General Anaesthesia – Mode of Administration, Classification

Introduction of General Anaesthesia

General Anaesthetics (GAs) are drugs which produce reversible loss of all sensation and consciousness.

Cardinal Features

• Loss of all sensation,especially pain
• Sleep (unconsciousness)
• Immobility and muscle relaxation
• Abolition of somatic and autonomic reflexes

In the modern practice combination of inhaled and i.v. drugs is used.

Mode of Administration

Drugs given to induce General Anaesthesia can be either as gases or vapours. (inhalational anaesthetics) or as injections (intravenous anaesthetics or even intramuscular)

1. Inhalational anaesthetic substances are either volatile liquids or gases.

Anaesthesia machine allows composing a mixture of oxygen, anaesthetics and ambient air, delivering it to the patient.

Desflurane, isoflurane and sevoflurane are the most widely used volatile anaesthetics today. They are often combined with nitrous oxide.

Less popular, volatile anaesthetics, include halothane, enflurane and methoxyflurane.

Xenon is also used as an anaesthetic.

2. Injectable anesthetic

It is used for the induction and maintenance of a state of unconsciousness. Anaesthetists prefer to use intravenous injections, as they are faster, generally less painful.

Stages of Anaesthesia

PHARMACOKINETICS OF INHALATIONAL ANAESTHETICS

Inhalational anaesthetics are gases or vapours that diffuse rapidly across pulmonary alveoli and tissue barriers.

Transfer of the anaesthetic between lung and brain depends on a series of tension gradients; as

Alveoli then Blood then Brain.

TECHNIQUES OF INHALATION OF ANAESTHETIC

1. Open drop method= Liquid anaesthetic is poured over a mask with gauze and its vapour is led with air. A lot of anaesthetic vapour escapes in the surroundings. It can be used only for a cheap anaesthetic: Use now is limited to peripheral areas.

2. Anaesthetic Machines= Use is made of gas cylinders Specialized graduated vaporisers, flow meters, unidirectional valves, corrugated rubber tubing and reservoir.

The gases are delivered to the patient through a tightly fitting face mask or endotracheal tube.

CLASSIFICATION

1. Inhalational

Gas

• Nitrous oxide; volatile liquid= Ether, Halothane, Isoflurane, Desflurane Sevoflurane

2. Intravenous

Fast acting drugs

• Thiopentone sod. , Methohexitone sod., Propofol, Etomidate

Slower acting drugs

• Benzodiazepines, Diazepam Lorazepam, medazolam, Dissociative anaesthesia Ketamine, Opioid analgesia Fentanyl.

INHALATIONAL ANAESTHETICS

1. Nitrous oxide

Nitrous oxide is the most commonly used inhalation Anaesthetic in dentistry and is commonly used in emergency centers and ambulatory surgery centers as well.

When used alone, it is incapable of producing general anaesthesia reliably, but it may be combined with other inhalation and/or intravenous agents in deep sedation/general anaesthetic techniques.

2. Ether (Diethyl ether)

Ether is a potent anaesthetic, produces good analgesia and marked muscle relaxation .

It is a highly volatile liquid, produces irritating vapours which are inflammable and explosive.

It is highly soluble in blood. Induction is prolonged and unpleasant with struggling, breath holding, salivation and marked respiratory secretions.

Atropine must be given as pre-medication to prevent the patient from drowning in his own secretions.

Ether is not used now in developed countries because of its unpleasant and inflammable properties.

3. Isoflurane

Sold under the trade name Forane among others, is a general anaesthetic. It can be used to start or maintain anaesthesia.

Often another medication is used to start anesthesia due to airway irritation with isoflurane.

4. Desflurane

It is a highly fluorinated methyl ethyl ether used for maintenance of general anaesthesia.

It has the most rapid onset and offset of the volatile Anaesthetic drugs used for general anaesthesia due to its low solubility in blood.

5. Halothane

Sold under the brand name Fluothane among others, is a general Anaesthetic.

One of its benefits is that it does not increase the production of saliva, which can be particularly useful in those who are difficult to intubate.

6. Sevoflurane

Is used to cause general anaesthesia (loss of consciousness) before and during surgery. It is inhaled (breathed in). Although sevoflurane can be used by itself, combinations of anaesthetics are often used together.

This helps produce more effective Anaesthesia in some patients.

COMPLICATIONS OF GENERAL ANAESTHESIA

A. During Anaesthesia

1. Respiratory depression and hypercarbia.
2. Salivation, respiratory secretions.
3. Cardiac arrhythmias, asystole.
4. Fall in BP
5. Aspiration of gastric contents, acid pneumonitis
6. Laryngospasm and asphyxia.
7. Delirium, convulsions and other excitatory effects are seen.

B. After Anaesthesia

1. Nausea and vomiting.
2. Persisting sedation impaired psychomotor functions.
3. Pneumonia
4. Cognitive defects
5. Organ toxicities: liver, kidney damage.
6. Nerve palsy (affect in eye movement)- due to faulty positioning.

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