Effects of hypercapnia on Breuer-Hering threshold for inspiratory termination

Abstract

The effects of CO2 concentration on the timing of inspiratory duration (TI) and expiratory duration (TE) and the responses to lung inflation were studied in decerebrate paralyzed cats. With lung volume held at functional residual capacity during the breath cycle, hypercapnia (fractional concentration of inspired CO2 = 0.04) caused variable changes in TI and significant increases in TE. To obtain the Breuer-Hering threshold relationship [tidal volume (VT) vs. TI] and the timing relationship between TE and the preceding TI (TE vs. TI), ramp inflations of various sizes were used to terminate inspiration at different times in the breath cycle. Hypercapnia caused the VT vs. TI curves to shift in an upward direction so that at higher lung volumes TI was lengthened. Also, the slope of the TE vs. TI relationship was increased. The results suggest that hypercapnia diminished the sensitivity of the Breuer-Hering reflex to the lung volume, thus allowing volume to increase with little effect on TI. In addition, TE appears to become more sensitive to changes in the preceding TI. A model is presented which provides a possible neural mechanism for these responses.

Lateral parabrachial nucleus mediates shortening of expiration and increase of inspiratory drive during hypercapnia

“…In anesthetized or awake dogs, cats and rats with intact or severed vagi, hypercapnia generally causes increases of f and shortening of T E , with slight decrease or no change in T I (Clark et al 1972; Gautier 1976; Ledlie et al 1981; Oliven et al 1985; Coles et al 2002). However, hypercapnia reportedly may also lead to a paradoxical prolongation of T E in decerebrate cats (St John 1979; Clanton et al 1984) and similar post-hypercapnia prolongation of T E in awake rats (Coles et al 2002). The central pathways mediating these varied effects during central and/or peripheral chemoreceptors activation by hypercapnia are unclear.…”