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. 2009 Feb;23(1):11-9.
doi: 10.1007/s10877-008-9158-4. Epub 2009 Feb 3.

Technical performance and reflection capacity of the anaesthetic conserving device--a bench study with isoflurane and sevoflurane

Andreas Meiser  1 , Martin Bellgardt, Javier Belda, Kerstin Röhm, Heinz Laubenthal, Clemens Sirtl
Affiliations

Technical performance and reflection capacity of the anaesthetic conserving device--a bench study with isoflurane and sevoflurane

Andreas Meiser et al. J Clin Monit Comput. 2009 Feb.

Abstract

Objective: The anaesthetic conserving device (AnaConDa), Sedana Medical, Sundbyberg, Sweden) facilitates administration of isoflurane or sevoflurane by liquid infusion. An anaesthetic reflector inside the device conserves exhaled anaesthetic and re-supplies it during inspiration. In this bench study, we examined the influence of infusion rates and ventilatory settings on the resulting anaesthetic concentrations on patient (C(pat)) and ventilator side of the reflector (C(loss)) to describe its technical performance.

Methods: A Puritan Bennett 840 ICU ventilator (Pleasanton, US), AnaConDa, and a test lung (3 l-chloroprene-bag) were assembled. Infusion rates (IR, 0.2-50 ml h(-1)), respiratory rates (RR, 5-40 breaths min(-1)), and tidal volumes (V(T), 0.3, 0.5, and 1.0 l) were varied. C(pat) was measured via a thin catheter in the middle of the 3 l-bag in steady state (online data storage and averaging over >10 min). C(loss) was calculated from IR (to yield the volume of vapour per unit of time), and expired minute volume (in which the vapour is diluted) on the assumption that, in the steady state, input by liquid infusion equals output through the reflector.

Results: At lower concentrations (C(pat) < 1 vol%) the ratio C(loss)/C(pat) was constant (R(C) = 0.096 +/- 0.012) for all combinations of IR, RR and V(T), both for isoflurane and sevoflurane. The device could efficiently reflect up to 10 ml vapour per breath (e.g. 2 vol% in 0.5 l). When exceeding this capacity, surplus vapour "spilled over" and R(C) markedly increased indicating decreased performance.

Conclusions: The triple product minute volume times R(C) times C(pat) describes anaesthetic losses through the reflector. It can easily be calculated as long as the 10 ml reflection capacity is not exceeded and thus R(C) is constant. Increased minute ventilation necessitates increasing the IR to keep C(pat) constant. When using large V(T) and high C(pat) "spill over" occurs. This effect offers some protection against an inadvertent overdose.

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Figures

Fig. 1
Fig. 1
Experimental setup. (1) Anaesthesia gas scavenging, (2) ICU ventilator, (3) inspiratory hose, (4) expiratory hose, (5) Y-piece, (6) anaesthetic conserving device, (7) catheter mount (with bronchoscopy port), (8) test lung (3 l chloroprene bag), (9) thin catheter, (10) gas sampling tube, (11) gas monitor, (12) redelivery of sample gas, (13) serial communication cable, (14) notebook computer for online data sampling, (15) syringe pump (16) syringe filled with liquid isoflurane or sevoflurane. MV expiratory minute ventilation, carrying anaesthetic vapour through the anaesthetic reflector. Closs mean anaesthetic concentration in the gas expired through the anaesthetic reflector. Cpat mean anaesthetic concentration inside the 3 l bag under steady state conditions.
Fig. 2
Fig. 2
(a) and (b) Characteristic curves of AnaConDa® for isoflurane (a) and sevoflurane (b) with different ventilatory settings. The mean anaesthetic concentrations on the ventilator side of the anaesthetic reflector (Closs, [vol%]) plotted against the mean anaesthetic concentrations in the test lung (Cpat, [vol%]). RR respiratory rates, VT tidal volumes. Each curve describes a different ventilatory setting, each point a different infusion pump rate. The straight line delineates a concentration ratio Closs/Cpat of 0.097 (isoflurane) and 0.096 (sevoflurane) respectively.
Fig. 3
Fig. 3
Comparison of the circle system (CS) with the Anaesthetic Conserving Device (ACD). A classical anaesthesia machine consists of the fresh gas supply with the interposed vaporizer, the CS with soda lime, and a ventilator usually in form of a bag in bottle system. In the absence of patient uptake and leaks, the outflow from the CS equals the FGF and the anaesthetic concentrations in fresh gas, inspired and expired air will all be the same after wash in (CCS). Losses of anaesthetic vapour can be described as the product FGF times CCS. With the ACD, the anaesthetic is delivered as a liquid via a syringe pump. Closs, the mean concentration on the ventilator side of the device, is 10.4 times lower than Cpat. Vapour losses can be calculated as the product minute ventilation (MV) times Closs. Vapour losses can also be imagined to be carried away by a clearance flow flushing the patient side of the ACD (semicircular arrow). The clearance is a fraction (Closs/Cpat = 0.097) of the minute volume. Vapour losses can be calculated as the product clearance times Cpat.
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