Can a drawover vaporizer be a pushover?

Abstract

SummaryBench testing was carried out to establish whether the vapour output from an OMV50 vaporizer, as used in the Triservice apparatus, difers according to whether the carrier gas is either drawn or pushed through the vaporizer. Results show that the diferences in output concentration between the two modes were clinically insign8cant. Key wordsAnaesthetic techniques; drawover, inhalation. Equipment; Triservice anaesthetic apparatus.The Triservice apparatus based on the use of the OMVSO vaporizer (Penlon) [l] was primarily designed to function as a drawover system for spontaneously breathing patients. When used for positive pressure ventilation this principle holds true with the vaporizer upstream of the self-inflating bag or suitable mechanical ventilator. The ventilator is used to suck the gas through the vaporizer instead of the patient's own inspiratory effort.The British Army field anaesthesia kit includes the Cape TC50 ventilator (Penlon) which can be used with the Triservice apparatus [2] (Fig. 1). This is a small, robust and simple ventilator. It works on the principle of a rubber bellows which sucks in fresh gas when it is expanded by a spring. The gas is then expelled from the bellows under the influence of a piston pushed by a cam. This is powered by a 240 v AC electric motor. There is a one-way valve on the gas intake and a Laerdal Resuscitator non-rebreathing valve at the patient end of the system. Both these valves are required to ensure the correct flow of anaesthetic gas. The ventilator is designed so that the two OMV50 vaporizers can be placed on the built-in back-bar. The apparatus is conventionally used as a drawover system, with the ventilator downstream of the supplementary oxygen and the vaporizers. This arrangement works very well [3].Unfortunately, in the field, this configuration of the apparatus is not always convenient. It may become necessary, for a variety of reasons, to have the vaporizer between the ventilator and the patient. These reasons include: (1) The Triservice apparatus was originally designed to use a self-inflating bag as the means of performing intermittent positive pressure ventilation (IPPV). The complete set-up could then be sat easily upon the shelf at the head of the Macvicar operating table. The TC50 ventilator is significantly heavier than a self-inflating bag, therefore in field use it is usually more convenient and practical to position it on the ground. In order to view the agent level window, or @ able to adjust the control lever whilst holding a mask on the patient's face, it is far easier and safer to position the OMV50 vaporizer on the shelf attached to the table rather than attach it to the back-bar of the TCSO. This means that the vaporizer must be between the ventilator and the patient. (2) The length of tubing required for the breathing system in drawover set-up is considerably greater than that required if the vaporizer can be positioned on the shelf between the ventilator and the patient. When lengths of tubing get lost or damaged in the field this ...

Drawbacks of pushovers

“…Under these circumstances, the output of the OMV 50 exceeds that set on the vaporiser scale. Other authors have described higher than expected agent outputs with the OMV 50 [3] and the Universal Portable Anaesthesia Complete vaporiser [4]; however, in these experiments vaporiser temperature was not controlled. In our experiments, the increased output in the pushover compared with the drawover mode disappeared at high minute volumes.…”

“…For reasons of convenience, ease of assembly and good working ergonomics, some anaesthetists assembled the Triservice apparatus using the TC 50 to push the fresh gas flow (pushover mode) through the Oxford Miniature Vaporiser (OMV 50). Taylor & Restall [3] studied this configuration and concluded that there was no difference in the performance of the OMV 50 in the pushover or drawover mode. Recently, the production of the TC 50 ventilator ceased as did servicing and sales of spare parts.…”

The Triservice Anaesthetic Apparatus: A Review

“…The ventilator is used to provide a gas piston from the distal end of the circuit which is described colloquially as the "pushover" arrangement. The output from drawover vaporisers and for the OMV is similar in both arrangements [3]. As shown in Figure 3, the SIB must be removed from the circuit during mechanical ventilation due to its compliance and the tendency for the system to predispose to "breath stacking".…”

Draw‐over or push over during manual ventilation with the Tri‐Service Anaesthetics Apparatus?

“…The authors discuss the coagulopathy induced by hydroxyethyl starch, but do not explain the rationale for its use as a volume replacement fluid in total hip replacement, despite their statement that 'there may be additive detrimental effects on haemostasis, leading to greater peri-operative blood loss'. While there is no systematic review of evidence for the risks and benefits of starch solutions in total hip replacement, their use in critically ill patients is not indicated [2]. Since alternative and less costly fluids are available, it is unclear why starch solution was used.…”

“…It was originally conceived as a draw-over system (gas pulled through the vaporiser by the patient or a self-inflating bag), but is increasingly used in a push-over mode (gas pushed through the vaporiser by a selfinflating bag or ventilator) for ergonomic reasons [2]. It has been recognised that the push-over configuration can lead to agent over-delivery [3].…”