Steady State Effects of CO₂ and Temperature on the Relationship between Lung Volume and Inspiratory Duration (Hering‐Breuer Threshold Curve)

Abstract

Abstract The time course of the volume threshold curve for the inspiratory off‐switch, i.e. the relationship between volume, (Vx) and inspiratory duration (Tj) with intact vagus nerves, was determined in steady conditions at different inspired CO2 concentrations and body temperatures in lightly anesthetized cats by means of ‘ramp’‐shaped inflations and deflations of different rise times. With intact vagal feedback the Vx–TT data points could be very well fitted to hyperbolas in all conditions studied. Changes in steady state FACO2 never caused any detectable influence on the volume threshold curves. Changes in body temperatures, in a range below the panting threshold, caused pronounced effects on the volume threshold curve which could best be characterized as a shift of the whole curve along the volume axis to lower values with increased temperature. Thus, at each rate of volume expansion the threshold for inspiratory off‐switch was reached earlier the higher the temperature. Also in the absence of vagal volume feed‐back Ti became correspondingly shorter with increased temperature which indicates that these temperature effects are mainly of central origin. The mechanisms for the CO2 and temperature effects on tidal volume and inspiratory duration are discussed.