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Introduction to anaesthesia

By Herbert Grant,2014-11-19 10:25
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Introduction to anaesthesia

    Introduction to anaesthesia

    (The loss of sensation)

Classical reasons:

     ? Humane aspects;

     - Prevention of pain.

     - Humane handling to prevent animal injury.

     ? Technical efficiency;

     - Ease of surgery.

     - Personnel protection.

Objectives of anaesthesia:

    1- provide relief from pain.

    2- Control and immobilization of the patient.

    3- Ensure patient safety and survival.

Types of anaesthesia:

     - General anaesthesia; unconsciousness.

     - Local anaesthesia; blocking nerves endings.

     - Analgesia.

     Preanaesthetic examination:

     (Minimize risk of anaesthetic morbidity and mortality)

? History.

    ? Respiratory system evaluation:

     - Respiratory pattern / effort.

     - Pulmonary diseases; Pneumonia.

     - Cough; a moist cough when is suppressed by anaesthesia, the airway secretion will

     obstruct the lung.

? Cardiopulmonary evaluation:

     -Auscultation of heart sounds; heart rate and rhythm; pulse rate and quality.

     - Cardiovascular failure.

     - Hypovolaemia and dehydration.

     - Dysrhythmia.

     - Anaemia; Hypoproteinaemia ? Hypoalbuminaemia ? Drug unbound ?

     Increase peak activity.

Hepatic system:

    - Metabolism of many drugs.

     - Hepatitis, Jaundice; lead to hypoalbuminaemia.

Renal system:

     -Regulate the body's fluid volume, mineral composition and acidity.

     - Dehydration, haemorrhage ? Arterial hypotension ? Renal ischaemia ?

     Uraemia ? Death.

    What would contraindicate general anaesthesia?

- Severe respiratory diseases.

    - Congestive heart failure.

    - Ventricular dysrhythmia.

    - Severe electrolyte or acid base disturbances.

    - Acute anaemia.

    - Full stomach.

Preparation of the patient:

     Preoperative starvation; An empty stomach reduces pressure on the lung, and

     reduces the chance of vomiting.

     -Time of starvation depends on species; about 12 hours for single stomached animals,

     more (24-48) for ruminants.

     -Water should be freely available.

     -Small mammals and birds are not usually starved.

Pain and the central nervous system:

    -CNS comprises; brain and spinal cord.

    -Peripheral nervous system consists of; cranial and spinal nerves, and consists of

    bundle of thousands of nerve fiber.

    - The nerves connect the receptors with the CNS and CNS with the muscles and organs. - Nerve fibers act as a wire transmitting messages as electrical pulses. (in one direction only)

    Synapse; the region where two or more neurons meets and where impulses are passed from one to another.

    Neurotransmitter; are chemicals that move electrical impulses across a synaptic gap to the next neuron. It is stored in vesicles on synaptic membrane.

    Neurotransmitter are ;

     - Excitatory " glutamate" or inhibitory "GABA",

     - Circulating hormones " dopamine, adrenaline, noradrenaline".

     - Acetylcholine.

    Principles of sedation

Definitions:

? Tranquillizers; drugs that reduce fear and apprehension. Example is the

    acepromazine. Overdose of these drugs does not increase the sedative effect but may cause CNS stimulation and muscle twitching.

    ? Sedatives; drugs that relieve anxiety and make the animal sleepy. Drugs in this group include the alpha 2-adrenoceptor agonists (e.g. Xylazine).

? Hypnotics; drugs that induce sleep. e.g. thiopentone.

Reasons for using sedatives:

    - To control normal animals for minor procedures.

    - To reduce fear and resistance to anaesthesia.

    - Help to ensure smooth induction, and to reduce the dose of the anaesthetic.

General rules for using the sedative agents:

    - Sensory input " noise, etc" should be minimized, as no animal will sedate effectively when is significantly disturbed " high adrenaline level".

    - Choose the correct drug and route, and consider the species differences, age and the condition of animal "disease, pregnancy".

    - Sedatives are NOT anaesthetics. Never trust the effect and still handle with care "horse kick, dog bite".

     ALPHAADRENOCEPTOR AGONISTS 2

     "Most important sedatives in Vet. Med."

Agents include: Xylazine and Detomidine.

Mechanism of action:

    Alpha-2 receptors are found in the CNS, and PNS. The drug’s mechanism of action is mainly through its agonist activity at presynaptic

    adrenergic alpha-2 receptors, that decreases noradrenaline release from adrenergic nerve

    terminals in CNS and PNS.

    Pharmacokinetic: These drugs are metabolized by the liver.

Clinical uses:

    -Excellent sedatives, provide moderate to heavy sedation. - Provide analgesia, and muscle relaxation.

    - Several combinations are in widespread use. (Synergistic with some anaesthetics agents

     like ketamine)

Side effects:

- Bradycardia (decreased heart rate).

    - Respiratory depression (rate and depth).

    - Hypoxaemia and pulmonary oedema in ruminants.

    - Induce vomiting in small animals (dogs and cats).

    - Stimulation of pregnant uterus, which may lead to abortion. (data sheet gives

    contraindication in late stage of pregnancy.

General considerations:

    - There is a marked species differences in dosage with xylazine (xylazine dosage for

    cattle is one tenth of horse and dog dose).

    - Detomidine is very potent in horses (approximately 10 times than xylazine, but very

    low effects in camel).

    - Detomidine is less ecbolic effect than xylazine.

     Phenothiazines derivatives

    ? Acepromazine (ACP)

Mechanism of action: Dopamine antagonism in CNS

    Pharmacokinetic: Metabolized by liver.

Clinical uses:

    - Calming effects, and sometimes sedation.

    - Anti emetic effect in dogs and cats.

    - Maximal sedative effects are obtained with the recommended doses. (excessive dose

    does not increase the degree of sedation)

Side effects:

    - Priapism in male horses.

     GENERAL ANAESTHESIA

Definition:

    It is a state o f unconsciousness produced by reversible drug that produces depression of the CNS, in which the patient neither feels nor remember painful stimuli.

Induction and maintenance of anaesthesia:

    General anaesthesia can be induced by injectable agents (IV/IM) or inhalation (gaseous) agents. Onset of anaesthesia is faster with IV injection than with inhalation.

    The duration of action of IV induction agents is generally 5 to 10 minutes, after which time spontaneous recovery of consciousness will occur. In order to prolong anaesthesia for the required duration (usually the duration of surgery), anaesthesia must be maintained.

    Usually this is acheived by the total inravenous anaesthesia (TIVA), or by the inhalation system.

Mode of action:

    - Unknown specific receptor sites. -No method of antagonism.

Stages of anaesthesia:

    Stage One: voluntary excitement might be observed.

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