General Anaesthesia

General anaesthesia is a state of controlled and reversible loss of consciousness characterized by analgesia (lack of pain), amnesia (lack of memory), muscle relaxation and immobilisation (relatively depressed reflex responses).

Anaesthesia

“Anaesthesia” word derived from Greek word “Anaesthesia” meaning insensibility
The word was coined by OLIVER WENDELL HOMES in 1846
It is reversible lack of awareness
Pharmacologically induced reversible state of amnesia, analgesia, loss of responsiveness, loss of skeletal muscle reflexes and decreased stress response

Anaesthetic protocol

Use of anaesthetic technique and agents to achieve general anaesthesia.

Balanced Anaesthesia

State of CNS depression achieved by the administration of CNS depressants (Preanaesthetics) to facilitate handling of the animal, smooth induction and reduce the anaesthetic requirement.

The anaesthetics which produce anaesthesia in one or two injection site to brain circulation time are called as “RAPIDLY ACTING DRUGS”. Eg: Thiopentone, thiamylal, methohexitone etc..

Because of their quick onset of action, I/V administration of these agents can be easily titrated to the depth of anaesthesia required and so are most suitable for induction of general anaesthesia.

Agents like pentobarbitone, ketamine, chloral hydrate etc. have slow onset of action even after I/V administration. Therefore such agents cannot be administered “ to effect” and a pre-calculated dose is required to be administered.

Dissociative Anaesthesia

A CNS state characterized by catalepsy, profound peripheral analgesia, and altered consciousness produced by cyclohexamine drugs (ketamine and tiletamine).

In Dissociative anaesthesia, the animal appears awake (eyes remain open) but unaware of its surroundings.

Catalepsy

A state in which there is malleable rigidity of limbs. The patient is unresponsive to aural, visual or minor painful stimuli.

Dissociation of thalamocortical and limbic systems occurs with dissociative anaesthesia.

Chemical Restraint

Drug induced state that produces favourable behaviour modification, sedation, and analgesia or muscle relaxation. Eg: tranquilizer, sedatives, non opioid analgesics, opioid analgesics and neuroleptanalgesics.

Tranquilization (Ataxia, neurolepsis)

A state of tranquility and calmness in which the patient is awake, relaxed and unconcerned with its surrounding.

Sufficient stimulation will arouse the patient from Tranquilizers. The tranquilizers act by depressing the hypothalamus and reticular activating systems. Eg: phenothiazines & butyrophenones.

Sedation

A mild degree of central depression in which the patient awake but calm. Sufficient stimulation will arouse the patient from sedation. Sedatives act by a dose dependent depression of the cerebral cortex. eg. Banzodiazepines.

Neuroleptanalgesia

A state of profound sedation and analgesia achieved by administering an opioid and a tranquillising agent.

Examples of Neuroleptanalgesia are-

Acepromazine – meridine
Acepromazine – oxymorphone
Droperidol – fentanyl
Xylazine – oxymorphone
Diazepam – morphine
Diazepam – oxymorphone
Diazepam – tentanyl

Glucose effect

A unique re-anaesthetizing action has been observed in animals recovering from barbiturate anaesthesia, that was subsequently given glucose.

The glucose effect presumably occurs with most barbiturates and thio-barbiturates but not with inhalation or other anaesthetics

Glucose causes a decrease in activity of the components of the microsomal electron chain resulting in decreasing microsomal metabolism.

MAC (Minimum alveolar concentration)

MAC is defined as the Minimum Alveolar Concentration of an inhalant anaesthetic at one atmosphere that produces immobility in 50% of subjects (patinents) exposed to a supramaximal stimuli.

Minimum alveolar concentration of an anaesthetic required to keep a patient from gross movement in response to a painful stimulus.

End tidal anaesthetic concentration midway between that allowing movement and that preventing it is minimum alveolar concentration.

MAC is equivalent to ED50.

MAC (Minimum alveolar concentration) of some gases-

Nitrous oxide – 104%
Halothane – 0.87%
Isoflurane – 1.28%
Methoxyflurane – 0.24%
Sevoflurane – 2%
Desflurane – 6%

Solubility co-efficient

Solubility co-efficient is the measure of a gas to dissolve in a solvent. For inhalation anaesthetics it is measured and expressed as a partition “co-efficient”.

Partition co-efficient

Partition co-efficient is the concentration ratio of an anaesthetic in the solvent and the gas phases (Blood gas partition co-efficient) or between two tissue solvents (brain and blood).

Blood Gas partition co-efficient

Blood Gas partition co-efficient is the concentration ratio of an anaesthetic in the blood and the gas phases. It provides a means of predicting the speed of anaesthetic induction, recovery and change of anaesthetic depth. Lower the blood gas partition co-efficient faster will be the induction and recovery. Blood gas partition co-efficient of different gases-

Nitrous oxide– 0.47
Isoflurane– 1.46
Halothane– 2.54
Methoxyflurane– 1.5

Oil gas partition co-efficient

Oil gas partition co-efficient is the concentration ratio of an anaesthetic ion oil (brain cells) and gas phase at equilibrium. It correlates inversely with anaesthetic potency. Very potent anaesthetics have low MAC and high oil gas partition co-efficient.

IPPV (Intermittent Positive Pressure Ventilation)

Application of positive pressure during the inspiratory phase when the patient has an artificial airway in place and is connected to a ventilation.

Indications for IPPV (Intermittent Positive Pressure Ventilation)-

Apnea
Significant ventilation during anaesthesia
Using of neuromuscular blocking agents
Intra thoracic surgery
General anaesthesia lasting more than 90 minutes
To facilitate inhalant anaesthesia
To eliminate oscillations between deep and light anaesthesia associated with depression of spontaneous ventilation.
To maintain normal carbondioxide tension in arterial blood (ECAPNIA) (Pco2 = 35-45 mmHg)

PEEP (Positive End Expiratory Pressure)

PEEP (Positive End Expiratory Pressure) improves the alveolar ventilation by increasing the functional residual capacity. High levels of PEEP however decrease cardiac return and lowers cardiac output.

Second Gas Effect

Second Gas Effect applies to the situation in anaesthesia where a gas such as nitrous oxide when given in a high enough concentration its rapid uptake from the alveolus, causes an increase in the concentration of the other gas.

Diffusion Hypoxia

A factor that influences of alveolar PO₂ gas. When soluble gases (such as N₂O) are breathed in large quantities can be dissolved in body fluids rapidly. This can lead to a temporary increase in the concentration of oxygen and CO₂ in the alveolus causing an increase in their respective partial pressures. When patient is recovering from N₂O anaesthesia, large quantities of this gas move from the blood to the alveolus (down its concentration gradient) and so far a short period of time the oxygen and CO₂in the alveolus are diluted by this gas.

This could cause the partial pressure of O₂to decrease and could lead to hypoxia. The decrease in CO₂could also potentiate this effect as ventilation would be suppressed leading to potential Hypoxaemia. Hypoxia can be avoided by increasing the O₂ when recovering from N₂O anaesthesia.