An Automation System Equivalent to the Douglas Bag Technique Enables Continuous and Repeat Metabolic Measurements in Patients Undergoing Mechanical Ventilation

Abstract

PurposeTo develop a system that is equivalent to the gold standard Douglas Bag (DB) technique for measuring oxygen consumption (V̇o2), carbon dioxide generation (V̇co2), and respiratory quotient (RQ) and to validate its use in clinical settings. MethodsThis was a prospective, observational study conducted at a suburban, quaternary care teaching hospital. Healthy volunteers and patients 18 years or older who received mechanical ventilation were enrolled. FindingsData from 3 healthy volunteers and 7 patients were analyzed in this study. The interrater reliability between the automation device and DB methods were 0.999, 0.993, and 0.993 for V̇o2, V̇co2, and RQ, respectively. In healthy volunteers, mean (SD) V̇o2, V̇co2, and RQ measured by DB were 411 (100) mL/min, 288 (79) mL/min, and 0.70 (0.03) at high fraction of inspired oxygen (Fio2) and 323 (46) mL/min, 280 (45) mL/min, and 0.85 (0.05) at normal Fio2, respectively. V̇o2 was significantly higher (P < 0.05) and RQ was lower (P < 0.01) in the high Fio2 group as compared to those in the normal Fio2 group. Values measured by the automation system were 227 (31) mL/min, 141 (18) mL/min, and 0.62 (0.04) at high Fio2 and 209 (25) mL/min, 147 (18) mL/min, and 0.70 (0.06) at normal Fio2, respectively. RQ was significantly lower (P < 0.05) in the high Fio2 group as compared to the normal Fio2 group. We also successfully performed continuous and repeat measurements by using the device. The longest measurement reached 12 hours 15 minutes, including 50 cycles of repeat measurements that are equivalent to the DB technique as described above. ImplicationsWe developed an automation system that enables repeat measurements of V̇o2, V̇co2, and RQ, and the accuracy was equivalent to the DB technique. High Fio2 may decrease RQ because of an increase in V̇o2.