Use of the ventilatory equivalent to separate hypermetabolism from increased dead space ventilation in the injured or septic patient.

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 VO2 of 0.132 +/- 0.022 L/min/m2, and VCO2 of 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 of CO2 excreted has been used instead of the dead space/tidal volume (VD/VT) ratio 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.CO2 increased 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.CO2 supplements what is learned from successive numbers representing the trend in VD/VT.