Using Volatile Anaesthetic Agents (page 3)

Issue 5 (1995) Article 5: Page 3 of 3

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Using volatile anaesthetic agents (Continued)

Trichloroethylene (Trilene)

Advantages: A non-irritant, safe agent. It provides good analgesia during and after surgery. It maintains cardiac output and is inexpensive.

Disadvantages: Trichloroethylene takes effect slowly. It has weak anaesthetic properties and may result in a rapid respiratory rate in spontaneously breathing patients. Arrhythmias may occur and adrenaline administration is contraindicated. If high doses are used a prolonged recovery will occur - particularly in elderly patients. Trichloroethylene is stabilised in solution by the addition of 0.01% thymol and should be protected from light.

Indications: Trichloroethylene is mainly used as an analgesic supplement to halothane or used on its own for minor procedures such as fracture manipulation. It has been used as the sole agent for tonsillectomy without intubation and for analgesia in labour.

Contraindications: Never use trichloroethylene in a circle with soda-lime as the toxic compounds phosgene and carbon monoxide are produced.

Dosage: 0.5-1% initially, reducing to 0.2-0.5%.

Vaporisers: A variety of vaporisers have been used with trichloroethylene, the Oxford Miniature Vaporiser (OMV) is recommended.

Practical points: A very easy agent to give but remember to turn it off well before the end of the operation to avoid prolonged sedative effects. It is most commonly used to give background analgesia for long cases or combined with halothane for short cases using inhalation induction. When combining vaporisers in this way always place the halothane vaporiser closer to the patient. ICI has ceased production of their blue "Trilene", though laboratory grade trichloroethylene from Germany can still be obtained.

Nitrous Oxide

Advantages: Nitrous oxide has a rapid onset and recovery. It is a good analgesic supplement for halothane and reduces the incidence of awareness. It produces minimal cardiovascular and respiratory effects.

Cylinders containing a 50% mixture of nitrous oxide in oxygen (named Entonox) are produced in some countries. The contents may be breathed by patients via a demand valve for analgesia following trauma, changes of dressings or childbirth.

Disadvantages: In developing countries nitrous oxide is expensive to produce and transport. It is delivered to the patient through a rotameter and is mixed with oxygen to produce an inspired mixture of not less than 30% oxygen. If the rotameters are set incorrectly a hypoxic gas mixture may be given to the patient. (This may be a particular problem if nitrous oxide is mixed with "oxygen" from an oxygen concentrator).

During anaesthesia nitrous oxide diffuses into any body cavity which contains gas. This includes air spaces in the gut, middle ear, endotracheal tube cuff and pneumothorax.

Diffusion hypoxia (Fink principle) may occur at the end of anaesthesia when nitrous oxide rapidly leaves the blood and tissues and passes out through the lungs. This may result in a dilution of the oxygen in the lungs for a few minutes and is prevented by administering extra oxygen at the end of anaesthesia.

In the developing world where resources are scarce and transport costs high, the use of nitrous oxide is an unnecessary extravagance. Before its use was discontinued at the Queen Elizabeth Hospital in 1988 nitrous oxide accounted for a quarter of the total pharmacy budget for the whole hospital (which included 3,000 outpatients a day)!

Contraindications: Nitrous oxide is not used in drawover circuits. It should never be given to a patient with an untreated pneumothorax or a patient who has been scuba diving within the previous 24 hours due to the potential for decompression sickness. Less than 50% nitrous oxide is largely ineffective.

How should volatile agents be used?

One method is to use them for both induction and maintenance of anaesthesia. The patient inhales the agent via a close-fitting facemask and provided the smell is accepted and the stage two excitement effects are not excessive, this is a very satisfactory method of inducing general anaesthesia for short, minor cases.

Another method is to induce anaesthesia intravenously and use the volatile agent for maintenance of anaesthesia. Often the intravenous induction will be followed by tracheal intubation. Most general anaesthesia for major cases may be done this way.

When muscle relaxants are used the concentration of anaesthetic agents may be reduced but care should be taken to avoid the patient becoming too light whilst paralysed.

This article contained links to the following additional information:

Table 1: Physical characteristics of volatile agents