Anesthesia

On this page: Definition | Selected Terms | Phases of General Anesthesia | Types of General Anesthesia | Stages of General Anesthesia 


Definition:

A total loss of sensation in a part of, or in the entire body, as a result of drug induced depression of local / regional (peripheral) or general (central) nervous tissue.

Components:

  • Analgesia
  • Amnesia
  • Immobilization
  • Permits rapid induction, quick alteration of anesthetic depth and smooth, fast recovery
  • No depression of cardiopulmonary function
Ideal Anesthetic In reality
  • Does not depend on metabolism for elimination
  • Non irritating to tissues
  • Easy to administer
  • Safe, inexpensive and readily available

Factors to consider:

  • Species
  • Size (limits volume administered)
  • Genetics (inbred vs.outbred)
  • Age
  • Sex
  • Nutritional plane
  • Stage of reproductive cycle (e.g. pregnancy, lactation)
  • Health status
  • Body score condition (obesity vs.ideal weight)
  • Invasiveness and duration of procedure
  • Familiarity with anesthetic technique and technical skill

 

  • Alleviation of pain and provision of muscle relaxation for surgery
  • Restraint and safe transportation of wild and exotic animals
  • Euthanasia - Includes humane slaughter of food animals
  • Patient evaluation (complete physical examination) and additional diagnostic tests as necessary
  • Preanesthetic fasting –Exception: rodents, rabbits due to a high metabolic rate, rapid dehydration, severe hypoglycemia and inability to vomit
  • Prophylactic antibiotics, analgesics, enemas etc
  • In non emergency situations, correction of underlying abnormalities, such as anemia, infections and dehydration
Biologic Variants
  • Species
  • Genetics
  • Health status
  • Mental state
  • Age
  • Sex
  • Weight
Physiologic impacts
  • Central nervous system depression
  • Cardiovascular depression
  • Respiratory depression
  • Hypotension
  • Hypothermia
  • Gastrointestinal hypomotility
  • Renal hypoperfusion

May result from administration of:

  • Multiple drugs in a fixed dose formulation
  • Multiple separate drugs administered simultaneously
  • Multiple drugs administered separately at specific intervals
Anesthetic depth can be assessed by:
  • Response to external stimuli
  • Pulse rate, frequency and strength
  • Respiratory rate, frequency and strength
  • Capillary refill time
  • Reflexes (palpebral, pedal, corneal)
  • Blood pressure

 

Other Parameters
  • Muscle and jaw tones
  • Ocular position
  • Core body temperature
  • Blood oxygen saturation
  • Blood gases

 


 

Selected Terms

Tranquilization
  • Relief of anxiety
  • Aware of surroundings
  • No analgesia provided
  • Mode of action: depression of hypothalamic and reticular systems
  • Example: Acepromazine (Atravet)
 Sedation
  • Drowsiness and relaxation
  • Unaware of surroundings
  • No analgesia provided
  • Mode of action: depression of cerebral cortex
  • Example: Diazepam (Valium)

 

Local anesthesia
  • Loss of sensation of a circumscribed body area (can be superficial tissue such as the epidermis or deeper structures such as the spinal cord)
  • Mode of action: blockage of nerve conduction impulses
  • Example: Lidocaine

 

Dissociated Anesthesia
  • State of altered consciousness rather than loss of consciousness
  • Catatonia (muscular rigidity)
  • Intact swallowing reflex
  • Mode of action: dissociation of thalamo-cortical and limbic systems
  • Example: Ketamine (Ketaset)
Neuroleptanalgesia
  • Trance like state characterized by immobilization and analgesia
  • Will remain somewhat responsive to movement and noise
  • Combination of a tranquilizer and opioid agonist
  • Example: Fentanyl – fluanisone (Hypnorm)
Balanced Anesthesia
  • A multiple drug approach in order to induce anesthesia
  • Each drug is targeted to specifically attenuate an individual component (unconsciousness, muscle relaxation, autonomic reflexes, analgesia) of the anesthetic state

 

General Anesthesia
  • Loss of consciousness and sensation in conjunction with muscle relaxation
  • Non arousable
  • Attenuation of sensory, motor and autonomic reflex functions
  • Mode of action: CNS depression
  • Single agent or combination drug approach

 

Surgical Anesthesia
  • Plane of general anesthesia that provides unconsciousness, analgesia muscular relaxation and immobilization sufficient for painless surgery.

 

Phases of General Anesthesia

1. Preanesthesia

The stage from full consciousness to a state of sedation or tranquilization with varying degrees of muscle relaxation and immobilization.

Reasons for preanesthesia:

  • Calm patient
  • Induce sedation
  • Reduce general anesthetic drug requirements
  • Provide analgesia and muscle relaxation
  • Decrease airway secretion, salivation, gastric fluid volume and acidity
  • Suppress vomiting or regurgitation
  • Obtund autonomic reflexes
  • Promote smooth and rapid induction and recovery phases

Preanesthesia Drugs:

  • Anticholinergics e.g. Atropine, glycopyrrolate
  • Tranquilizers e.g. Acepromazine
  • Sedatives e.g. Diazepam

2. Induction

  • The stage following the preanesthetic stage characterized by a loss of consciousness and complete muscular relaxation and immobilization.
  • Typically a very short phase (5-10 minutes)
  • Often associated with an excitement phase prior to achievement of general anesthesia
  • Both injectable and inhalant drugs are available

Induction drugs:

  • Barbituates e.g. Thiopental, thiamylal, methohexital
  • Alkylphenol e.g. Propofol
  • Inhalant gases e.g. Isoflurane, halothane

3. Maintenance

  • A plane of general anesthesia that is achieved prior to surgical intervention.
  • Associated with the greatest physiological impact (hypothermia, hypotension, depression of cardiopulmonary parameters).

Maintenance Drugs:

Examples:

  • Inhalant gases (isoflurane, halothane)
  • Barbituates (pentobarbital)
  • Neuroleptanalgesics (fentanyl-fluanisone, Hypnorm®
  • Tribromoethanol (Avertin®)
  • Drug combinations (e.g.ketamine, xylazine and acepromazine)

 

Types of General Anesthesia:

Injectable

  • Anesthetic drugs can be administered topically, intravenously, intramuscularly, intraperitoneally, subcutaneously, epidurally or intrathecally
  • Can be used to induce local, regional or general anesthesia
  • Anesthetic depth can be more difficult to recognize
  • Controlling anesthetic depth can be problematic
  • Minimization of special equipment

Inhalant

  • Anesthetic gases in combination with 100% oxygen are inhaled
  • Easier to monitor anesthetic depth
  • Gas concentrations delivered to patient can be continually adjusted to ensure appropriate plane of anesthesia
  • Expertise in oral endotracheal intubation may be required
  • Occupational health issues (appropriate scavenging of waste gases)
  • Requires special equipment

Stages of General Anesthesia

A continuum of progressive CNS depression defined by the neuromuscular signs exhibited by the patient while under the influence of inhalant anesthesia. 
Patient response dependent on factors such as physical status of patient, preanesthetic medication(s) and parameters such as carbon dioxide retention and blood oxygen saturation.

Stage I

  • Termed “stage of voluntary movement”
  • Time from initial drug administration to loss of consciousness
  • May demonstrate ataxia, struggling, excitation, breath holding, salivation, defecation and urination

Stage II

  • Termed “stage of delirium or involuntary movement”
  • Loss of all voluntary control
  • From loss of consciousness to onset of regular respiratory pattern
  • External stimulation can cause exaggerated reflex responses

Stage III

  • Termed “stage of surgical anesthesia”
  • Progressive reflex depression
  • Ventilatory rate becomes slower and regular
  • Loss of vomiting and swallowing reflexes
  • Divided into 4 planes:
    > Plane 1: Analgesia and muscle relaxation sufficient only for minor noninvasive procedures of short duration
    > Plane 2: Analgesia and muscle relaxation sufficient for most surgical procedures
    > Plane 3: Paralysis of intercostal muscles & Abolishment of all reflexes
    > Plane 4: Severe cardiopulmonary depression & Pupillary dilatation

Stage IV

  • Respiratory arrest and cardiovascular collapse

 

 

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