Anesthetic Agents

Anesthetic Agents

  • Anesthetic Agents: any drug used to induce a loss of sensation (with or without unconsciousness)

  • Adjunct:

    • Effects include: sedation, muscle relaxation, analgesia, reversal, neuromuscular blockade, or parasympathetic blockage

    • Adjuncts are part of our goal of balanced anesthesia (therefore included in our study here)

Classifications of Anesthetic Agents

  1. Route of Administration: such as injectable, oral, or inhalant agents

  2. Timing: such as premedication, induction agents, and maintenance agents

  3. Principal effects: such as local anesthetics or general anesthetics

  4. Chemistry: drugs grouped in this way (classes) will share similar properties and effects

  • We will utilize timing and chemistry to group our studies!

Quick Review

  • What is pharmacokinetics?

  • What is pharmacodynamics?

  • What is an agonist drug versus an antagonist drug?

Effects of Anesthetic Agents and Adjuncts

  • Need to understand major effects of each drug to use them safely and effectively

  • Dose Dependent:

    • Some are desirable: loss of consciousness, muscle relaxation, etc

    • Some are undesirable: respiratory depression, hypotension, etc

      • Often referred to as “adverse effects” or “side effects”

Analgesic Effects of Anesthetics and Adjuncts

  • Analgesia needs to be considered for any painful procedure

  • Many commonly used general anesthetics produce unconsciousness but do not provide pain control

    • Not considered “true analgesics”

  • Animals under anesthesia are not able to perceive pain until they are awake again

    • Pain is still occurring: physiological changes

    • Acutely perceived when patient regains consciousness; now difficult to get under control

  • Therefore - analgesic medications should be included as part of anesthetic protocol

Using Drugs in Combination

  • Many anesthetic drugs used in combination for balanced anesthesia

  • Some can safely mix in syringe; some cannot

  • Mixing incompatible drugs can cause:

    • Loss of potency

    • Change in chemistry

    • Precipitation of one or more of the drugs

    • Other untoward interactions

  • Generally: most are water soluble and can be mixed

  • Exception: Diazepam; can mix with ketamine

  • Best Practice Rule: do not mix anesthetic drugs unless you have scientific knowledge that you are safe to do it!

Pre-Anesthetic Medication

  • Drugs administered during the pre-anesthesia period

  • Classes include:

    • Anticholinergics

    • Tranquilizers And Sedatives

      • Phenothiazine drugs

      • Benzodiazepine drugs

      • Alpha-2 adrenoreceptor agonists

    • Opioids

Reasons for Using Pre-Anesthetic Medication

  • To calm or sedate an excited, frightened, or vicious animal

  • To minimize adverse effects of concurrently administered drugs

  • To reduce required dose of concurrently administered agents

  • To produce smoother anesthetic inductions and recoveries

  • To decrease pain and discomfort before, during, and after surgery

  • To produce muscle relaxation

Pre-Anesthetic Drugs

  • Also have other uses!

  • Examples:

    • Tranquilizers - calm patients for transport, wound care, radiographs, etc

    • Benzodiazepines - given to stop seizures

    • Opioids - cough suppression

Pre-Anesthetic Drug Routes

  • Given a variety of routes

  • Will affect onset of action, duration of action, and dose

  • Patients given pre-anesthesia IM or SC: leave undisturbed until peak action is reached

    • Excitement or stimulation can override effects of these agents

  • IV dosing: works within second to minutes - watch carefully

    • Generally about 1/2 of IM or SC dose of same drug

  • Oral dosing is usually unpredictable - avoid unless necessary (too aggressive to handle)

Anticholinergics (AKA: Parasympatholytics)

  • Non-controlled drugs

  • Used to prevent and treat bradycardia and decrease salivary secretions that occur due to stimulations of parasympathetic nervous system

  • Most commonly used drugs in this category:

    • Atropine

    • Glycopyrrolate

  • Approved routes of administration: IV, IM, SC, IT

  • Mode of Action: How does it work?

Nervous System Review

  • Primary neurotransmitter of the parasympathetic nervous system = acetylcholine

  • Two receptors for acetylcholine:

    • Nicotine Receptors (located on postganglionic neurons at junction with preganglionic neurons)

    • Muscarinic receptors (located on target organs)

Anticholinergics: Mode of Action

  • Competitively block the binding of acetylcholine at the muscarinic receptors

  • Vagus nerve: main component of the PNS

    • Innervates many important organs including:

      • Heart, lungs, GI tracts, and iris of eye

    • Surgery can irritate vagus nerve (intubation, visceroagal reflux occurring with organ manipulation, manipulating the eye during ocular surgery, and some common anesthetic drugs)

      • Causes physiological changes such as bradycardia, bronchoconstriction, excessive tear and salivary production, excessive respiratory secretions, increased GI motility, miosis

      • Blocking this = reverse and prevent these effects

Anticholinergics:Onset of Actions and Durations

  • After IM injection:

    • Atropine - begins to work in about 5 minutes; peak effect 15-20 mins; duration of 60-90 minutes

    • Glyco - similar onset as atropine; peak effect 35-40 min; duration of 2-3 hours

    • Should be given 20-30 min before induction (to allow for peak effects)

  • Given IV:

    • Atropine: onset of action at about 1 min; peak effect at 3-4 min

      • Ideal drug for emergency situations (bradycardia; CPR)

Anticholinergics: Effects

  • CNS:

    • Not sedatives so limited CNS effects

    • Atropine can cause mild sedation in some patients

    • Glyco does not cross blood brain barrier

  • Cardiovascular:

    • Prevent bradycardia (which is often associated with anesthesia/surgery)

    • At low doses, atropine can cause bradycardia (blocks another receptor that inhibits acetylcholine release)

    • Arrhythmias: after IV admin, can cause temporary first or second degree atrioventricular (AV) block, followed by sinus tachycardia

      • Can cause other arrhythmias (atropine more likely than glyco)

      • Contraindicated in animals with pre-existing rapid heart rates, or heart disease

  • Respiratory System:

    • Reduces and thickens respiratory and salivary secretions

      • Because of this - contraindicated in cats and ruminants

      • Thick mucous secretions seen with anticholinergics used in cats

        • Blocks airway

      • Causes saliva to become thick and ropy in ruminants, increased risk of respiratory obstruction

    • Bronchodilation: increases diameter of bronchioles

      • Increases anatomic dead space (area of breathing circuit not participating in gas exchange)

        • Increase risk for hypoventilation and hypoxemia (low blood oxygen)

Anticholinergics: Other Effects

  • Mydriasis: seen in cats more than dogs

  • Reduced GI and salivary secretions

    • Can lead to dry mouth

    • Reduction of lacrimal secretions

    • Can lead to drying of corneas - corneas

    • Use eye lubricant!

  • Inhibit intestinal peristalsis

    • Gut stasis and colic in horses

    • Bloat in ruminants

    • Avoid in these species

Use of Anticholinergics

  • Still commonly used, through less often than previously

  • Beneficial for some patients but have potential of significant adverse effects

  • Atropine vs Glycopyrrolate:

    • Glyco slightly less likely to induce cardiac arrhythmia

    • Glyco suppresses salivation better than atropine

    • Gylco only minimally crosses placental barrier (less than atropine)

    • Atropine more affordable

    • Atropine better in emergencies (faster onset of action)

Tranquilizers and Sedatives

  • Reminder:

    • Tranquilizer:

    • Sedatives:

    • Effects overlap

  • Three classes of tranquilizers/sedatives:

    • Phenothiazines

    • Benzodiazepines

    • Alpha-2 adrenoceptors agonists