Increased airway resistance due to surfactant dysfunction can be alleviated with aerosol surfactant

To investigate the contribution of pulmonary surfactant to a low airflow resistance through narrow conducting airways, a system was developed with which it was possible to determine the resistance meeting a steady flow of air at 0.5 mL/min. The airflow, delivered by an infusion pump, entered the extreme periphery of a conducting airway in an excised rat lung and exited through the trachea. The resistance was determined by measuring the pressure of the air entering the lung. If the airway remained open, the pressure was only slightly above zero; when a blocking liquid column formed in the lumen of the airway, the pressure increased rapidly but dropped abruptly as the liquid was pushed away into a wider airway section. When endogenous pulmonary surfactant was removed with a saline lavage, the airway was blocked almost constantly by an endless re-formation of liquid columns. Consequently, during a 4-min period of pressure recording, free airflow was observed only rarely. However, administration of aerosol surfactant increased the duration of free airflow in relation to the volume administered. After an injection of 80 mL of aerosol surfactant, the airway stayed open 89 +/- 3% of the 4-min recording time compared with only 28 +/- 5% when the same volume of air (80 mL) without surfactant had passed through the airway (p < 0.0001). We conclude that surfactant contributes to a free airflow through conducting airways and may have an important role in the maintenance of low airway resistance.