To determine the acute physiologic effects of removing oxygen from patients with chronic obstructive pulmonary disease (COPD) who are receiving long-term oxygen therapy, we made serial measurements in 20 patients during and after stopping low-flow oxygen therapy. Removing oxygen caused an increase in pulmonary vascular resistance, requiring 2 to 3 h to reach a new steady state. Removing oxygen therapy increased pulmonary vascular resistance index (PVRI) by 31% during rest (8.14 +/- 0.61 versus 6.23 +/- 0.51 units, p less than 0.001) and by 29% during exercise (8.11 +/- 0.9 versus 6.31 +/- 0.7, p less than 0.001). The increase in PVRI occurred because of an increase in pulmonary arterial pressure without a change in pulmonary capillary wedge pressure or cardiac index. At rest the increase in pulmonary arterial pressure caused by stopping oxygen correlated with the decrease in arterial oxygen saturation (r = 0.70, p less than 0.01). Removing oxygen decreased stroke volume index during rest and exercise. Although removing oxygen increased pulmonary vascular resistance, it did not affect systemic arterial pressure or vascular resistance. Stopping oxygen reduced arterial and mixed venous oxygen tension and oxygen delivery during rest and exercise. In patients who had a normal PaCO2 while breathing room air, removing oxygen therapy increased their oxygen consumption; conversely, in those patients who had an elevated PaCO2 while breathing room air, stopping oxygen therapy reduced oxygen delivery and oxygen consumption.(ABSTRACT TRUNCATED AT 250 WORDS)