After anaesthesia has been induced with an injectable drug it is often maintained with an inhalation drug. This also allows the animal to be given supplementary oxygen (30% minimum). Anaesthesia can also be induced with inhalation agents but is usually slow and unpleasant for the animal since these drugs usually produce an excitement phase before surgical anaesthesia. Sevoflurane and desflurane are much better but it is still difficult to get animals to cooperate when you say “take a deep breath...”. One exception is young foals and calves, where it may be possible to place a nasopharyngeal tube and induce anaesthesia with gas.
Advantages of inhalation anaesthesia are control of the airway (since the animal is nearly always intubated), ventilation (IPPV) and uptake and elimination of drug. The disadvantages are that expensive equipment is needed and possible equipment failure is dangerous for the animal (especially the obsolete and unmaintained equipment which seems to end up in veterinary practices).
Halothane is the most important inhalation anaesthetic in veterinary anaesthesia by a long way (at the moment). It is a good general purpose anaesthetic but not a very good analgesic (use with nitrous oxide or injectable analgesic). It is also a poor muscle relaxant. It is best used as the hypnotic part of a balanced anaesthesia technique. Its side effects include:
When used alone, to provide surgical anaesthesia (ie, high doses), cardiorespiratory depression can be severe. If not enough halothane is used, the combination of halothane and adrenaline can cause tachyarrhythmias, especially in cats.
Halothane hepatitis is a cause of (unnecessary?) worry in human anaesthesia - halothane has been largely replaced by isoflurane (particularly in the USA) and may be withdrawn in the future. Hepatitis after halothane is not a problem in domestic animals but concern about operating theatre staff may cause controls on atmospheric pollution (see scavenging below).
Malignant hyperthermia occurs in some breeds of pig (and people), particularly Pietrain and some families of Landrace pigs. If the pig starts to go hot and rigid, turn off the halothane immediately and ventilate with 100% oxygen. It may also need to be hosed down with cold water. The problem is caused by a mutation of the ryanodine receptor, which controls calcium flux out of the sarcoplasmic reticulum. The definitive treatment is dantrolene (ryanodine receptor antagonist), but it is rarely available. Malignant hyperthermia can also occur in other species but is rare. Halothane is sometimes deliberately used in pigs to detect if they are carrying the MH genes (MH is also induced by stress and lowers the value of the meat). About 30% of commercial pigs in NZ have the MH gene. Pigs have been extensively studied as a model for MH in people; but little is known about it in other species.
Isoflurane is similar to halothane but gives a faster induction and recovery. Many dogs object to the smell. It is expensive, but can be useful in sick animals or where a very fast induction and recovery are required. If halothane is taken off the market, isoflurane is the obvious sucessor. It may also cause MH. Like halothane, it has little analgesic effect. Enflurane is a chemical isomer of isoflurane but often produces excitatory effects which make judgement of depth of anaesthesia difficult. It has no obvious advantages and is not often used.
Nitrous oxide (N2O,
laughing gas) has been around since the 18th century. It was used as a recreational
drug until its analgesic properties were discovered. It is a weak anaesthetic
although a good analgesic so it must be used with other agents. It produces
a rapid induction - it equilibrates in about 10 mins. A number of problems are
associated with nitrous oxide:
• diffusion hypoxia - Fink effect - the opposite of the second gas effect
• diffuses into air filled spaces - beware pneumothorax and colic
• can build up in circle systems - oxygen analyser must be used or run
the system semi closed.
• depresses folate metabolism (long term use > 8 hours)
Since it is a gas, it comes in cylinders and requires pressure regulators, flow
regulators etc (more equipment to break down!). However, it is well worth the
bother. The main reason to use it is that its analgesic properties mean that
much less halothane can be used. This either means smoother anaesthesia or faster
recoveries, depending on how it is used.
The newer agents desflurane and sevoflurane are not yet fully evaluated in animals (but give a very fast induction and recovery in man). Desflurane is very good in theory but requires special (very expensive) vaporisers. Sevoflurane has been used in horses, pigs and birds. Some paediatric anaesthetists use them for induction (takes 10 - 60 seconds) and then switch to halothane or isoflurane for maintenance. They may have their use depending on price. Sevoflurane is catching on in human anaesthesia and getting cheaper all the time.
Older agents include trichoroethylene which is almost obselete but worth using in some circumstances. It is a very
good analgesic but a poor muscle relaxant and is best used in combination with
another agent such as halothane - low dose halothane keeps the animal asleep
and trichloroethylene provides the analgesia. It gives a very slow induction
and recovery but good analgesia during recovery. Do not use in a closed system
with soda lime - reacts to produce phosgene. (Anaesthetic or HPLC grade trichloroethylene
should be used for anaesthesia; it is also one of the commonest industrial solvents
but solvent grade is full of toxic impurities.)
Diethyl ether (= ether) is almost obselete
and should be avoided in most circumstances. It is a good anaesthetic but very
inflammable in air, and explosive in oxygen. Its vapour is heavier than air
and will roll along the floor and under doors. It gives a slow induction and
recovery. The only reason it is still around is that it is the only agent which
can be used without supplementary oxygen as it stimulates respiration (but if
oxygen is available use it). It is irritant to the airways (peroxide degradation
products) - anticholinergic premed required.
Methoxyflurane is not available in NZ
and is almost obselete. It gives a very slow induction and recovery, but is
a very good analgesic and a good muscle relaxant so is useful for long orthopaedic
ops. It is extensively metabolised releasing free fluoride ions which may cause
kidney damage (particularly in combination with NSAIDs).
Chloroform used to be used in horses because
it was cheap. It causes massive liver necrosis in hypoxia and is very good at
sensitising the heart to adrenaline. Do not use.
| drug | induct % | maint % | MAC % | mL vap/mL liq | svp (kPa) | blood:gas | $/100mL | problems |
| halothane | 2 - 5 | 0.5 - 2 | 0.9 | 228 | 33 | 2.4 | $30 | malignant hyperthermia |
| isoflurane | 2 - 3 | 0.5 - 2.5 | 1.5 | 189 | 32 | 1.4 | $120 | |
| sevoflurane | 5 - 7 | 1 - 3 | 2.5 | 143 | 21 | 0.6 | $180 | |
| enflurane | 4 - 5 | 0.5 - 3 | 2.2 | 196 | 23 | 1.9 | ($60) | excitation |
| methoxyflurane | not easy!! | 0.2 - 0.5 | 0.2 | 210 | 3 | 11 | ($222) | strong smell |
| desflurane | 4 - 11 | 2 - 9 | 8 | 206 | 89 | 0.4 | $60 | heated vaporiser |
| ether | up to 20 | 3 - 10 | 3 | 232 | 59 | 12 | $12.50 | inflammable / explosive |
| chloroform | 1.5 - 2 | 0.5 - 1.5 | 0.8 | 302 | 21 | 8 | $9 | liver toxicity |
| trichloro ethylene |
max | 0.2 - 1.5 | 0.6 | 267 | 8 | 9 | $39.50 | + soda lime = phosgene |
| nitrous oxide | not possible | 66 | 105 (man) 220 (dog) 255 (cat) |
656 | 5100 | 0.47 | 0.7c (vapour) |
diffusion hypoxia |
| xenon | 70 | 70 | 60 - 71 (man) 120 (dog & pig) |
- | - | 0.11 | $3 (gas) | cost |
There has been concern about the effects of occupational exposure to small amounts of anaesthetic drugs in the atmosphere. In NZ, OSH have recentlu tightened guidelines on the maximum amounts permissible in room air; in Britain and the USA, there are similar guidelines but with different numbers. There is no good evidence that exposure to trace amounts of these drugs harms human health (with the possible exception of nitrous oxide causing abortion in women) but they smell nasty and are better removed. OSH have recently caught on to the fact that vets do not worry about this - so be warned!
A number of things can be done to reduce pollution:
| NZ 8 hr average | NZ 15 min | UK | USA | |
| halothane | 0.5 | 1.5 | 10 | 2 |
| nitrous oxide | 25 | 75 | 100 | 25 |
Exposure limits (ppm). In NZ, 3 x 15 minute exposures are allowed per day, as long as they are within the day’s limit. NZ exposure limits are legally binding; they are only guidelines overseas.
When fish have to be anaesthetised, they are given drugs in their water which are absorbed across the gills and could thus be classified as “inhalation anaesthetics”.
Tricaine methanesulphonate (tricaine mesylate, MS222) is commonly used and probably best. It comes as a powder which needs to be dissolved in aerated fresh water (not tap water) or sea water as appropriate. Phenoxyethanol is an oily liquid which can be useful but may be carcinogenic. It dissolves some plastics. Benzocaine (dissolved in ethanol or acetone stock solution) has been used, deep anaesthesia not reliable. A mixture of clove oil and detergent (Aqui S) has been developed in NZ to allow handling of salmon. It is sedative rather than anaesthetic and may cause stress (fish generally do not like detergent - it damages their gills). Ether can be used at a pinch.
