Effect of vagal cooling on lung functional residual capacity in rats with pneumonia.

We examined the effect of cold vagal block on the functional residual lung capacity (FRC) in control rats and in rats with experimental pneumonia induced by intratracheal administration of the herbicide paraquat. The measurements were performed in a body plethysmograph in anaesthetized and intubated rats. Rats with pneumonia had tachypnoea and increased minute ventilation. Their FRC was 3.6 +/- 0.7 ml in comparison with 2.5 +/- 0.6 ml in controls (mean +/- SD). Cooling the cervical vagi, so that the Hering-Breuer inflation reflex was abolished (approximately 8 degrees C), resulted in a marked decrease of the rate of breathing and an increase of tidal volume in both groups of animals. The value of FRC did not change during vagal cooling (3.7 +/- 0.9 ml in rats with pneumonia, 2.8 +/- 0.8 ml in controls). In rats with model pneumonia, bilateral cervical vagotomy was followed by a normalization of the FRC (2.3 +/- 0.7 ml). In control animals, the FRC did not change after vagotomy (2.5 +/- 0.6 ml). We conclude that increased FRC in rats with paraquat pneumonia depends on intact conduction through thin, slowly conducting vagal fibres.