Inspiratory resistance effects on exercise breathing pattern relationships to chemoresponsiveness

This study examined the effects of added inspiratory resistance (R5 5 cm H2O.L-1.S-1) on the relationship between exercise breathing pattern and resting hypercapnic ventilatory responsiveness (HCVR). Twelve men completed an HCVR test and two progressive intensity exercise tasks with minimal (R0) and elevated (R5) resistance. Peak oxygen uptake, and peak power output were not different, but peak VE was decreased with the R5 load. Exercise ventilation (VE was tightly coupled to VCO2 (r = 0.97) as was mean inspiratory flow rate (VT/TI, r = 0.95), but not duty cycle (TI/TTOT, r = 0.39). With imposition of R5, VT/TI was depressed (p < 0.05) at mild (approximately 40% VO2peak) to peak exercise intensities, whereas TI/TTOT was relatively unaffected. At both moderate (approximately 60% peak VO2) and peak exercise intensities, VE was positively correlated (r = 0.62, p < 0.05 and r = 0.82, p < 0.01, respectively) to subjects' HCVR. However, when normalized, VE.VCO2(-1) was significantly correlated to HCVR only at peak exercise ventilation during the R0 load. Analysis of the exercise breathing pattern revealed that at both moderate and peak exercise intensities, VT/TI was positively correlated to HCVR, but TI/TTOT was not. The imposition of R5 decreased the slope of the relationship between exercise VT/TI and HCVR at both moderate and peak exercise intensities, and weakened the positive correlation at the moderate exercise intensity. Our analysis indicates that: 1) the positive correlation between exercise hyperpnea and HCVR is mediated by the mean inspiratory flow rate rather than the duty cycle component of the breathing pattern and, 2) at moderate exercise the relationship between mean inspiratory flow rate and resting HCVR is more sensitive to added inspiratory resistance than minute ventilation per se. These findings suggest that the degree of influence resting HCVR has on exercise hyperpnea is dependent upon the magnitudes of both the ventilatory hyperpnea and mechanical loading placed on the ventilatory system.