Interaction between Expiratory Time and Inspiration in Conscious Humans

Abstract

A breath can be divided into inspiratory and expiratory drive and timing variables. The regulation of expiratory time (T_E) in conscious humans is important in speech and many non-metabolic respiratory functions. T_E is linked to inspiratory variables via mechanical, vagal reflex, chemical and central neuronal mechanisms. However, the mechanisms of these linkages are poorly understood in conscious humans and very little is known about the influence that T_E has on the following breath, especially inspiratory time (T_I). To study these linkages, respiratory patterning variables were measured for each breath in real time by computer in various combinations of 17 normal awake humans breathing mildly hyperoxic (P_I.O2 35%) and hypercapnic gas mixtures via a pneumotachograph into an open circuit. Computer controlled auditory feedback allowed gradual and imperceptible 3 minute step alterations at a constant end-tidal PCO₂ (P_ET,CO2) over 45–60 minutes of (1) T_I over a range of (800 msec at constant inspired tidal volume (V_TI),(2) V_TI up and down in repeated steps of 200 ml at constant T_I, and (3) T_E over a range of (2000 msec at constant V_TI. In each case, the timing of the uncontrolled half of the respiratory cycle was free to be determined by the subjects’ automatic respiratory control mechanisms. Variables were averaged over the final minute of each step, and in protocol 2 the average was obtained of 8 up and down steps respectively. In protocol 1, T_E, changed significantly (P<0.05) from 1.94 to 3.10 sec in parallel with a mean achieved change of T_I from 1.48 to 2.91 sec (N = 10) despite constant V_TI. In protocol 2, T_E and time for expiratory flow did not change significantly in response to a mean step change of V_TI. of 150 ml at constant T_I averaged over 10 steps in 7 subjects.