Isolation valve increases work of breathing in a mechanically ventilated pediatric animal model

BACKGROUND: Accurate measurement of minute ventilation and, hence, the accurate calculation of oxygen consumption (V̇(O₂)), using open-circuit indirect calorimetry may require separation of inspired and expired gases with an isolation valve when continuous bias flow is present. We hypothesized that the use of an isolation valve in the ventilator circuit would increase work of breathing (WOB). METHODS: We studied the effect of a valve apparatus on WOB in 5 mechanically ventilated, spontaneously breathing, healthy lambs. Animals were ventilated with a continuous-flow, volume ventilator (F(IO₂) 0.21, pressure support 5 cm H₂O). WOB of the animal (WOB(pt)) and pressure- time product (PTP) were determined with a Bicore CP-100 respiratory monitoring device. RESULTS: WOB and PTP were elevated by approximately 100% with the isolation valve present (p = 0.04). CONCLUSIONS: We conclude that the use of this isolation valve to separate inspired and expired gases significantly increases WOB in intubated healthy lambs. We speculate that this increase in WOB may be associated with increased V̇(O₂) in critically ill patients, thereby altering the results of metabolic monitoring.