Evaluating the efficiency of desflurane reflection in two commercially available reflectors.

ABSTRACT

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 (VT), respiratory rates (RR), and 'patient' concentrations (CPat) in a bench study. A test lung was ventilated with four settings (volume control, RR × VT 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 (flowin), the input concentration (Cin) was varied (1-17 vol%), and the same flow (flowex, Cex) was suctioned from the test lung. After equilibration, CPat was stored online and averaged; efficiency [%] was calculated [Formula see text]. Second (washout), flowin and flowex were stopped, the decline of CPat 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 CPat and VT. Efficiency inversely correlates with the product of CPat and VT 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.