A novel device for target controlled administration and reflection of desflurane--the Mirus™

Abstract

The Anaconda™ system is used to deliver inhalational sedation in the intensive care unit in mainland Europe. The new Mirus™ system also uses a reflector like the Anaconda; however, it also identifies end-tidal concentrations from the gas flow, injects anaesthetics during early inspiration, controls anaesthetic concentrations automatically, and can be used with desflurane, which is not possible using the Anaconda. We tested the Mirus with desflurane in the laboratory. Compared with an external gas monitor, the bias (two standard deviations) of the end-tidal concentration was 0.11 (0.29)% volume. In addition, automatic control was reasonable and maximum concentration delivered was 10.2%, which was deemed to be sufficient for clinical use. Efficiency was > 80% and was also deemed to be acceptable, but only when delivering a low concentration of desflurane (≤ 1.8%). By modifying the reflector, we improved efficiency up to a concentration of 3.6%. The Mirus appears to be a promising new device for long-term sedation with desflurane on the intensive care unit, but efficiency must be improved before routine clinical use becomes affordable.

Figure 1

(a) Anaconda device. (b) Set-up of the Anaconda: the Anaconda is inserted between the Y-piece (1) and the tracheal tube (2). Liquid isoflurane or sevoflurane is administered by a syringe pump into a hollow rod (Evaporator). An external gas monitor must be used (3 = sample gas line). Active and passive scavenging systems are available and should be connected to the gas outlet of the ventilator (black arrow). Sample gas should also be scavenged (4 = sample gas scavenging line). (c) Mirus device. (d) Set-up of the Mirus system: the interface (Mirus Exchanger) is inserted between the Y-piece (1) and the tracheal tube (2). The interface is connected with the control unit by a multilumen cable (blue line). An active anaesthesia gas scavenging system (black arrow), connected to the vacuum system, is provided by the manufacturer. (e) Volatile anaesthetic reflector of the Anaconda (two layers, left side) and of the Mirus (right side). In our second experiment, the Mirus reflector was replaced by a two-layered cut-out of the Anaconda reflector (dashed lines).

Figure 2

Experimental set-up of bench study. The Mirus control unit for desflurane, a ventilator, a carbon dioxide source and a gas monitor are connected to a test lung as shown. The control unit is positioned on a precision balance. Data from balance and gas monitor are stored online on a personal computer (PC): (1) ventilatory tubing; (2) Mirus interface; (3) bronchoscopy port; (4) Mirus interface cable; (5) carbon dioxide line; (6) sample gas line; (7) sample gas refeeding line and (8) serial communication cable.

Figure 3

(a) High-resolution recording (every 100 ms) of the concentration of desflurane (left axis, blue-green-red line and dots) and carbon dioxide (CO₂, right axis, black line) during three breaths, recorded by the Vamos gas monitor. During the beginning of the inspiratory flow phase (IF), CO₂ is still high because the first 100 ml of gas from the Mirus interface is passing the gas sampling port. Thereafter, the CO₂ trace decreases not down to zero, but to 0.2% (small orange arrow) indicating minor CO₂ reflection by the Mirus reflector. The desflurane trace clearly shows the expiratory plateau. By averaging each plateau (green lines), end-tidal concentration of all 1800 single breaths were determined. During the first inspired breath, the concentration decreases to about 50% (large orange arrow), indicating desflurane reflection. After the last two expirations, there is a high peak of the desflurane concentration during the inspiratory flow phase, which lasts only 0.5 s (red lines: mean concentration during this peak). Thereafter, the desflurane concentration drops to similar values as during the previous expiration, indicating the end of desflurane injection. (b) High-resolution recording (every 100 ms) of the desflurane concentration recorded by the Vamos gas monitor over 5 min, with a target concentration of 2.4% (black line) including the three breaths from Fig. 3a (shaded area). From the second breath onwards, 13 breaths with peaks during the initial inspiratory flow phase indicating desflurane vapour injection can be seen. At an end-tidal concentration of 3.11% (red arrow), the last injection is applied. Thereafter, the concentration decreases down to 2.02% (black arrow), after which desflurane is once more injected during the following nine breaths.

Figure 4

Bland–Altman diagram for comparison of the end-tidal concentrations measured by the gas monitor and those displayed by the Mirus. The difference between the measurements for each single breath is plotted against the mean of the two measurements. With high concentrations, there is a small overestimation by the Mirus, however, bias (0.11%, dashed line) and random measurement disagreement (0.29%, shaded area) are small and not clinically relevant.

Figure 5

Consumption of desflurane over the range of the target concentration set using the Mirus with the standard reflector (filled diamonds) and using the modified Mirus reflector (unfilled diamonds). The increase in consumption is disproportionately high with increasing concentrations, indicating a decrease in efficiency. The red line indicates an efficiency of 80%.