INHALANT ANAESTHETICS

INHALANT ANAESTHETICS

·         Inhalation anaesthetic drugs are commonly used to produce general anaesthesia

·         Inhalation anaesthetics offer many advantages over Injectable anaesthetics for maintenance (mainly) and induction (some cases) of general anaesthesia.

·         Depth of anaesthesia during maintenance can be controlled by adjusting the total flow rate of anaesthetic drug.

·         Duration of clinical effect and elimination of inhalation anaesthetics are not dependent on the body detoxification mechanisms.

·         The incidence of anaesthetics related toxicity is rare and is not considered a problem during normal anaesthesia.

·         Most of them have short anaesthetic hangover as a result of which animals can be send home soon after recovery from anaesthesia.

·         The potency of inhalation anaesthetics is defined quantitatively as minimum alveolar concentration (MAC).

·         The minimum alveolar concentration is defined as the minimum concentration of an anaesthetic in pulmonary alveoli that produces immobility to a painful stimulus in 50% of animals exposed to it. Eg: Isofulrane – 1.28%

·         Inhalation anaesthetics are gases or vapors that diffuse rapidly across pulmonary alveoli and tissue barriers to produce anaesthesia.

·         Induction – time period between the onset of the anesthetic administration and development of effective surgical anaesthesia

·         Recovery – Time period between the cessation of the anaesthetic administration and consciousness of the patient.

·         During induction partial pressure or tension of the gas in inhaled air is more, so the anaesthetic agent moves from alveolar air into circulatory blood and then from blood into brain according to partial pressure gradient till equilibrium or steady state reached.

·         During recovery direction of flow is reverse, from brain into blood then into alveolar air.

·         Solubility of anaesthetic agent in different media is expressed as partition coefficient and defined as the ratio of concentration of the agent in two phases at equilibrium.

·         Expressed as oil: gas, blood:gas and tissue:blood partition coefficient.

·         Oil:gas partition coefficient gives lipid solubility of the agent i.e., higher oil gas partition coefficient  higher the lipid solubility of the agent.

·         Blood gas partition coefficient: It is given by the ratio of concentration of anaesthetic agent in blood to that in the gas phase at equilibrium is the index of solubility of the agent in blood.

·         Tissue blood partition coefficient: It is given by the ratio of concentration of anaesthetic agent in tissue to that of in blood at equilibrium.

Nitrous oxide

·         Odorless, non inflammable, inorganic gas at room temperature.

·         Blood gas partition coefficient is very low. Induction and recovery are rapid.

·         Non irritant

·         Has good analgesic effect.

·         Liver and kidney functions are unaffected.

·         It readily crosses the placenta; hypoxemia may develop if a neonate is allowed to breath room air immediately after the cesarean delivery.

·         Concentration of 50-60% is commonly used and the remainder of the anaesthetic dose is provided by some other inhalant drug (Eg: Isoflurane).

·         Second gas effect: Physical movement of nitrous oxide out of the alveolus into the blood stream tends to concentrate the other components of the alveolus enhancing their absorption.

·         During recovery when the nitrous oxide in the blood quickly moves from the blood into the gas dilutes other components of the alveolus (Eg: Oxygen) and may lead to diffusion hypoxia. To prevent hypoxia 100% oxygen should be administered to all patients for atleast 5 min after nitrous oxide is discontinued.

·         Adverse effects: Hypoxemia administration of nitrous oxide at concentration greater than 70% may place the animal at risk of developing hypoxemia.

·         Contraindicated in animals with pockets of trapped gas.

·         Pernicious anemia and neurologic dysfunction have been reported in humans following chronic exposure to nitrous oxide, which inhibits the activity of vitamin B₁₂ dependant enzymes in the body.

·         Minimum 20% oxygen in the inspired air is required to avoid hypoxia and for large animal it is 40%.

Ether

·         High blood gas partition coefficient hence very slow induction and recovery.

·         Vapour is pungent and irritant to mucus membrane

·         Potent analgesics, provides excellent smooth muscle relaxation.

Chloroform

·         Irritant to mucus membrane.

·         Compared to ether recovery and induction are fast.

·         Metabolized in liver to highly reactive free radicals which is highly hepatotoxic.

Halothane

·         Halogenated ethane derivative.

·         Vapour has pleasant odor and non-irritant to mucus membrane.

·         Halothane may be broken down by different reductive pathways with the production of toxic metabolites. These metabolites results in halothane induced hepatitis.

·         Analgesia and skeletal muscle relaxation - moderate to good.

Isoflurane

·         Halogenated methyl ethyl ether for use in dogs and horses.

·         Isoflurane rapidly induce anaesthesia and recovery is also rapid.

·         Metabolism is very low (0.17%). Thus can be considered for anaesthesia in animals with hepatic or renal disease.

·         Decreases arterial blood pressure in a dose dependant manner.

·         Heart rhythm remains relatively normal

·         Cardiac arrhythmias occur, but to lesser extent than with other inhalant anaesthetics

·         Dose dependant CNS depression noticed.

·          Decreased respiratory rate noticed.

·         Hepatic function is reversibly depressed. No cases of hepatic necrosis reported.

·         Renal function is also reversibly depressed.

·         Because of its inertness, isoflurane has not been associated with any organ toxicities.

Desflurane

·         It is a new inhalant anaesthetic agent that is approved for use in human. Its use in veterinary medicine has been very limited.

·          Induction and recovery are rapid.

·         Potency is low.

·         Cardiovascular effects similar to isoflurane

Sevoflurane

·         It is approved for veterinary use in dogs.

·         Blood gas partition coefficient is low so induction and recovery is rapid.

·         2-5% metabolized and eliminated through kidney

·         Cardiovascular system is depressed

·         Dose dependant decrease in renal blood flow and glomerular filtration.