Can a drawover vaporizer be a pushover?

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.

Evaluation of an Oxford Miniature Vaporizer Placed in-Circuit during the Maintenance Phase of Low-Flow Anaesthesia

The aim of the study was to assess Oxford Miniature Vaporizer output when mounted in-circuit during the maintenance phase of anaesthesia, using isoflurane, controlled ventilation and a fresh gas flow rate less than 1 l/min.

Twenty patients of ASA Physical Status I and II were recruited from routine general surgical lists. All patients were paralysed and ventilated. An out-of-circuit isoflurane vaporiser was used during the induction period (first 20 to 30 minutes). Anaesthesia was maintained using an Oxford Miniature Vaporizer placed in-circuit, using a fresh gas flow of 500 ml/min. The end-tidal isoflurane concentration was recorded for 90 minutes at five-minute intervals using a sidestream agent analyser. Two groups were compared, with the Oxford Miniature Vaporizer dial setting at either the 0.5 mark (low output setting) or at the 1.0 mark (higher output setting).

At a dial setting of 0.5, the Oxford Miniature Vaporizer produced a steady end-tidal isoflurane of 0.63% (95% confidence interval 0.60 to 0.66). However, when the dial was turned to 1.0 the output was almost always excessive and had to be reduced.

These findings indicate that a stable, predictable and clinically useful output can be achieved when the Oxford Miniature Vaporizer is positioned in-circuit using low-flow and controlled ventilation.

Sevoflurane output from the Oxford Miniature Vaporizer in drawover mode

The sevoflurane output from the Oxford Miniature Vaporizer (OMV) was measured in a series of bench tests in a drawover system. Using a range of settings on two OMVs in series, output ranged from 0.5 to 5.9% with a 600 ml tidal volume at a rate of 12 breaths/min produced by a Penlon Multivent. The OMVs' output was steady over 40 minutes. In tests with tidal volumes ranging from 300 ml to 1200 ml, the OMVs' output was lower at higher minute volumes. When continuous flow of carrier gas at 7.2 l/min was used, the output was lower than at the equivalent intermittent flow rate. Using two OMVs with a drawover system appears to be a feasible technique for the induction and maintenance of sevoflurane anaesthesia. Clinical tests of sevoflurane drawover anaesthesia are now required so that the advantages of sevoflurane can gain wider application in field and military anaesthesia.

Drawbacks of pushovers

Some military anaesthetists have started to use the Oxford Miniature Vaporiser in a pushover configuration with the Triservice anaesthetic apparatus. This vaporiser performs identically in the pushover and drawover configurations with the Cape TC 50 ventilator. We tested the Oxford Miniature Vaporiser with three other ventilators and found variable performance. When used in the pushover configuration with the Laerdal bag at normal minute volumes, the Oxford Miniature Vaporiser delivers a higher than set output.