Pulmonary surfactant maintains patency of conducting airways in the rat

The hypothesis was tested that after extrusion of the liquid columns that often block the lumen of conducting airways, the latter will remain open because of well-functioning pulmonary surfactant preventing the liquid columns from returning. The extirpated lungs of 22 Wistar rats were studied. Via a tracheal tube a very fine catheter (PE 10) was inserted and advanced until it pierced the pleura. It was extracted until only 2 mm remained in the lung parenchyma. A pressure transducer measured the resistance that met a steady flow of air through the series of tubes: the PE 10 tube, the conducting airway of the lung, and the tracheal tube. The airway resistance was studied for 240 s after three airway flushings, two with saline solution and one with calf lung surfactant extract (CLSE), 3 mg/ml. The pressure recording showed that a low pressure, indicating airway patency, occurred for only 31 ± 8 s (mean ± SEM) after the first saline flush, and for 26 ± 8 s after the second. After the CLSE flush the airway remained open for 174 ± 12 s, which indicated a significantly reduced resistance (p < 0.0001). The results imply that well-functioning pulmonary surfactant is required for a low airway resistance.