Anaesthetic machine


An anaesthetic machine or anesthesia machine is a medical device used to generate and mix a fresh gas flow of medical gases and inhalational anaesthetic agents for the purpose of inducing and maintaining anaesthesia.
The machine is commonly used together with a mechanical ventilator, breathing system, suction equipment, and patient monitoring devices; strictly speaking, the term "anaesthetic machine" refers only to the component which generates the gas flow, but modern machines usually integrate all these devices into one combined freestanding unit, which is colloquially referred to as the "anaesthetic machine" for the sake of simplicity. In the developed world, the most frequent type in use is the continuous-flow anaesthetic machine or "Boyle's machine", which is designed to provide an accurate supply of medical gases mixed with an accurate concentration of anaesthetic vapour, and to deliver this continuously to the patient at a safe pressure and flow. This is distinct from intermittent-flow anaesthetic machines, which provide gas flow only on demand when triggered by the patient's own inspiration.
Simpler anaesthetic apparatus may be used in special circumstances, such as the triservice anaesthetic apparatus, a simplified anaesthesia delivery system invented for the British Defence Medical Services, which is light and portable and may be used for ventilation even when no medical gases are available. This device has unidirectional valves which suck in ambient air, which can be enriched with oxygen from a cylinder, with the help of a set of bellows.

History

The original concept of continuous-flow machines was popularised by Boyle's anaesthetic machine, invented by the British anaesthetist Henry Boyle at St Bartholomew's Hospital in London, United Kingdom, in 1917, although similar machines had been in use in France and the United States. Prior to this time, anaesthesiologists often carried all their equipment with them, but the development of heavy, bulky cylinder storage and increasingly elaborate airway equipment meant that this was no longer practical for most circumstances. Contemporary anaesthetic machines are sometimes still referred to metonymously as "Boyle's machine", and are usually mounted on anti-static wheels for convenient transportation.

Many of the early innovations in anaesthetic equipment in the United States, including the closed circuit carbon-dioxide absorber and diffusion of such equipment to anaesthesiologists within the United States can be attributed to Richard von Foregger and The Foregger Company.

Flow rate

In anaesthesia, fresh gas flow is the mixture of medical gases and volatile anaesthetic agents which is produced by an anaesthetic machine and has not been recirculated. The flow rate and composition of the fresh gas flow is determined by the anaesthetist. Typically the fresh gas flow emerges from the common gas outlet, a specific outlet on the anaesthetic machine to which the breathing attachment is connected.
Open circuit forms of equipment, such as the Magill attachment, require high fresh gas flows to prevent the patient from rebreathing their own expired carbon dioxide. Recirculating systems, use soda lime to absorb carbon dioxide, in the scrubber, so that expired gas becomes suitable to re-use. With a very efficient recirculation system, the fresh gas flow may be reduced to the patient's minimum oxygen requirements, plus a little volatile as needed to maintain the concentration of anaesthetic agent.
Increasing fresh gas flow to a recirculating breathing system can reduce carbon dioxide absorbent consumption. There is a cost/benefit trade-off between gas flow and use of adsorbent material when no inhalational anaesthetic agent is used which may have economic and environmental consequences.
  • High flow anesthesia supplies fresh gas flow which approximates the patient’s minute ventilation, which is usually about 3 to 6 litres per minute in a normal adult.
  • Low flow anesthesia supplies fresh gas flow of less than half the patient's minute ventilation of the patient, which is usually less than 3.0 litres per minute in a normal adult.
  • Minimal flow anesthesia supplies fresh gas flow of about 0.5 litres per minute.
  • Closed system anesthesia supplies fresh gas flow as needed to make up the recirculated gas volume to compensate for the patient’s need for oxygen and anesthetic agents.

    Anaesthetic vapouriser

An anesthetic vaporizer or anaesthetic vapouriser is a device generally attached to an anesthetic machine which delivers a given concentration of a volatile anesthetic agent. It works by controlling the vaporization of anesthetic agents from liquid, and then accurately controlling the concentration in which these are added to the fresh gas flow. The design of these devices takes account of varying: ambient temperature, fresh gas flow, and agent vapor pressure. There are generally two types of vaporizers: plenum and drawover. Both have distinct advantages and disadvantages. The dual-circuit gas-vapor blender is a third type of vaporizer used exclusively for the agent desflurane.

Plenum vaporizers

The plenum vaporizer is driven by positive pressure from the anesthetic machine, and is usually mounted on the machine. The performance of the vaporizer does not change regardless of whether the patient is breathing spontaneously or is mechanically ventilated. The internal resistance of the vaporizer is usually high, but because the supply pressure is constant the vaporizer can be accurately calibrated to deliver a precise concentration of volatile anesthetic vapor over a wide range of fresh gas flows. The plenum vaporizer is an elegant device which works reliably, without external power, for many hundreds of hours of continuous use, and requires very little maintenance.
The plenum vaporizer works by accurately splitting the incoming gas into two streams. One of these streams passes straight through the vaporizer in the bypass channel. The other is diverted into the vaporizing chamber. Gas in the vaporizing chamber becomes fully saturated with volatile anesthetic vapor. This gas is then mixed with the gas in the bypass channel before leaving the vaporizer.
A typical volatile agent, isoflurane, has a saturated vapor pressure of 32kPa. This means that the gas mixture leaving the vaporizing chamber has a partial pressure of isoflurane of 32kPa. At sea-level, this equates conveniently to a concentration of 32%. However, the output of the vaporizer is typically set at 1–2%, which means that only a very small proportion of the fresh gas needs to be diverted through the vaporizing chamber. It can also be seen that a plenum vaporizer can only work one way round: if it is connected in reverse, much larger volumes of gas enter the vaporizing chamber, and therefore potentially toxic or lethal concentrations of vapor may be delivered., what it actually delivers is a partial pressure of anesthetic agent ).
The performance of the plenum vaporizer depends extensively on the saturated vapor pressure of the volatile agent. This is unique to each agent, so it follows that each agent must only be used in its own specific vaporizer. Several safety systems, such as the Fraser-Sweatman system, have been devised so that filling a plenum vaporizer with the wrong agent is extremely difficult. A mixture of two agents in a vaporizer could result in unpredictable performance from the vaporizer.
Saturated vapor pressure for any one agent varies with temperature, and plenum vaporizers are designed to operate within a specific temperature range. They have several features designed to compensate for temperature changes. They often have a metal jacket weighing about 5 kg, which equilibrates with the temperature in the room and provides a source of heat. In addition, the entrance to the vaporizing chamber is controlled by a bimetallic strip, which admits more gas to the chamber as it cools, to compensate for the loss of efficiency of evaporation.
The first temperature-compensated plenum vaporizer was the Cyprane 'FluoTEC' Halothane vaporizer, released onto the market shortly after Halothane was introduced into clinical practice in 1956.

Drawover vaporizers

The drawover vaporizer is driven by negative pressure developed by the patient, and must therefore have a low resistance to gas flow. Its performance depends on the minute volume of the patient: its output drops with increasing minute ventilation.
The design of the drawover vaporizer is much simpler: in general it is a simple glass reservoir mounted in the breathing attachment. Drawover vaporizers may be used with any liquid volatile agent. Because the performance of the vaporizer is so variable, accurate calibration is impossible. However, many designs have a lever which adjusts the amount of fresh gas which enters the vaporizing chamber.
The drawover vaporizer may be mounted either way round, and may be used in circuits where re-breathing takes place, or inside the circle breathing attachment.
Drawover vaporizers typically have no temperature compensating features. With prolonged use, the liquid agent may cool to the point where condensation and even frost may form on the outside of the reservoir. This cooling impairs the efficiency of the vaporizer. One way of minimising this effect is to place the vaporizer in a bowl of water.
The relative inefficiency of the drawover vaporizer contributes to its safety. A more efficient design would produce too much anesthetic vapor. The output concentration from a drawover vaporizer may greatly exceed that produced by a plenum vaporizer, especially at low flows. For safest use, the concentration of anesthetic vapor in the breathing attachment should be continuously monitored.
Despite its drawbacks, the drawover vaporizer is cheap to manufacture and easy to use. In addition, its portable design means that it can be used in the field or in veterinary anesthesia.