The Effects of Chronic Airflow Limitation, Increased Dead Space, and the Pattern of Ventilation on Gas Exchange during Maximal Exercise in Advanced Cystic Fibrosis

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Abstract
Although exterional hypercapnea has been observed in patients with advanced cystic fibrosis (CF), the causes have not been fully elucidated. In 14 patients aged 15 to 35 yr of age with advanced CF, the effects of chronic airflow limitation (CAL), increased physiologic dead space (VD), and the timing components of ventilation .ovrhdot.VE) on gas exchange during maximal exercise were assessed. The patients were divided into those who retained CO2 during exercise, the CO2R group, and those who did not, the CO2NR group CO2 retention was defined as a rise in end-tidal CO2 tension of 5 mm Hg or more or to a value greater than 50 mm Hg during a progressive exercise test on a cycle ergometer. CO12 retention occurred in half the subjects, usually by the halfway mark of the test, and did not rise progressively as exercise continued. It was associated with a low .ovrhdot.VE caused by a low tidal volume (VT) that was the result of a short inspiratory time to total respiratory time ratio (0.33 .+-. 0.03 versus 0.38 .+-. 0.04, P < 0.02), whereas there was no difference in mean inspiratory flow or respiratory rate. Although the CO2R group had the worst CAL, with a FEV1 of 28 .+-. 7 versus 41 .+-. 12% predicted (p < .05) and a FVC of 42 .+-. 12 versus 61 .+-. 9% predicted (P < 0.01), the VT at maximal work expressed as a percentage of FVC was lower (45 .+-. 13 versus 60 .+-. 11, p < 0.005). VD did not correlate with CO2 retention but did correlate with VT (r = 0.79, p < 0.001). It was concluded that exertional CO2 retention in advanced CF occurred in the context of severe CAL and a pattern of ventilation associated with a cutoff in inspiration resulting in a reduced VT and hence a reduced .ovrhdot.VE.