Role of the mechanical impairment on the ventilatory response to CO2 in chronic airway obstruction.

The purpose of this study was to assess the relationship between the breathing pattern response to CO2 and the severity of mechanical impairment in twenty patients with COLD. The CO2 response was compared to that of a control group of twelve normal subjects. All patients had airway obstruction (FEV1 = 40 +/- 14% of predicted; means +/- SD) and hyperinflation (FRC = 154 +/- 23% of predicted). Tidal volume (VT), inspiratory and total cycle duration (TI, TT), occlusion pressure (P0.1) and endtidal PCO2 were measured at rest and during hyperoxic CO2 rebreathing. On the same day, in all patients, arterial blood gas analysis, spirometric and plethysmographic measurements were made. The slope (S) of the P0.1 response (SP 0.1) to increasing endtidal PCO2 was negatively correlated with airway resistance (r = -0.59; p less than 0.01). Although the flow response, S(VT/TI), was positively and closely correlated with SP 0.1 (r = 0.88; p less than 0.001), it also appeared to be independently influenced by obstruction (p less than 0.01). The tidal volume response, SVT, was principally correlated with inspiratory capacity (r = 0.90; p less than 0.001) and also, independently, with Vmax50 (p less than 0.01). SVT was diminished in seventeen patients, ten of whom only had a decreased S(VT/TI). The shortening in TI during hypercapnia was most marked in patients with the greatest S(P0.1), who did not have arterial hypercapnia at rest. These results suggest: that the poor VT response to CO2 in COLD patients is principally caused by a limitation in inspiratory volume expansion.(ABSTRACT TRUNCATED AT 250 WORDS)