Principles of Anesthesia and Anesthetic Pharmacology

Principles of Anesthesia

  • Definition: Anesthesia means "without sensation."
  • Mechanism of Action: Anesthetic drugs work by depressing the central nervous system (CNS).
  • Concept: Anesthesia is often referred to as "controlled death" due to its depressive effects on vital body systems.
Challenges of Anesthesia
  • Physiological Depression: Causes depression of the neurologic, cardiovascular, and pulmonary systems.
  • Drug Management: Requires precise drug calculation and administration.
  • Rapid Assessment: Demands quick evaluation of visual, tactile, and auditory information from the patient and monitoring devices.
  • Multitasking: Anesthetic procedures necessitate significant multitasking.
  • Risk vs. Surgery: The risks associated with anesthesia are frequently greater than those of the surgical procedure itself.
Pre-Anesthetic Exam
  • Medical History: A thorough medical and past anesthetic history is obtained for every animal before anesthesia.
  • Pre-existing Conditions: Any pre-existing health conditions are the most critical knowledge to consider when evaluating a patient for anesthesia.
  • System Focus: Special attention is paid to the cardiovascular and pulmonary systems during the exam, as they undergo the most stress during anesthesia.
Pre-Anesthetic Lab Testing
  • Response Assessment: Lab values assess an animal's ability to respond to the depressive effects of anesthesia.
  • Metabolism & Excretion: They also evaluate the patient's capacity to metabolize and excrete anesthetics and other drugs.
  • Underlying Conditions: Lab values can provide clues indicating underlying health issues.
Anesthetic Risk Assessment
  • Review Process: The patient history, physical exam, and pre-anesthetic diagnostic results are reviewed by the Doctor of Veterinary Medicine (DVM).
  • Risk Assignment: This information is used to determine the patient's anesthetic risk, assigned using the ASA (American Society of Anesthesiologists) scale.
  • Postponement: Anesthesia will be postponed if any abnormalities are discovered during the pre-anesthetic exam.
ASA Scale
  • P1 (Minimal Risk):
    • Criteria: Normal, healthy patient.
    • Representative Conditions: Patients undergoing elective procedures (e.g., ovariohysterectomy, castration, declaw).
  • P2 (Low Risk):
    • Criteria: Patient with mild systemic disease.
    • Representative Conditions: Neonatal, geriatric, or obese patients; mild dehydration; skin tumor removal.
  • P3 (Moderate Risk):
    • Criteria: Patient with severe systemic disease.
    • Representative Conditions: Anemia; moderate dehydration; compensated major organ disease (e.g., ruptured bladder).
  • P4 (High Risk):
    • Criteria: Patient with severe systemic disease that is a constant threat to life.
    • Representative Conditions: Internal hemorrhage; pneumothorax; pyometra.
  • P5 (Extreme Risk):
    • Criteria: Moribund patient that is not expected to survive without the operation.
    • Representative Conditions: Severe head trauma; pulmonary embolus; gastric dilatation-volvulus (GDV); end-stage major organ failure.

Stages of Anesthesia

Anesthesia traditionally has four stages:

Stage 1 – Disorientation/Induction
  • Transition: The patient moves from consciousness into unconsciousness.
  • Induction Methods: Anesthesia can be induced by injectable or inhaled drugs.
  • Critical Period: This is the time when cardiac arrest is most likely to occur.
  • Respirations: Respiration rate can vary from normal to panting.
Stage 2 – Excitement Phase
  • Behavioral Signs: The animal becomes excited, may struggle/paddle, vocalize, chew, and yawn.
  • Breathing: Respirations become irregular, potentially involving breath-holding or hyperventilation.
  • Cardiovascular: Heart rate increases.
  • Muscle Tone & Reflexes: Good muscle tone is present, and all reflexes are present and may be exaggerated.
Stage 3 – Ideal Anesthesia
  • Goal: This is the ideal or desired stage for surgical patients, representing a balance of not too light and not too deep.
  • Planes: Stage 3 is further divided into four planes:
    • Plane 1 (Light Anesthesia):
      • Breathing: Becomes regular.
      • Heart Rate: Within normal range.
      • Response: Animal may respond to surgical manipulation.
      • Muscle Tone: Good.
      • Reflexes: Diminished.
    • Plane 2 (Moderately Light Surgical Anesthesia):
      • Goal: This represents the ideal plane of the ideal stage of anesthesia.
      • Breathing: Regular, may be shallow.
      • Heart Rate: Lower end of normal range; pulse is strong; HR and respiration rate may increase with surgical manipulation.
      • Muscle Tone: Relaxed.
      • Reflexes: Patellar, palpebral, and corneal reflexes may be present.
    • Plane 3 (Deep Surgical Anesthesia):
      • Respirations: Shallow and at the low end of the normal range.
      • Cardiovascular: Heart rate is slowed; capillary refill time (CRT) is increased.
      • Muscle Tone: Greatly reduced.
      • Response: No response to surgical manipulation.
      • Reflexes: Diminished or absent.
    • Plane 4 (Overdose Stage):
      • Respirations: Jerky.
      • Heart Rate: Below normal values.
      • Circulation: Capillary refill time (CRT) is prolonged; mucous membranes are pale.
      • Muscle Tone: Flaccid.
      • Reflexes: No reflex activity.
Stage 4 – Moribund
  • State: The animal is "death bound" (moribund).
  • Respirations: Apneic (cessation of breathing).
  • Cardiovascular: Cardiovascular collapse.
  • Muscle Tone & Reflexes: Flaccid muscle tone and no reflex activity.
  • Urgency: THIS IS AN EMERGENCY!
Signs of Excessive Anesthetic Depth
  • Respiration: Low respiration ( < 8 ext{ breaths per minute}).
  • Heart Rate: Low heart rate ( < 80 ext{ beats per minute}).
  • Oxygen Saturation: Low oxygen saturation ( < 90 ext{%}).
Patient Monitoring
  • Frequency: Patients should be monitored a minimum of every 55 minutes.
  • Vitals to Monitor:
    • Heart Rate
    • Pulse Rate
    • Respiratory Rate
    • Blood Pressure
    • Percent Oxygen Saturation (extSPO2ext{SPO}_2)
    • Temperature

The Nervous System

  • Analogy: Tissue receptors in the body are like "locks"; neurotransmitters and CNS drugs are the "keys."
Divisions of the Nervous System
  • Central Nervous System (CNS): Comprises the Brain and Spinal Cord.
  • Peripheral Nervous System (PNS):
    • Somatic Nervous System (SNS): Responsible for voluntary control.
    • Autonomic Nervous System (ANS): Responsible for involuntary control.
      • Parasympathetic Nervous System: Involved in "rest-and-digest" activities.
      • Sympathetic Nervous System (SNS): Also known as the adrenergic system.
Sympathetic Nervous System (SNS)
  • Function: Controls the body's response to emergency or stressful situations.
  • Nickname: Referred to as the "Fight-or-Flight" system.
  • Physiological Effects:
    • Dilates pupils.
    • Dilates bronchioles of the lungs.
    • Increases heart rate (HR) and force of contraction.
    • Constricts blood vessels.
    • Activates sweat glands.
    • Inhibits peristalsis (digestion).
Sympathomimetics
  • Mechanism: Drugs that mimic the effects of the sympathetic nervous system by acting like SNS neurotransmitters (e.g., epinephrine, norepinephrine, dopamine, catecholamines).
  • Uses:
    • Cardiac arrest.
    • Severe allergic reactions (anaphylactic shock).
    • Hypotension.
    • Capillary bleeding.
    • Urinary incontinence (by constricting the bladder sphincter).

Pre-Anesthetic Agents

  • Purpose: Medications administered prior to induction of anesthesia.
  • Reasons for Administration:
    • To calm or sedate the patient.
    • To reduce the amount of general anesthetic needed.
    • To produce smoother transitions into and out of general anesthesia.
    • To minimize adverse effects of general anesthesia.
Types of Pre-Anesthetic Drugs
  • Anticholinergics
  • Sedatives (e.g., Alpha-2 Antagonists like Xylazine, DexDomitor)
  • Tranquilizers (e.g., Benzodiazepines, Phenothiazines)
  • Opioids (Narcotic analgesics)
  • Note: These can be given alone or in combination.
Anticholinergics
  • Mechanism: Block the action of the neurotransmitter acetylcholine, which is primarily used by the parasympathetic nervous system for "rest-and-digest" activities.
  • Effects (similar to sympathomimetics but different mechanisms): Cause tachycardia, hypertension, pupil dilation, and mental status changes in overdose.
  • Benefits:
    • Decrease gastrointestinal (GI) motility.
    • Decrease upper airway secretions, reducing salivation.
    • Prevent bradycardia (slow heart rate).
    • Dilate the pupils.
  • Side Effects:
    • Drowsiness, disorientation, anxiety.
    • Tachycardia.
    • Photophobia.
    • Constipation.
    • Burning at injection site.
  • Common Drugs: Glycopyrrolate and atropine sulfate are the two main anticholinergics used in veterinary medicine.
Sedatives
  • Function: Calm the patient and reduce irritability and excitement.
  • Analgesia: Typically do not possess analgesic properties, except for Alpha2 antagonists.
  • Alpha2 Antagonists:
    • Mechanism: Inhibit the release of norepinephrine, which maintains alertness.
    • Effects: Provide excellent sedation, muscle relaxation, and some minimal analgesia.
    • Side Effects: Pale/grey mucous membranes, extremely low heart rate, vomiting.
    • Examples:
      • Xylazine: Mainly used in large animals; very harmful to humans.
      • DexDomitor: Mainly used in dogs & cats.
    • Reversal Agents:
      • Yohimbine reverses Xylazine.
      • Antisedan reverses DexDomitor.
Tranquilizers
  • Function: Narcotics that relax a patient and induce mood changes.
  • Common Types:
    • Phenothiazines (e.g., Acepromazine):
      • Do NOT produce analgesia but enhance the analgesia of other drugs when used in combination.
      • Duration: Lasts 2−42-4 hours.
      • Contraindications: Should not be used in pets with liver problems or a history of seizures (as they lower the seizure threshold). Also, not recommended for very young or old animals due to respiratory depression.
    • **Benzodiazepines (e.g., Diazepam): ** * Provide less tranquilization than acepromazine or alpha-2 antagonists.
      • Common Use: Typically used with opioids or with ketamine to counteract muscle rigidity.
      • Patient Suitability: Good for patients with heart disease or a history of seizures.
      • Diazepam (Valium): A commonly used benzodiazepine tranquilizer. Causes burning if given subcutaneously (SC) or intramuscularly (IM), so it is only given intravenously (IV).
    • Opioids: Some opioids (e.g., butorphanol, hydromorphone, morphine) can produce tranquilization in addition to analgesia.

Anesthetic Drugs (General Anesthetic Agents)

  • Definition: General anesthetic agents create a loss of all sensation in the patient, including pain, cold, heat, pressure, or touch.
Types of Anesthetic Agents
  • Barbiturates
  • Dissociatives
  • Inhalants
  • Local Anesthetics
Dissociative Anesthetics
  • Characteristics: Short-acting, injectable anesthetics.
  • Mechanism: Produce a unique form of anesthesia where the animal feels dissociated from its body.
  • State Induced: Can put an animal into a cataleptic state with heightened emotional responses and overall muscular rigidity.
  • Common Use: Most commonly used for induction of anesthesia.
  • Specific Uses: Can be used alone in cats as an immobilizer.
  • Side Effects: Causes muscle rigidity and excessive salivation; often used with a muscle relaxant to counteract these effects.
  • Analgesia: Produces immobilization and peripheral/superficial analgesia.
  • Reflexes: Muscle tone is increased while swallowing and ocular reflexes remain.
  • Administration: Can be given IM in cats or IV in dogs & horses.
  • Reversibility: These drugs are NOT reversible.
  • Contraindications: High dosages should be avoided in patients with liver and kidney disease.
  • Recovery: Cause increased sensitivity to stimuli; behavioral changes may be observed during recovery.
  • Ketamine: A commonly used dissociative; used to induce anesthesia in dogs when combined with diazepam (a muscle relaxant).
Barbiturates
  • Uses: Produce short-term anesthesia, control seizures, and euthanize animals.
  • Onset: Rapid onset of anesthesia due to quick dispersion and easy penetration of the blood-brain barrier.
  • Euthanasia: Effective euthanasia agents because they produce unconsciousness before respiratory and cardiac arrest.
  • History: The oldest class of anesthetic agents, developed and used since the 1930s.
  • Classes (based on duration of action):
    • Ultra-short-acting:
      • Provide rapid loss of consciousness.
      • May require intubation and respiratory support due to respiratory depression.
      • Tissue Damage: Tissue necrosis will occur if given outside the vascular space.
      • Contraindications: Should not be given to patients with compromised livers; contraindicated in sighthounds (e.g., Greyhounds).
    • Short-acting
Non-Barbiturates
  • Propofol:
    • Type: A non-barbiturate that produces a rapid loss of consciousness.
    • Chemistry: Not related to other sedatives.
    • Pharmacokinetics: Non-cumulative and rapidly cleared from the body.
    • Administration: Can be infused for maintenance or given in bolus.
    • Side Effects: Apnea (temporary cessation of breathing) and cardiac depression.
    • Patient Suitability: Safe alternative for sighthounds.
    • Dosage: Often the high-end of the dose is drawn up and given "to effect."
    • Appearance: A milky, white emulsion containing soybean oil, egg lecithin, and glycerol.
    • Handling: Must be handled specifically due to an affinity for bacteria growth; refrigerate after opening and DISCARD AFTER 66 HOURS.
    • Uniqueness: The ONLY non-clear substance that can be given intravenously.
Inhalant Anesthetic Agents
  • Mechanism: Gas form of an anesthetic inhaled by the lungs, absorbed from the alveoli, and then distributed to the brain.
  • Primary Use: Used to maintain anesthesia after induction (which typically uses dissociatives, non-barbiturates, or barbiturates).
  • Effects: Creates muscle relaxation and unconsciousness.
  • Advantages: Allows for control over the depth of anesthesia; provides a more rapid recovery than injectable drugs; metabolized faster.
  • Common Inhalants:
    • Isoflurane: Most commonly used.
    • Sevoflurane: Alternate to isoflurane, offering a more rapid induction and recovery.
Local Anesthesia
  • Function: Promote a loss of sensation in a regional area of the body without a loss of consciousness.
  • Mechanism: Prevent the conduction of nerve impulses in peripheral nerves.
  • Indications: Used when general anesthesia is not advised or necessary.
  • Application Methods: Can be topical, intradermal, intra-articular, or intravenous.
    • Topical gels and transdermal patches exist.
    • Injectable solutions will cause numbness if they come in contact with the skin.
    • Epidurals are a form of intra-articular local anesthetic.
  • Specific Uses:
    • 2% Lidocaine: Used to numb and paralyze the larynx and vocal folds of cats prior to intubation.
    • Laryngeal Spasm in Cats: Cats are very prone to laryngeal spasm, where vocal folds clamp shut if the larynx is irritated (e.g., during intubation). This can cause asphyxiation in minutes to seconds.
    • Post-Procedure Monitoring: Cats need to be monitored for laryngeal spasm after intubation. Failure to intubate can also lead to asphyxiation.