Evaluating the efficiency of desflurane reflection in two commercially available reflectors

With the AnaConDa™ and the MIRUS™ system, volatile anesthetics can be administered for inhalation sedation in intensive care units. Instead of a circle system, both devices use anesthetic reflectors to save on the anesthetic agent. We studied the efficiency of desflurane reflection with both devices using different tidal volumes (V_T), respiratory rates (RR), and ‘patient’ concentrations (C_Pat) in a bench study. A test lung was ventilated with four settings (volume control, RR × V_T: 10 × 300 mL, 10 × 500 mL, 20 × 500 mL, 10 × 1000 mL). Two different methods for determination of reflection efficiency were established: First (steady state), a bypass flow carried desflurane into the test lung (flow_in), the input concentration (C_in) was varied (1–17 vol%), and the same flow (flow_ex, C_ex) was suctioned from the test lung. After equilibration, C_Pat was stored online and averaged; efficiency [%] was calculated \((100 \times \left( {1 - \left( {{\text{flow}}_{\text{in}} \times \left( {{\text{C}}_{\text{in}} - {\text{C}}_{\text{ex}} } \right)/{\text{C}}_{\text{Pat}} \times {\text{RR}} \times {\text{V}}_{\text{T}} } \right)} \right)\). Second (washout), flow_in and flow_ex were stopped, the decline of C_Pat was measured; efficiency was calculated from the decay constant of the exponential regression equation. Both measurement methods yielded similar results (Bland–Altman: bias: −0.9 %, accuracy: ±5.55 %). Efficiencies higher than 80 % (>80 % of molecules exhaled are reflected) could be demonstrated in the clinical range of C_Pat and V_T. Efficiency inversely correlates with the product of C_Pat and V_T which can be imagined as the volume of anesthetic vapor exhaled by the patient in one breath, but not with the respiratory frequency. Efficiency of the AnaConDa™ was higher for each setting compared with the MIRUS™. Desflurane is reflected by both reflectors with efficiencies high enough for clinical use.