Inhaled anaesthetics

Inhaled anaesthetics, also called inhalational or inhalation anaesthetics, are drugs used to induce and maintain general anaesthesia which are administered by inhalation. These are lipid-soluble, hydrophobic drugs. The more lipid-soluble the drug, the:

• Higher the potency (the lower the minimal alveolar concentration (MAC))
• Slower the induction of anaesthesia
• Slower the recovery from anaesthesia

However, even the fastest-acting inhalation anaesthetics take a few minutes before anaesthesia is induced.

In addition to their medical effects, the climate effects of these drugs must be considered. Desflurane and nitrous oxide are potent greenhouse gases. Sevoflurane is the inhaled anaesthetic with the least greenhouse potency.

Compounds

• Halogenated hydrocarbons
  • Halothane
• Halogenated ethers
  • Sevoflurane
  • Desflurane
  • Isoflurane
• Gases
  • Nitrous oxide (N2O)

Halothane is no longer used in the developed world due to the risk of halothane hepatitis, which has a very high mortality.

The halogenated ethers, mostly sevoflurane, are widely used anaesthetic drugs.

Indications

Halogenated hydrocarbons can either be used for induction of general anaesthesia alone (and using an intravenous anaesthetic for maintainance), or be used for both induction and maintainance. Sevoflurane is often used for induction of anaesthesia, especially in children and in short interventions.

Nitrous oxide (N2O) is the least potent inhaled anaesthetic. It’s used for smaller procedures where complete loss of consciousness is not necessary, like during labour or dental procedures. It may be self-administered by the patient. It’s necessary to give pure O2 for a few minutes after the operation to prevent diffusional hypoxia. N2O is the only inhaled anaesthetics that doesn’t depress respiration, and it can’t cause malignant hyperthermia. It does not induce muscle relaxation, so it should be used with a muscle relaxant.

Mechanism of action

General anaesthetics cause CNS depression by hyperpolarizing neurons, which inhibits the synaptic transmission.

The exact mechanism of how this is achieved depends on the exact general anaesthetic. Generally, these drugs either stimulate inhibitory receptors like the GABAA and glycine receptors or they inhibit excitatory receptors like neuronal acetylcholine and NMDA glutamate receptors. The inhibitory receptors GABAA and glycine receptors are ligand-gated Cl– channels. When these are activated by the general anaesthetics, chloride ions enter the neurons, which hyperpolarizes the membrane.

The excitatory receptors neuronal acetylcholine receptor and NMDA glutamate receptors are ligand-gated Na+ channels. When these are inhibited by general anaesthetics the neurons are hyperpolarized. By hyperpolarizing neurons in the thalamus, hypothalamus, cortex, spinal cord and hippocampus these drugs achieve loss of consciousness, immobility, analgesia and amnesia.

MAC is the concentration of drug in the alveolar space which prevents a motor response to pain in 50% of patients. It’s inversely proportional to the potency. During general anaesthesia, a concentration of 1,2 – 1,3 x MAC is usually used.

Pharmacokinetics

These drugs are rapidly absorbed by diffusion across the alveolar membrane. More lipophilic drugs will immediately dissolve in lipids in the blood. This causes them to enter the brain more slowly, which is why more lipophilic inhalation anaesthetics induce anaesthesia more slowly and vice versa.

These drugs are mainly eliminated by exhalation. More lipophilic drugs are more retained in the CNS and adipose tissue and are therefore eliminated more slowly. More lipophilic drugs therefore cause the patient to recover from the general anaesthesia more slowly and vice versa.

Adverse effects

Inhaled anaesthetics can cause cardiac depression and respiratory depression. The halogenated ethers vasodilate cerebral vessels, which increases the CBF and therefore the ICP, generally making them unsuitable for brain surgery. They all also have a cardiodepressive and bronchodilatory effects, the latter of which may be useful in an ICU setting. They also cause vasodilation in systemic vessels, causing hypotension. Isoflurane has the least cardiodepressive effect of the three.

All of them cause airway irritation, except for sevoflurane, so only sevoflurane can be used for induction of anaesthesia.

Repeated use of N2O causes vitamin B12 deficiency, because N2O can oxidize the vitamin. This is mostly only a problem in medical personnel who abuse the gas repeatedly. After the surgical procedure is finished and the nitrous oxide anaesthetic is removed, N2O will rapidly diffuse back into the alveolar space. This causes it to displace oxygen, which causes so-called “diffusional hypoxia”. This effect is counteracted by giving the patient pure O2 for a few minutes after the procedure.

Halothane hepatitis

Halothane induces a massive hepatic necrosis called halothane hepatitis in 1 out of 10 000 cases. It has a mortality rate of 80%. This development of halothane hepatitis was very unpredictable, so the use of halothane has been mostly discontinued where possible. Halothane is metabolized into a compound called TFA in the liver. TFA acts as a hapten, so it binds to surface proteins on hepatocytes and make them immunogenic. The immune system will then attack the hepatocytes.

Malignant hyperthermia

Halogenated inhaled anaesthetics can induce malignant hyperthermia in people who have a mutation in the ryanodine receptor. The ryanodine receptor is a calcium channel in the muscle, and in susceptible people halogenated inhaled anaesthetics or depolarizing muscle relaxants like succinylcholine will cause calcium to enter skeletal muscles. This causes hyperthermia and potentially rhabdomyolysis. Treatment is with dantrolene, which blocks the ryanodine receptor.