17. DRAWOVER ANAESTHESIA

Drawover anaesthesia is simple. The equipment is robust, versatile, easily maintained, relatively inexpensive, portable and does not need a pressurised gas supply, regulators or flow meters. In many parts of the world a regular supply of compressed gas is not available. The drawover vaporisers are less complex and have basic temperature compensation.

Drawover equipment is designed to provide anaesthesia without requiring a supply of compressed gas. In drawover systems the carrier gas (air or air/oxygen) is drawn though the vaporiser (adding the vapour from the liquid) either by the patient’s own respiratory efforts or by a self-inflating bag or manual bellows with a one-way valve placed downstream from the vaporiser. (Supplemental oxygen is administered via a T-piece connection mounted on the intake port of the vaporiser). Drawover systems operate at less than, or at ambient pressure, and flow though the system is “intermittent”, varying with different phases of inspiration and ceasing in expiration. A one-way valve prevents reverse flow in the circuit.

This is different to plenum anaesthesia in which a carrier gas (compressed gas) is pushed though the vaporiser at a constant rate (continuous flow). In plenum systems the carrier gas and vapour is then collected in a breathing system with a reservoir bag or bellows. Plenum systems are more technically complex and need a well-regulated, constant, positive pressure gas supply. If the compressed gas supply ends, so does the anaesthetic. They require a more sophisticated anaesthetic machine (e.g. Boyles machine).

Supplemental Oxygen

The 21% oxygen in air is diluted by the addition of vapour in the vaporiser, allowing a potentially “hypoxic mixture” to be delivered to the patient. This is a theoretical problem rather than a practical one, as the vapour concentration is small, and it is unlikely that the inspired oxygen concentration would fall below 18%.

It is important to consider the respiratory physiological effects of general anaesthesia that tend to reduce ventilation and increase shunting of blood within the lung (V/Q mismatch). Therefore hypoxia becomes a clinical problem with inhalation agents that decrease ventilation (e.g. halothane, isoflurane, enflurane) with spontaneous ventilation (SV) in air and supplemental oxygen is required. The problem is reduced, but not abolished when applying intermittent positive pressure ventilation (IPPV). Ether can be used in air (without supplemental oxygen), though for IPPV when used without oxygen in air with spontaneous respiration, some patients may become hypoxic.

In drawover systems supplemental oxygen is administered via a T-piece connection mounted on the intake port of the vaporiser. To maximise the inspired oxygen concentration a “reservoir tube” is attached to the T-piece. A one metre length of tubing with an internal volume of 415 ml allows an inspired oxygen concentration of at least 30% with a flow rate of 1.0 l/min, and 60% at 4 l/min, at normal adult ventilation. With higher respiratory rates and/or tidal volumes, the inspired oxygen concentration falls due to increased air dilution.

Breathing System

The drawover vaporiser is connected by 22 mm tubing to a self-inflating bag or bellows. This is then connected by tubing to the patient’s airway device. The breathing system must contain at least two valves to make the gas flow in the correct direction. There should be one valve at the patient end to ensure that expired gas passes to the atmosphere. Another valve is needed to prevent gas flowing back up into the vaporiser rather than down to the patient. The PAC vaporiser has a built in valve and the self-inflating bag is mounted on a T-piece limb.

Drawover Vaporisers

The volume of carrier gas passing though the vaporiser is determined by the patient’s tidal volume and respiratory rate. A proportion of the carrier gas is allowed to enter the vaporiser chamber and the remainder flows though a bypass channel. The gas flows then combine. The ratio of the flows and the saturated vapour pressure of the inhalation agent will determine the final concentration. Increasing the area of the vaporising chamber by inserting wicks will improve vaporisation but also increase airflow resistance. The ideal drawover vaporiser needs to have low internal resistance to gas flow to allow easy spontaneous ventilation, while the vapour output should be constant for a given dial setting over a wide range of minute volumes and ambient temperatures.

Plenum vaporisers have a constant driving pressure and predictable flow rates. They will operate effectively with increased internal complexity and resistance. Modern plenum vaporisers still have performance limitations at extremes of flow rate and temperature, but they are generally more accurate than drawover vaporisers.

Temperature Compensation

As vapour is liberated, the temperature of the liquid volatile agent falls due to the latent heat of vaporisation. This causes a fall in the saturated vapour pressure and lowers the output of the vaporiser. Temperature compensation is managed in two basic ways. The first is to provide a large heat-sink of conductive material (water bath or mass of metal), the dimensions of which are limited by size and portability. Heat is conducted from the heat-sink to the volatile liquid to minimise the fall in temperature. The second method is to vary the vapour chamber output with temperature, so that more carrier gas is allowed to pass though the vapour chamber as the temperature falls, and less as it rises. This is achieved by bimetallic strips and/or ether filled bellows in plenum vaporisers, but they cause an increase in the internal resistance. Some drawover vaporisers have basic thermo-compensation devices (EMO, PAC).

Drawover vaporisers theoretically should not be used as a plenum vaporiser, as the output may not be the same as the setting. Most plenum vaporisers cannot be used for drawover anaesthesia because their internal resistance is too high.

If a drawover vaporiser needs filling during an anaesthetic, the vaporiser must be turned to the zero position before opening the filling port. If the vaporiser is left “on” and the filling port opened, air will be drawn into the vaporising chamber and a dangerously high concentration of inhalation agent can be delivered to the patient.

EMO

EMO (Epstein Macintosh Oxford) is designed for use with ether and must not be used with halothane.

The temperature compensation device of the EMO vaporiser is a sealed canister containing liquid ether attached to a spindle, automated by opposing springs. The thermo-compensation valve is automatic and can be seen though a small window on top of the vaporiser. When the temperature of the vaporiser is within its working range (10 to 30 ºC) a black ring is visible in the window. If the vaporiser overheats a red ring also appears. If the vaporiser is too cold the black ring disappears and only the aluminium disc is visible. The metal disc will also be visible if the thermo-compensation device breaks. The vaporiser should not be used if it is too hot or cold.

The splitting system comprises two concentric brass cylinders with holes, one of which rotates with the dial setter, thus altering the overall ratio between vapour chamber and bypass flow. The pointer may stick after prolonged use due to a build-up of sticky deposits around the brass cylinders. These can be removed and cleaned. A setting gauge is available from Penlon to position the splitting device correctly. Alternatively a 0.1 inch (2.6 mm, 8 French gauge, 12 Stubs needle gauge) wire can be used. To calibrate the dial properly, the central screw should be loosened and the dial placed in the 6% position. The setting gauge is placed in the aperture, though the temperature compensator portal, and the screw is tightened until the gauge is lightly gripped.

The vaporising chamber sits in a water bath that acts as a heat sink. (New vaporisers will have an empty water bath and must be filled before use). The chamber can be emptied for transport.

In plenum mode the EMO only begins to perform reasonably accurately with flow rates around 10 l/min.

OMV

The OMV (Oxford Miniature Vaporiser) is the most portable and versatile drawover vaporiser, but its size does create performance limitations. The original model contained only 20 ml of volatile agent. Newer models contain 50 ml but this can empty rapidly when in use.

It is suitable for a number of agents. A different dial is attached to the OMV for each agent. A pointer that is moved over the scale controls the concentration of the agent. A build-up of thymol (the preservative in halothane) can cause the pointer to stick. A temporary repair is to fill the OMV with some ether and move the pointer until it is free. The OMV must be emptied of ether and blown dry before adding another agent.

The OMV has basic thermal compensation made up of a reservoir of glycol within a metal heat-sink.

Metal mesh wicks increase the output without significantly increasing internal resistance. It suffers a reduction in vapour pressure at lower temperatures, with a maximum output varying from 2 to 4% with halothane between 0 and 30 degrees Celsius.

It is common to use 2 OMVs in series to increase the output, as is standard in the Triservice apparatus, which was originally used with trichloroethylene in one vaporiser and halothane in the other.

The OMV can operate as a plenum vaporiser. Output reflects the dial setting at 25 ºC, in either continuous or drawover use, but falls dramatically at 15 ºC and rises steeply when above 35 ºC.

It is reasonably accurate over a wide range of flow rates and tidal volumes and, in particular, performs well at small tidal volumes. With continuous flow it is best to keep the fresh gas flow above 4 l/min.

The OMV should not be used in a circle system. It is efficient and can produce very high concentrations.

PAC

The PAC (Portable Anaesthesia Complete. Now called TEC) was originally released as a series of individual vaporisers designed for specific volatile agents. A multi-agent version, the Ohmeda Universal PAC is now also available. It may be used with halothane, isoflurane, enflurane and ether. The PAC vaporisers have automatic bimetallic strip thermo-compensation. Unfortunately the output is less accurate at small tidal volumes, or when used as a plenum vaporiser with gas flows below 2 to 4 l/min. Therefore it is not as useful for paediatric anaesthesia.

Self-Inflating Bag/Bellows

Self-inflating bag/bellows allow controlled ventilation. The Oxford inflating bellows (OIB) comes with the EMO system. The bellows sit vertically with a residual internal volume maintained by a spring. This allows movement of the bellows during spontaneous respiration providing a useful indicator of breathing.

All self-inflating bags have a one-way valve upstream of the bag to prevent gas flowing back to the vaporiser. The OIB also has a one-way valve located downstream from the bellows. The OIB was originally designed for use with a simple spring-loaded valve (e.g. Heidbrink valve). This arrangement works well for spontaneous ventilation, but is less satisfactory for IPPV as the Heidbrink valve must be constantly re-adjusted. Because the Heidbrink valve has no mechanism to prevent the patient’s expired gas from flowing backwards the OIB has the valve downstream.

Non-rebreathing valves (e.g. Laerdal, Ambu) can be used effectively at the patient end of the drawover circuit to facilitate IPPV, and are equally suitable for SV. These non-rebreathing valves will prevent expired gas flowing backwards.

The anaesthetist must be careful with this adaptation of the OIB because unless the downstream valve on the OIB is disabled with the magnet provided, the OIB is prone to jam. When the OIB jams the patient cannot exhale as an air lock develops between the non-rebreathing valve and the OIB valve. The patient must be disconnected to allow exhalation. The problem is more common with IPPV, but may occur with SV. When in use the magnet holds the distal OIB flap valve in the open position and stops the air lock developing. Some anaesthetists remove the downstream valve to prevent this problem. A simpler, single flap valve bellows called the Penlon Bellows Unit has been developed to prevent this problem and avoid confusion concerning when the magnet should and should not be used. Remember, when using modern valves use the magnet.

The tap on the side of the OIB is intended for connection to supplemental oxygen when using the bellows for resuscitation. However, during anaesthesia it is preferable to leave this closed and supply oxygen upstream of the vaporiser. Adding oxygen at the bellows dilutes the anaesthetic vapour.

With IPPV the OIB is operated by a rocking motion rather than direct up and down. This creates less fatigue and produces less variability in tidal volume. The bellows should not be lifted to its maximum capacity. This would produce a tidal volume of 2 litres. If the bellows is pushed down too hard a clip will engage and lock the bellows.

[The majority of this section has been reproduced from the excellent article in World Anaesthesia, Update in Anaesthesia Issue 15 (200) article 6 by Dr Scott Simpson and Dr Iain Wilson.]