hour after the last lavage and thereafter the surfactants were intratracheally instilled. During the whole experimental period the ventilation was not changed.
4. Partial arterial oxygen pressures (PaO₂, mmHg) at 30 min and 120 min after treatment were used for statistical comparison. All protein containing surfactants caused a dose-dependent increase of the reduced PaO₂ values at 30 min after treatment. The protein-free surfactants showed only weak dose-dependent increase in PaO₂ values at this time. This difference between the protein-containing and the protein-free surfactants was even more pronounced when comparing the PaO₂ values at 120 min after treatment. Only rSP-C surfactant, bLES and Infasurf showed a dose-dependent increase in PaO₂ at this time.
5. With this animal model of late treatment it is possible even to differentiate between bovine derived surfactants. The differences between protein-containing and protein-free surfactants become even more pronounced. From the comparison of rSP-C surfactant with bovine-derived surfactants and the PL surfactant without rSP-C, it can be concluded that addition of rSP-C is sufficient to achieve the same activity as that of natural surfactants.