Regulation of frequency and depth of breathing during expiratory threshold loading in cats

In six spontaneously breathing anesthetized cats, intermittently subjected to inspiratory elastic loads, we have studied the relationships between tidal volume (VT) and the durations of inspiration (Ti) and breath duration (Ttot) obtained during spontaneous ventilation from resting lung volume (FRCc) and from elevated end-expiratory levels. The latter was elevated by submerging the expiratory breathing line into a column of water, representing the addition of an expiratory threshold load (ETL). The VT vs. Ti relationships obtained at different end-expiratory levels were similar, indicating that during ETL the vagal mechanism regulating Ti responds only to lung volume changes above the new end-expiratory level and is independent of the absolute end-expiratory lung volume. Single vagal fiber recordings suggest that this effect on Ti control may be explained on the basis of adaptation occurring at the level of the pulmonary stretch receptors. The control of Ttot, on the other hand, was found to depend both on the Ti of the preceding breath (phasic component) and on a separate vagal mechanism specifically affecting the duration of expiration (Te) in response to changes in the absolute end-expiratory lung volume. The latter mechanism is functionally inoperative at FRCc.