Effects of end-expired pressure on phrenic output in servo-ventilated dogs

The pattern of breathing induced by increases in end-expired lung volume (EEVL) was determined in 9 anesthetized dogs. The pulmonary and systemic circulations were separately pump-perfused and the lungs were ventilated with a servo-ventilator actuated from the phrenic neurogram. EEVL was increased as a continuous ramp by slowly raising end-expired transpulmonary pressure from 1.5 to 12 cm H2O. Tidal volume (VT), inspiratory time (TI), and expiratory time (TE) were measured at vagal temperatures of 39 degrees C and 7 degrees C and following vagotomy. At a vagal temperature of 39 degrees C, increasing EEVL produced significant reductions in VT and TI while greatly prolonging TE. Vagal cooling to 7 degrees C, substantially altered the reflex response to increased EEVL. At 7 degrees C, VT decreased as EEVL increased, but the reduction was not so pronounced as at 39 degrees C. In addition, both TI and TE shortened. Increasing EEVL following vagotomy had no consistent effects on breathing pattern. We conclude that increasing EEVL stimulates tachypneic promoting pulmonary afferent nerves, most likely pulmonary C-fibers, but at normal vagal temperature their effect is masked by the stronger reflex inhibition of slowly adapting pulmonary stretch receptors.