Circulatory transport of oxygen in patients with chronic airflow obstruction exercising maximally

Twenty patients with chronic bronchitis were exercised maximally on a bicycle ergometer. At exhaustion, arterial partial pressure of O2 (Pao2) decreased and that of CO2 (Paco2) increased in 8 patients (Subgroup 1). Arterial PO2 increased and Paco2 did not change in the remaining 12 patients (Subgroup 2). Forced expiratory volume in one second was lower in Subgroup 1 than in Subgroup 2. The decrease in arterial pH and the response of cardiac output were the same in both subgroups. Mixed venous PO2 decreased to 23 mm Hg or less in all patients in Subgroup 1 and in 3 patients in Subgroup 2; it stayed above 25 mm Hg in the remaining 9 patients. Our data imply that the ventilatory response to exercise is insufficient not only in Subgroup 1 but also in Subgroup 2 where the patients failed to hyperventilate in response to exercise-induced acidosis and that an unused oxygen reserve is still present at exhaustion in some patients with chronic airflow obstruction.