We verified the utility of an oxygen economizer (Pendant Oxymizer) in assuring greater protection than nasal prongs against worsening of oxyhemoglobin resting desaturation (delta SaO2) induced by muscular exercise in 16 patients (ten with chronic obstructive pulmonary disease [COPD] and six with restrictive pulmonary disease). This worsening was quantified as desaturation surface accumulated within five minutes of exercise and was expressed in arbitrary units (au). Each patient carried out the same exercise three times, in a randomized fashion (breathing air or breathing supplemental oxygen [3 L/min] delivered by either nasal prongs or by oxygen economizer). In patients with obstructive disease, delta SaO2 was reduced from 38 +/- 12.0 au when they were breathing air to 18.1 +/- 11.7 au when breathing oxygen by nasal prongs (p less than 0.001) and to 10.1 +/- 9.5 au when breathing oxygen by economizer (p less than 0.001). In patients with restrictive disease, delta SaO2 was reduced from 35.6 +/- 9.9 au when breathing air to 14.9 +/- 10.2 au breathing oxygen by nasal prongs (p less than 0.01) and to 13.7 +/- 10.3 au breathing oxygen by economizer (p less than 0.01). The difference between breathing by economizer and nasal prongs was significant (paired t-test; p less than 0.01) only in patients with COPD. One explanation could lie in the different values of the respiratory rate, which was significantly greater in patients with restrictive disease (20.7 +/- 1.2 breaths per minute at rest and 25.8 +/- 1.5 with exercise) than in patients with obstructive disease (15.3 +/- 1.2 breaths per minute at rest and 20.8 +/- 1.4 with exercise).