Inhalational anesthetics are used for the induction and maintenance of general anesthesia as well as sedation. The exact mechanisms by which they act are still unknown. The most common inhalational anesthetics are sevoflurane, desflurane, and nitrous oxide. Of these, sevoflurane is the most common because of its rapid onset of action and the fact that patients recover quickly from it. Inhalational anesthetics cause respiratory depression, a decrease in arterial blood pressure and cerebral metabolic demand, and an increase in cerebral blood flow. While side effects differ based on the substance (e.g., halothane can cause hepatotoxicity), the most common side effect is nausea.
- Inhalational anesthetics provide both analgesia and narcosis and can be used for induction and maintenance of general anesthesia.
- The exact mechanism of action of inhalational anesthetics is still unknown.
- The most widely used inhalational anesthetics are:
- Nitrous oxide
- Enflurane (no longer marketed in the US)
- Halothane (no longer marketed in the US)
Pharmacokinetics and pharmacodynamics
Uptake into the blood: inhalational anesthetics are taken up passively via diffusion, which depends on:
- Blood solubility of the anesthetic
- Lung ventilation, volumes, and perfusion
- Distribution and uptake into the brain: Transport to and uptake into the brain depend on cerebral perfusion and the fat solubility of the inhalational anesthetic.
- Onset of effect: : The lower the blood-gas partition coefficient of an inhalational anesthetic, the faster the substance takes effect (less induction time)
- Inhalational anesthetics are eliminated by the lungs
- Inhalational anesthetics are metabolized only to a small degree
- With prolonged duration of anesthesia in obese patients, inhalational anesthetics with a high fat solubility can accumulate in adipose tissue and slow down recovery from anesthesia (increased context-sensitive half-life).
Measure of potency of inhalational anesthetics: minimum alveolar concentration (MAC)
- MAC is the fraction of volume of the anesthetic present in the inspired air that provides sufficient analgesia in 50% of patients, meaning that patients will not respond to an extremely painful stimulus such as surgical skin incision.
- MAC is inversely related to anesthetic potency (potency = 1/MAC) and represents the value.
- The lower the MAC value, the more fat soluble the anesthetic. For example:
Pharmacokinetics and pharmacodynamics of common inhalational anesthetics
|Blood-gas partition coefficient||Brain-blood partition coefficient||Minimum alveolar concentration (MAC)|
|Nitrous oxide|| || || |
|Desflurane|| || || |
|Sevoflurane|| || || |
|Isoflurane|| || || |
|Enflurane|| || || |
|Halothane|| || || |
- ↓ Respiration
- ↓ Arterial blood pressure
- Myocardial depression
- ↓ Cerebral metabolic demand
- ↑ Cerebral blood flow
- ↑ ICP
- Postoperative: nausea and vomiting
Specific characteristics of common inhalational anesthetics
|Nitrous oxide|| |
- General side effects
Side effects of specific substances
- Can diffuse into gas-filled body compartments and cause expansion of the gas present there → potential damage to organs/tissues → should not be used in patients with conditions such as pneumothorax
- Causes mild myocardial depression and increases pulmonary vessel resistance → should not be used in patients with conditions such as pulmonary hypertension
- Desflurane: sympatho-adrenergic reaction → ↑ blood pressure and ↑ heart rate
- Sevoflurane: : interacts with soda lime → nephrotoxic breakdown products (known as compounds A–E)
- Methoxyflurane: nephrotoxic
- Enflurane: proconvulsive
Halothane: hepatotoxic → halothane hepatitis 
- Pathophysiology: underlying mechanism not fully understood
- Clinical features
- Diagnostics: diagnosis of exclusion
- Treatment: : depending on the severity of liver damage, ranges from supportive treatment to liver transplantation 
- Nitrous oxide
We list the most important adverse effects. The selection is not exhaustive.