High frequency oscillation of the lungs alone lengthens expiration in dogs.

High frequency (15 Hz), low volume (approximately 40 ml) oscillation of the respiratory system lengthens expiration in anesthetized dogs even in the absence of changes in mean expiratory lung volume or blood gases (Banzett et al., 1983). Although vagotomy abolishes this response, we felt it might mask the reflex rather than interrupt the afferent pathway. To ascertain whether pulmonary and airway afferents alone are capable of evoking this reflex, we confined the oscillatory stimulus to the lung by widely opening the chest. The dogs were paralyzed but breathed 'spontaneously' by means of a solenoid valve opened and closed by the phrenic neurogram. End-expiratory lung volume and PCO2 were held constant. All eight dogs that were tested after thoracotomy lengthened expiration during oscillation. Although quantitative comparisons of responses before and after thoracotomy are not entirely reliable, we could see no consistent change in the strength of response with paralysis or with thoracotomy. We conclude that the principal afferent limb of the ventilatory reflex response to oscillation arises in the lungs and intrathoracic airways.