Critically ill patients must often breathe spontaneously through an endotracheal tube that acts as a fixed inspiratory and expiratory tubular airway resistor. Although this practice is common, its effect on the pattern of breathing is not known. The mean breathing patterns of seven normal, healthy male subjects and eight male patients who had undergone upper abdominal surgery 2-4 days previously were studied breathing through a mouthpiece fitted in random order with a 5, 6, 7, 8, or 15 mm diameter (17 mm long) resistor. These diameters were selected because they simulate the pressure-flow relationships of adult endotracheal tubes. With the 15 mm aperture, the patients had a greater breathing frequency (f) than did the normal subjects (21 +/- 5 [SD] vs. 14 +/- 4 breaths/min, P less than 0.01) as well as a smaller mean tidal volume (VT). In both groups, minute ventilation (VE) and f progressively decreased as resistance was increased by decreasing the aperture size from 15 to 16 mm. In the normal subjects but not the patients, VT also progressively decreased. When the diameter was decreased from 6 mm to 5 mm, there were increases in VT and decreases in f that were more marked in the normal subjects. In both groups, the changes in VE were accompanied by decreases in mean and peak inspiratory and expiratory flow rates. Throughout the study, oxygen consumption (VO2) and carbon dioxide production (VCO2) did not change. This, coupled with the decreases in VE resulted in decreases in the ventilatory equivalents to CO2 and O2 (VE/VCO2, VE/VO2).(ABSTRACT TRUNCATED AT 250 WORDS)