anesthesiology

...ANESTHESIA ...

Satisfactory anesthesia is very important for both humanitarian and technical efficacy. Humanity ensures gentle handling of the animal with minimal restraint to minimize possible injury to the animal, while technical efficacy ensures protection of personnel from bite, scratch, kick, inhalation of volatile anesthetics in the animal breath, or accidental self injection by sedative or addictive drugs.
The veterinary anesthetist deals with variety of animal species that exhibit variation in size, temperament, and anatomical and physiological development. Response to anesthesia not only varies according to species and breed, but it also varies among individuals of the same breed. Fear and aggressive reaction of animals to casting (prior to anesthesia) and struggling to escape breathing irritant gases, increase the difficulty of anesthetic administration and brain activity that in turn affects the amount of required anesthetic. Accordingly, sedatives and tranquilizers are used pre-anesthetic to reduce brain activity, fear and struggling to reduce anesthetic dose and to help smooth induction and recovery of 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-Local analgesia: -
It is the loss of sensation in a limited area of the body
7-Regional analgesia: -
It is insensibility in a large area of the body, or it is a process through which certain region retained insensible
8-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.
9-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.
10-Sedative: -
It is a narcotic agent that can be used to calm a nervous, excited, or vicious animal, and these drugs cause drowsiness.
11-Ataractic or tranquilizer: -
It is substance that produces sedation without drowsiness.
AIMS OF ANESTHESIA: -
1-Humanity point of view like prevention of pain during surgical interference
2-Creation of a safe state under which the surgeon and assistants can work
EXAMINATION OF THE ANIMAL: -
The general condition of the animal should be evaluated and recorded prior to anesthesia and surgery, including history, temperature, pulse rate, and respiration rate etc....
PREPARATION OF ANIMAL FOR ANESTHESIA: -
1-Fastening 24 hours prior to operation
2-Fluid therapy according to animal state
3-Preanesthetic medication according to the nature of anesthesia, surgery, and animal species
TYPES OF ANESTHESIA: -
I-Substances have selective transient paralytic action on sensory nerves
I-1-Local analgesia
I-1-A-Surface application
I-1-A-i-Topical application
I-1-A-ii-Intra-synovial analgesia
I-1-B- Intra-dermal and Subcutaneous infiltration analgesia
I-1-B-i-Intra or sub-dermal infiltration
I-1-B-ii-Linear infiltration analgesia
I-1-B-iii-Field block analgesia
I-1-B-iii-1-Cup shape
I-1-B-iii-2-Inverted-L block
I-1-B-iii-3-Ring block
2-Regional analgesia
2-A-Perineural nerve block
2-A-i-Peripheral nerve block
2-A-ii-Paravertebral nerve block
2-B-Spinal nerve block
2-B-i-Epidural analgesia
2-B-i-1-Caudal epidural
2-B-i-2-Segmental lumbar epidural
2-B-ii-Intrathecal analgesia or sub arachnoid
II-Sedation, narcosis, and pre-anesthetic medication
II-1-In combination with local or regional analgesia
II-2-In adjunction to general anesthesia
III-Substances have depressant paralytic action on the cns producing progressive loss of consciousness and voluntary motor function (general anesthesia)
III-1-By inhalation (volatile anesthesia)
III-2-By intravenous injection
III-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 is sufficient 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, temperament of the animal, or severity and intensity of surgical procedure.
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 and breed of animal: -
Not only size and temperament 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.