TITLE: A NEW DEVICE TO REDUCE INHALATIONAL AGENT EXPENDITURE

AUTHORS: Mats G. Enlund, MD, PhD¹, Hans R. Lambert, MSc², Lars Wiklund, MD, PhD¹
AFFILIATION: ¹Uppsala University, Uppsala, Sweden; ²Louis Gibeck AB, Upplands Väsby, Sweden.

INTRODUCTION: A notable amount of a vaporized inhalational agent is evacuated to the atmosphere, and a certain part does not reach the patient. This is uneconomical, and it might also contribute to the green-house-effect (1). In a laboratory setting, a new anesthesia saving device (ASD), which is a development of the well-known heat and moisture exchanger (HME) (2), seems to have the capacity to significantly reduce the waste of isoflurane. In the present study, this new device was tested clinically.
METHODS: After Regional Ethics Committee approval 16 patients admitted for arthroscopic crucial ligament repair were anesthetized. Propofol was used for induction, followed by a remifentanil infusion to a target blood concentration of 4-6 mg/ mL. After endo-tracheal intubation isoflurane was given to an initial end-tidal concentration (Et) of 0.5%, adjusted according to EEG/EMG and clinical responses (3). The tidal volume was set to 10 mL/kg BW, the respiratory rate was 12 br/min (I/E 1:2), and the fresh gas flow was adjusted to give Et-CO₂ of 5.3 kPa in all patients. Randomly, isoflurane was given either through a vaporizer and a Bain coaxial system (control, C), or through the same system without a vaporizer, but with an ASD connected between the endo-tracheal tube and the Bain system (test, T). A syringe pump delivered fluid isoflurane to the ASD, in which the agent was vaporized. The isoflurane concentration was measured at the proximal end of the endo-tracheal tube (Et) and in the evacuation system (mixed). The vaporizer was weighed before and after anesthesia (C) (isoflurane density 1.495). The volume of isoflurane delivered to the ASD by the syringe pump was measured (T). A Datex AS 3 monitor was used for gas measurements. The M-W U-test was used for statistics.
RESULTS: Preliminary results of the first 11 patients, 5 C and 6 T, indicated that the Et-isoflurane concentration, the fresh gas flow and Et-CO₂ were comparable in the groups. The isoflurane concentration in the evacuation system differed significantly between the groups, median value 0.21 and 0.10% in group C and T, respectively (p<0.01). The median consumption of isoflurane was 0,38 and 0,22 mL/kg/h/%Et-isoflurane in group C and T, respectively (p<0.01).
DISCUSSION: The efficiency of the ASD was evident. It remains to investigate its cost and environmental saving capacity in comparison with a low-flow system.
REFERENCES:

1. Acta Anaesthesiol Scand 1991;35:40.

2. Anaesthesia 1997;52:144. 3. Br J Anaesth 1994:73:174.