☻Anesthesiology

ANESTHESIA

Terminology:

1-Anesthesia It is the art and the science related to production of insensibility.

2-General anesthesia It is a state of unconsciousness as a result of controlled reversible intoxication of the central nervous system, and characterized by lowered sensitivity to external stimuli with diminished motor response to such stimuli.

3-Anesthetic agent It is the substance that produces controllable loss of consciousness and absence of motor response to noxious stimuli.

4-Analgesic agent It is the substance that temporarily abolishes awareness of pain.

5-Local analgesic It is a substance that when applied to the nerve endings or nerve fibers, temporarily prevents the conduction of impulses by the nerve, by interference with transmission of impulses concerned with appreciation of pain.

6-Narcotic agent It is the substance that produces insensibility, or stupor bordering upon it, and simple stimuli like noise can only produce temporary partial arousal. Accordingly all anesthetic agents are narcotic but many narcotics are not anesthetics.

7-Hypnotic agent It is a narcotic agent that produces sleeping, which is a state of physiological unconsciousness, from which the animal can easily be awakened by wide variety of stimuli.

8-Sedative It is a narcotic agent that can be used to calm a nervous, excited, or vicious animal, and these drugs cause drowsiness.

9-Ataractic or tranquilizer It is substance that produces sedation without drowsiness.

TYPES OF ANESTHESIA:

I-Substances Have Selective Transient Paralytic Action On Sensory Nerves

      1-Local Analgesia

 A-Surface application         i-Topical application                

                                  ii-Intra-synovial analgesia

B-Intra or sub-dermal infiltration

C-Subcutaneous infiltration        i-Linear infiltration               

                                         ii-Field block analgesia         1-Cup shape

                                                                                       2-Inverted-L block 

                                                                           3-Ring block

      2-Regional Analgesia

A-Perineural nerve block          i-Peripheral nerve block

                                         ii-Paravertebral nerve block

            B-Spinal nerve block                                                     

                               i-Epidural analgesia     1-Caudal epidural  

                                                                    2-Segmental lumbar epidural   

                              ii-Intrathecal analgesia or sub arachnoid

II-Sedation, Narcosis, And Pre-Anesthetic Medication

                     1-In Combination With Local Or Regional Analgesia

                     2-In Adjunction To General Anesthesia

                  1-By Inhalation – Volatile Anesthesia      

      2-By Intravenous Injection

      3-By Combination Of The Mentioned Types With Or Without Premeditation

GENERAL CONSIDERATIONS IN SELECTING ANESTHETIC METHOD:

1-Nature And Magnitude Of The Operation: -

Local infiltration suffices for simple interferences like incision of superficial abscess or neoplasm, or castration in immature animals. However some simple surgical operations can’t be performed by local infiltration as a result of severe fibrosis.

2-Site Of Operation: -

Presence of some critical structures in vicinity of site of operation may render local infiltration insufficient as the movement of the animal may endanger his life, and the example is the surgery for retropharyngeal abscess.

3-Duration Of Operation: -

The duration of operation affects the choice of anesthetic method, especially when adopting general anesthesia. Short-duration, simple dental operations, can be performed by using ultra-short acting barbiturates, while longer interferences can be performed by using longer-acting barbiturates with local analgesia, or inhalation anesthesia. Pre-anesthetic medication should be considered when the operation is a major operation with long duration and it is required that the animal remain quite for several hours after surgery. Pre-anesthetic medications not only reduce the amount of anesthetic agent and increase duration of anesthesia, but also control undesirable effect of some anesthetics like salivation.

4-Species: -

Not only size and temper of the animal affect the choice of anesthetic method, but also the anatomy and physiology of some species affect that choice. Generally the larger size animals have greater difficulties and dangers in induction and maintenance of general anesthesia. The safe satisfactory methods for general anesthesia in pets may be unsuitable for large animals, especially for heavy vigorous one, as the upset of locomotor coordination and prolonged recumbency may entail risks.

Not all species react to drugs in the same manner, as horse may be excited when administered sub-anesthetic dose of barbiturate, and cat may become maniac when given large dose of morphine.

Increased susceptibility for toxicity by anesthesia affected by two main factors, the prolonged fasting (predisposes to depletion of liver glycogen and reduced detoxication capacity of non-volatile anesthetic agents), and disease condition (toxemia predisposes to degeneration of parenchymatous organs as liver leading to reduction of its detoxication capacity, also toxemic animal seems to need smaller dose).

A-The Horse: -

The animal should be adequately restrained to ensure safety of the veterinarian and the animal himself. Casting methods of conscious animal frightening and expose him to injury, accordingly, many muscle paralyzing drugs can be sued to induce casting without endangering the animal.

The possibility of blocking of many peripheral nerves should be considered, and on using anesthetic method that is associated with slow recovery, veterinarians have to ensure that this recovery period will be free from excitement.

Some sedatives and non-volatile anesthetics are precluded from use in horse, as they don’t have some requirements like; the ability of the animal to rise to his feet soon after surgery; and at the outside within 1-2 hours with strong enough locomotor power and coordination to prevent the animal from falling.

B-Ruminants: -

Generally they are unsuitable candidate for inhalation anesthesia unless endotracheal tube is used, but under field condition, light general anesthesia by intravenous injection has satisfactory results. However the simpler regional analgesic techniques in this species and side effects of general anesthesia make regional anesthesia more popular in these species.

C-The Dog: -

General anesthesia has a high degree of perfection in this species that make this method so popular for veterinarians not only for surgery but also for examination procedures in the animal. Some breeds have brachicephalic skull predisposing them to asphyxia as a result of relaxation of jaw muscles during general anesthesia and accordingly endotracheal tube should be considered on inducing general anesthesia in such breeds.

D-The Cat: -

Cat is a difficult subject to be anesthetized quietly and safely as restraint provokes violent struggling. Accordingly, cat should be handled quietly with minimal restraint then general anesthesia can be induced by slow intravenous barbiturate.