Effects of hypercapnia and inspiratory flow-resistive loading on respiratory activity in chronic airways obstruction

Abstract

The respiratory responses to hypercapnia alone and to hypercapnia and flow resistive loading during inspiration were studied in normal individuals and in eucapnic and hypercapnic patients with chronic airways obstruction. Responses were assessed in terms of minute ventilation and occlusion pressure (mouth pressure during airway occlusion 100 ms after the onset of inspiration). Ventilatory responses to CO2 (ΔV/ΔPCO2) were distinctly subnormal in both groups of patients with airways obstruction. The 2 groups of patients, however, showed different occlusion pressure responses to CO2 (ΔP100/ΔPCO2): ΔP100/ΔPCO2 was normal in the eucapnic patients but subnormal in the hypercapnic patients. Flow resistive loading during inspiration reduced ΔV/ΔPCO2 both in normal subjects and in patients with airways obstruction. The occlusion pressure response to CO2 increased in normal subjects during flow resistive loading but remained unchanged in both groups of patients with chronic airways obstruction. These results indicate that while chemosensitivity as determined by ΔP100/ΔPCO2 is impaired only in hypercapnic patients with chronic airways obstruction, an acute increase in flow resistance elicits a subnormal increase in respiratory efferent activity in both eucapnic and hypercapnic patients.