Use of the Ventilatory Equivalent to Separate Hypermetabolism from Increased Dead Space Ventilation in the Injured or Septic Patient

Abstract

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. Patients [29] undergoing uncomplicated elective operation had a mean preoperative minute ventilation of 3.44 .+-. 0.84 l/min per m2, a VO2 [O2 uptake] of 0.132 .+-. 0.022 l/min per m2, and VCO2 [CO2 uptake] of 0.105 .+-. 0.017 l/min per m2, and the postoperative values on the 3rd-5th day were not statistically different. The ventilatory equivalent (VECO2) or the liters of air moved per liter of CO2 excreted was 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%. VECO2 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 VECO2 supplements what is learned from successive numbers representing the trend of VD/VT.