Exercise performance with added dead space in chronic airflow obstruction

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Abstract
Individuals with chronic airflow obstruction (CAO) are thought to have limited exercise tolerance primarily because of impaired ventilatory mechanics. The effects of added external dead space (DS) on exercise capacity [maximum O2 consumption (.ovrhdot.VO2 max)], maximum exercise ventilation (.ovrhdot.VEmax) and blood gases (arterial PO2, PCO2, pH) were studied in 22 patients with CAO [forced expired volume at 1s (FEV1) = 0.96 .+-. 0.41 l]. Maximum exercise testing (Emax) was performed by incremental cycle ergometry. Patients exercised at base line (BL) and with DS (0.25 l if FEV1 < 0.8 and 0.50 l if FEV1 > 0.8 l), in random-order single-blind fashion. DS resulted in a 12.2% increase in .ovrhdot.Vemax (P < 0.001); tidal volume increased (P < 0.025) while respiratory frequency was unchanged. The .ovrhdot.VO2 max and maximum CO2 production decreased (P < 0.001) with DS. Arterial PCO2 at rest and at exhaustion increased with DS (P < 0.001). The pH and arterial PO2 showed small declines at rest and at Emax. Thus, at the lower maximum work load achieved with DS, the patients ventilated more and tolerated a higher arterial PCO2 and a lower arterial PO2 and pH before stopping from dyspnea as compared with the BL exercise run. The .ovrhdot.VO2 max of 9 normal control subjects was unaffected by the addition of DS. Although .ovrhdot.VEmax can be increased in CAO patients with DS, this increase is not sufficient to prevent further CO2 retention or a decrease in exercise capacity. Exercise performance is limited primarily by impaired ventilatory mechanics in CAO.

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