Normal subjects and surgical patients were studied with a noninvasive canopy-spirometer system which provides prolonged measurements of gas exchange and pattern of breathing. Values for normal subjects agreed with published values. Twenty-nine patients undergoing uncomplicated elective operation had a mean preoperative minute ventilation of 3.44 $$ 0.84 L/min/m2, a Vo2of 0.132 $$ 0.022 L/min/m2, and Vco2of 0.105 $$ 0.017 L/min/m2, and the postoperative values on the third to fifth day were not statistically different. The ventilatory equivalent (V.E.co2) or the liters of air moved per liter $$ for the indication of levels of minute ventilation, which are excessive for the associated metabolic demands for gas exchange. Thirty-eight runs on 18 acutely ill surgical patients showed mean increases in minute ventilation of 85%; the associated increases in metabolism averaged 17%. Therefore, their V.E.co2increased from a normal of 31 $$ 6 to 50.7 $$ 8, indicating a sharp increase in dead space ventilation. The additional clinical information provided by the serial graphic presentation of V.E.co2supplements what is learned from successive numbers representing the trend in VD/VT.