Anaesthesia Inhalation Agents and reactions with absorbents

Part 1 - Inhalation agents

The inhalation agents that are commonly used in Africa and other places where resources are limited are ether and halothane. When it is available, trichloroethylene is also used.

In the West halothane has been displaced by newer agents: isoflurane and sevoflurane. (Halothane is still widely used in paediatric anaesthesia.) These are far more costly than halothane and will not be considered in detail, though if you get the chance to use isoflurane you will be impressed how good the recovery is compared to halothane. Ether, of course, is never used in the western world and trichloroethylene has a diminishing number of users world-wide and is hard to get. (Laboratory grade is still available).

Ether (diethyl ether)

This is a very cheap agent as it is non-halogenated, made from sugar cane via ethanol using recycled sulphuric acid. With suitable fire precautions, it could easily be made locally in any country with the will to be self-sufficient. Dr William T.G.Morton, a dentist from Boston, Massachusetts is considered as the person responsible for discovering/inventing the practical use of anaesthesia for surgery. He demonstrated its effects on a famous occasion in Boston, USA, in 1846 and this event has become recognised world-wide as the "first anaesthetic".

Physical properties:
Low boiling point: 35°C.
High SVP (Saturatated Vapour Pressure) at 20°C : 425 mm Hg.
Blood/Gas partition coefficient: 12 (high), MAC (Mean Alveolar Concentration): 1.92% (low potency).

Ether is highly volatile and inflammable. In oxygen, it is explosive. It has a very strong and characteristic smell.

Advantages:

Stimulates respiration and cardiac output, maintaining blood pressure and causes bronchodilatation, all due to its sympathomimetic effect mediated by adrenaline release.
A good sole anaesthetic agent because of its analgesic effect.
Does not relax the uterus like halothane but gives good abdominal relaxation.
A safe agent.

Disadvantages:

Flammable
Slow onset
Slow recovery
Secretions needing atropine
Bronchial irritation, so inhalation induction of anaesthesia by mask is very difficult because of coughing.
P.O.N.V. (postoperative nausea and vomiting) is sometimes seen in Africa but is a major disadvantage in the West, where patients vomit much more.

Indications: Any general anaesthetic, but especially good for Caesarean section (because the baby tolerates it and the uterus contracts well), and major cases with intubation. It is life saving for poor risk cases using a low dose. Also indicated when no supplementary oxygen is available.

Contra-indications: There are no absolute contra-indications for ether.

Scavenging should be carried out (where possible) to avoid contact between heavy inflammable ether vapour and diathermy apparatus or other electrical devices that may spark and also to prevent exhaled vapour blowing at the surgeon.

Practice points: The best method is to give a high concentration to a paralysed, intubated patient. Thus after atropine, thiopentone, suxamethonium and intubation, generous IPPV (Intermittent positive pressure ventilation) is commenced with ether 15-20% and then according to the patient's needs, the ether is reduced after about 5 minutes to 6-8%. Remember vaporiser performance is variable. Poor risk, septic or shocked patients may need only 2%.

It seems to be purely fortuitous, but the patients that benefit most from ether anaesthesia, such as Caesarean section and emergency laparotomy (which comprise over 90% of all major surgery in Africa2) do not need diathermy. Where diathermy is essential, eg. in paediatric surgery, halothane is a better drug, so the conflict between ether and diathermy rarely arises. At our hospital, we do not allow ether to be used with diathermy.

Halothane ("Fluothane")

Physical properties: Boils at 50°C, SVP at 20°C: 243mmHg.
Blood /Gas partition coefficient:2.3, MAC 0.75%.

Advantages:

Well tolerated, non-irritant, potent (low MAC) agent, which is relatively insoluble in blood, giving rapid induction
low dose maintenance and rapid recovery
There is predictable, dose-related depression of the respiratory and cardiovascular systems.
The ideal inhalation induction agent.

Disadvantages:

Perhaps too potent, and overdose is easy.
Poor analgesic properties necessitating deep planes of anaesthesia before surgery and especially intubation can be tolerated.
No post-operative analgesia.
Uterine relaxation and haemorrhage at concentrations above 2%
Hypotension, dysrhythmias and especially dangerous with adrenaline where cardiac arrest in VF (Ventricular Fibrillation) readily occurs.
Post-operative shivering.
"Halothane hepatitis" may very rarely occur. It is extensively metabolised in the body and is best avoided within three months of a previous halothane anaesthetic unless the indications to use halothane are considered to override the risk of this rare condition.

Indications: almost all general anaesthesia, especially paediatrics. Inhalation induction especially in upper airway obstruction.

Contra-indications: simultaneous administration with adrenaline, especially during spontaneous breathing. High dose for Caesarean section or uterine evacuation. History of unexplained hepatitis following a previous anaesthetic.

Dosage: Induction with 3%, reducing to 1.5% for maintenance. Children need 2% for maintenance. Over 4% for more than a few minutes will produce an overdose.

Practice Points: Halothane alone is not ideal because it has no analgesic properties. You need high concentrations to abolish reflex activity, eg. straining on the endotracheal tube. This becomes expensive and may also be unsafe. The common clinical situation of an intubated patient breathing spontaneously high concentrations of halothane in oxygen and air is potentially hazardous in the presence of heart disease.

Nitrous oxide is commonly used for analgesia; opioids or regional blocks are alternatives.

Supplementary oxygen is mandatory when using halothane to avoid hypoxia.

Trichloroethylene ("Trilene")

Physical properties: Boils at 87°C (high), SVP at 20°C: 60 mmHg.
Blood/Gas partition coefficient: 9 (high), MAC 0.17% Advantages:

Non-irritant
Safe
Good analgesia during and after surgery
Cardiovascular stability
Very cheap, because one uses so little

Disadvantages:

Low volatility, slow onset of effect because of high blood solubility and low boiling point making it impossible to get concentrations that are high enough
It is a potent agent because you need little to produce an effect, BUT it is a weak anaesthetic because, despite this, vaporisers cannot produce high enough concentrations because the volatility is so low.
Dysrhythmias may occur with adrenaline
Prolonged recovery, because of high blood solubility.

Indications: analgesic supplement to halothane or used on its own for minor procedures such as fracture manipulation, debridement etc.

Contraindications: overdosage in the elderly. Closed circuit with soda-lime. Best avoided in very small babies.

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

Practice Points: Switch off 20-30 minutes before the end of a long operation to avoid prolonged sedative effects. Its ideal function is to give background analgesia for long cases using halothane as the main anaesthetic but it is also very good given with halothane for a fast turn-over of short cases using inhalation induction.

The newer agents:

Enflurane: was a replacement for halothane, now used infrequently.

Isoflurane: Boils at 48°C. SVP at 20°C: 250mmHg, Blood/Gas partition coefficient: 1.4, MAC: 1.15. In general use, good recovery because of relatively low blood solubility, but induction difficult because of irritating bad smell, minimal metabolism, no arrhythmias but causes hypotension, six times the cost of halothane. Big cost reductions when used in a low flow system.

Desflurane: Boils at 23.5°C, SVP at 20°C: 673 mmHg, Blood/Gas partition coefficient 0.4 (low), MAC: 5-10%. Replacement for enflurane, requires a specially designed vaporiser, has come and gone without me ever seeing it!

Sevoflurane: Boils at 58.5°C, SVP at 20°C: 160 mmHg, Blood/Gas partition coefficient 0.6 (low), MAC: 1.7-2%. Expensive, but costs can be reduced if used in a low flow system. There may be problems with sevoflurane and carbon dioxide absorbers, baralyme in particular, but these are currently being investigated. Ultra low solubility resulting in ultra rapid induction and recovery especially as it is non-irritant and sweet smelling. High volatility and high percentage required.

How should volatile agents be used?
One way is to use them for inhalation induction of anaesthesia followed by maintenance with the same or another agent as your sole anaesthetic. The patient puts him or herself to sleep by breathing via a close-fitting mask and provided the smell is accepted and the stage II excitement effects are not excessive, this is a very satisfactory method of inducing general anaesthesia for minor cases without gastric aspiration risk. Lung disease, smoking or drinking habit, obesity and high uptake situations (see above) will make this method slower and prolong stage II effects. Loss of airway in an obese patient may be dangerous. Ideal for a fast turn-over of lots of short procedures on thin patients.

The other way is to give an intravenous induction followed by the volatile agent for maintenance of anaesthesia. Very often the intravenous induction will include intubation of the trachea as well. All general anaesthesia for major cases will be done this way.

Part Two >>