1-1-8 one-step sevoflurane wash-in scheme for low-flow anesthesia: simple, rapid, and predictable induction

Diamedica Draw-over Vaporiser: bench testing the UK Defence Anaesthesia System in the deployed environment

INTRODUCTION

The Diamedica Draw-over Vaporiser 2 (DDV2) is the sevoflurane vaporiser used by the UK Defence Medical Services to provide deployed volatile general anaesthesia. The Defence Anaesthesia System employs the DDV2 with a turbine-driven ventilator as a 'push-over' vaporiser, a modification from the manufacturer's design. We investigated sevoflurane delivery at varying minute volumes (MVs), vaporiser settings and temperatures in this configuration.

METHODS

A range of DDV2 settings (1%, 2%, 3%, 4% and induction) and MVs (2, 4, 6 and 8 L/min at 12 ventilations per minute) were tested at two ambient temperatures (20 and 30±3°C) over 30 min. A supplemental experiment, simulating anaesthesia during damage control surgery, was also completed, where he DDV2 was set to 2% with a 6 L/min MV for 90 min.

RESULTS

In both experiments, two distinct phases of sevoflurane delivery were noted, a 'wash-in phase' followed by a 'maintenance period'. The wash-in phase normally lasted less than 5 min. During the maintenance period at low MVs and vaporiser settings the DDV2 delivered a constant output, while at higher MVs and settings vapour output fell predictably. At 20±3°C, using DDV2 settings likely to be encountered in clinical practice, sevoflurane delivery was within 20% of that set. Higher vaporiser settings, MVs and temperatures resulted in greater variation between vaporiser setting and agent delivery. This variation is explained by the incomplete temperature compensation of the DDV2.

CONCLUSIONS

The DDV2 functions predictably at a range of settings, MVs and temperatures. Anaesthetic delivery in the defence anaesthesia configuration is like that previously described in the draw-over configuration. The equipment was found to be reliable and robust. This experimental work supports the continued use of the Defence Anaesthesia System for the delivery of and training in deployed general anaesthesia.

The effect of different flow levels and concentrations of sevoflurane during the wash-in phase on volatile agent consumption: a randomized controlled trial

PURPOSE

The standard procedure for low-flow anesthesia usually incorporates a high fresh gas flow (FGF) of 4-6 L/minute during the wash-in phase. However, the administration of a high FGF (4-6 L/min) increases the inhaled anesthetic agent consumption. This study was designed to compare the sevoflurane consumption at 2 rates of flow and vaporizer concentration during the wash-in period.

METHODS

Patients were randomly enrolled into high FGF (HFGF) (n = 30) and low FGF (LFGF) (n = 30) groups. During the wash-in, the HFGF group received 4 L/minute FGF with a sevoflurane vaporizer setting of 2.5%, and the LFGF group received 1 L/minute FGF with a vaporizer setting of 8%. Once the wash-in was complete, anesthesia maintenance was performed with 0.5 L/min FGF with a vaporizer setting of 2.5-4.5% in both groups. The patient demographic data, bispectral index values, hemodynamic variables, wash-in time, sevoflurane consumption during the wash-in phase, and total sevoflurane consumption were analyzed.

RESULTS

The median sevoflurane consumption in the wash-in phase was 8.2 mL (7.1-9.3) in the HFGF group and 2.7 mL (2.2-3.1) in the LFGF group (p = 0.001). The mean total sevoflurane consumption was 17.41 ± 3.58 mL in the patients who received HFGF and 14.93 ± 3.57 mL in the LFGF group (p = 0.001). The mean wash-in completion time was 12.49 ± 2.79 min in the HFGF group and 3.35 ± 0.67 min in the LFGF group (p = 0.001).

CONCLUSIONS

The anesthetic agent consumption during the wash-in phase was approximately 3 times lower with the administration of sevoflurane at 1 L/minute FGF than the use of 4 L/minute FGF.