Inhalation anaesthesia

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GENERAL ANAESTHESIA - INHALANT ANAESTHETIC AGENTS AND MAINTENANCE

INTRODUCTION

·  The inhalant anaesthetics are chloroform, ether, trilene, cyclopropane, enflurane, desflurane, halothane, isoflurane and sevaflurane.

·  Their uptake and distribution determine the anaesthetic action of these inhalant agents.

·  The uptake and distribution depends on

·  Solubility coefficient - At any given temperature the mass of a gas dissolved in a solution (i.e. its concentration in the solution).

·  Blood flow - The inhaled anaesthetic gas is diluted in the residual air when it enters pulmonary ventilation and then distributed to alveolar membrane. The arterial and venous tension of the anaesthetic slowly increases towards the virtual equilibrium with the inspired air.

·  Solubility of inhalant anaesthetic is defined as a concentration distribution ratio between alveolar concentration and the tissue concentration.

·  Methoxyflurane is highly soluble than isoflurane in the body tissues hence the induction and recovery will be slow.

·  Mininimum alveolar concentration (MAC) - MAC is the concentration of the inhalant anaesthetic in the alveoli to produce lack of response in 50% of the subjects to a standard stimulus. This term is used to mention the potency of the anaesthetic. The MAC of each inhalant anaesthetic varies in each species.

·  The other factors are

o Physical and chemical properties of the agent

o Absorption

o Pulmonary blood flow

o Cardiac output perfusion

o Lipid content of tissues

o Final elimination

NITROUS OXIDE

·  Nitrous oxide is the oldest anaesthetic gas available as liquid at room temperature.

·  Its MAC is more than 100% in animals.

·  It has got good analgesic property.

·  Nitrous oxide is used as the principle anaesthetic at a level of 80% in combination with 20% oxygen for dental extraction in human.

·  It is eliminated rapidly from the body.

·  Nitrous oxide moves rapidly through tissues faster than carbon dioxide.

·   It is not used in ruminants. as it will diffuse into the rumen and cause bloat.

·   It induces tachypnoea at high concentration.

·  During recovery it may induce diffusion hypoxia.

·   Occupational hazards to humans, as prolonged exposure to nitrous oxide causes bone marrow depression.

·  Nitrous oxide is administered at 66 to 70% of the total inspired air. Oxygen is given at 30% concentration.

 

DIETHYL ETHER

·  It is a colourless, highly volatile and inflammable liquid with a boiling point 35^oC.

·  The MAC is 1.92%.

·  It gives an irritating vapour and may cause salivation.

·  In low concentration the vagal activity is decreased and at higher concentration it induce arrhythmia.

·  Catecholamine level increases following ether administration.

·  Health hazards are more in human.

METHOXYFLURANE

·  Methoxyflurane is a halogen-substituted ethyl ether.

·  It is nonexplosive.

·   It has fruity odour.

·  The MAC is 0.23.

·  It reacts with metal, rubber and soda lime.

·   Methoxyflurane induces dose dependent central nervous system depression.

·  It does not alter the cardiac function.

·  Concurrent use of epinephrine are contraindicated.

·   Compared to halothane, cardiac arrhythmia are less.

·  Methoxyflurane induces respiratory acidosis.

·  It is highly soluble in fat.

·  Methoxyflurane is contraindicated in patients with renal disease.

·  In veterinary practice its use is restricted to small animals.

HALOTHANE

·  Halothane is colourless volatile liquid.

·   It is non-flammable and nonexplosive.

·  Halothane reacts with metal and soda lime.

·   It is marketed in amphor coloured bottles with thymol.

·  The MAC varies in various species.

o Dogs 0.87%,

o Cats 0.75%,

o Horses 0.9%

o Pigs 1.25%

·  The MAC is reduced with  morphine, xylazine and nitrous oxide.

·  Halothane reduces cerebrospinal fluid production.

·  It suppress adrenal cortical hormone.

·  Halothane depress cardiac out put.

·  Oxygen exchange is reduced in patients with pulmonary diseases.

·  Halothane induces hepatic hypoxia.

·  Halothane undergoes biotransformation in the liver.

·  Halothane has got teratogenic and mutogenic properties.

·  Halothane produces immune suppression.

ISOFLURANE

·  Isoflurane is the new inhalant anaesthetic widely used in human anaesthesia.

·  Its induction and recovery are quick.

·  It is non-inflammable and does not react with metal, rubber or soda lime.

·  It does not decomposed if exposed to ultra violet light.

·  It provides cardiac stability.

·  It has better muscle relaxation property than halothane.

·  It induces more respiratory depression than halothane.

·  It is recommended in patients with liver diseases.

SEOFLURANE

·  It is the newest inhalant anaesthetic used in humans. Still trials are conducted in veterinary anaesthesia.