Pre-anesthetic agents

General anesthesia process

1. pre-medication

2. induction (injectable anesthetics_

3. maintenance (inhalant/injectable)

WHY?

• calm, free from anxiety = sedation

• proper and ease of the patients restraint = chemical restraint

• pre-emptive analgesia

• decrease drug dosage

• smooth induction/ smooth recovery

• counteract side effect caused by other premedication drugs

Sedation

• to decrease stress, anxiety from the patient

chemical restraint

• to provide proper restraint and to decrease the chance from getting injury for both your patient and yourself

Pre-emptive analgesia

• before pain occurs, control and treat pain

• recovery from surgery is faster

• control pain, decrease other drug dosage, like inhalant maintenance

decrease drug dosage

• not only induction and inhalant anesthesia dosage, but also premedication drug dosage

• decrease side effect caused by each drug

Example #1

• sedation eith hydromorphone (0.1 mg/kg, IM) and acepromazine (0.02 mg/kg, im)

• sedation with acepromazine (0.2 mg/kg, IM)

  • side effect: peripheral vasodilation = hypotension

• Both equal same sedation quality

example #2

• propofol

  • without pre-medication: 8-10 mg/kg, IV

  • with pre-medication: 4-6 mg/kg, IV

Example #3

• if you want to perform OHE…

  • without pre-medications:

    • isoflurane maintenance: 4~5%

  • With Pre-medication:

    • isoflurane maintenance: 1~2%

• less waste gas = more economic and less vasodilation = less hypotension

counteract side effect caused by other premedication drugs

• example: look at slide

Pre-medication drugs:

1. phenothiazine (major tranquilizers)

2. benzodiazepines (minor tranquilizers)

3. Alpha 2 agonists

4. opioids

5. anticholinergics

Major tranquilizers

• phenothiazines:

  • acepromazine

• butyrophenones:

  • droperidol (in innovar-vet)

  • azaperone (stresnil)

• Mechanism of action:

  • dopamine antagonists

  • high doses have negative effects: tremor, rigidity, catalepsy

Acepromazine (10mg/ml)

• note: the bottle dose is 10 times the recommended dose!

• in our small animal clinic, we dilute acepromazine 1:5, so the concentration is 2mg/ml instead of 10mg/ml: this allows greater accuracy in drug dosing

• Dosage:

  • dog: 0.01-0.05 mg/kg, IM, SQ, IV

  • Cat: 0.02-0.1 mg/kg, IM, SQ, IV

  • Horse: 0.005-0.02 mg/kg, IV, IM

• Clinical uses:

  • approved to use in small and large animals

  • decrease anxiety, produce a “tranquil” state= Produce Sedation!!

  • NO ANALGESIA

  • ONLY FOR SEDATION

  • dose-dependent effects: use at low doses

  • Cheap!

  • 3 “anti-”

    • anti arrhythmogenic

    • antiemetic

    • antihistamine

• Cardiovascular effects

  • HYPOTENSION

    • due to alpha 1-adrenergic blockade

    • dose-dependent effect

    • phenothiazine > butyrophenones

  • TX:

    • reduce the level of general anesthesia

    • IV Fluids

    • 1-adrenergic agonists

      • Phenylephrine

      • methoxamine

• Other effects

  • minimal respiratory effects

  • penile prolapse in stallions (can be permanent)

    • do not use phenothiazine in breeding stallion

  • Mild H1-antihistaminic properties

  • extrapyramidal signs at high doses (rigidity, tremors, catalepsy); butyrophenones > phenothiazines

  • lower seizure threshold (controversial)

• Duration of effects:

  • most of these agents last for several hours (relatively long-acting and dose-dependent)

  • not reversible

    • double-check dosage before you give it to the patient

• Method of elimination:

  • elimination due to hepatic metabolism

  • effects may be prolonged with hepatic disease or in the neonate / geriatric patient

    • use lower dosage

More tranquilizers

• butyrophenones

  • droperidol: very similar to phenothiazine

  • azaperone: use in swine primarily

    • approved for use when mixing groups of swine to produce sedation and control aggression

    • used commonly IM

    • very similar to acepromazine effect

    • can be combined with other drugs

      • BAM: butorphanol, azaperone, and medetomidine

Minor tranquilizers

• benzodiazepines:

  • diazepam

  • midazolam

  • zolazepam

  • flumazenil - antagonist

• Mechanism of action:

  • agonists at benzodiazepine receptors sites in the CNS; these receptors potentiate the effects of GABA (an inhibitory neurotransmitter) in the CNS

• Clinical uses

  • mild sedation effects ( not dependable) see adverse side effects below

  • combine with another drug to potentiate sedation effects

  • central muscle relaxant properties

  • useful sedatives in debilitated animals or neonates

  • good sedative in camelids, small ruminants

  • potent anticonvulsants

• Cardiopulmonary effects:

  • in general: NONE!

    • often we choose this group of drugs as a premedication for cardio procedures, geriatric & pediatric, or debilitated patients, etc

  • occasionally mild hypotension and respiratory depression

  • may see apnea following bolus injection when combined with other respiratory depressant agents

• adverse effects:

  • may see excitement or aggression when given alone

  • do not use these drugs alone unless patient is pediatric/ geriatric or debilitated

  • do not use these drugs in cats

Diazepam Vs. Midazolam

• Diazepam

  • insoluble in water

  • comes dissolved in propylene glycol

    • irritating to tissues

    • erratic absorption from IM injection

      • IV or Rectally

    • often incompatible with other solutions

• Midazolam

  • water soluble

  • non irritating

  • well absorbed from IM and SQ injections

  • physically compatible with many other solutions

Benzodiazepines

• duration of action: relatively short

  • diazepam: up to 1 hour

    • antiseizure effects may be shorter, duration of effects may be greatly prolonged in geriatrics

  • midazolam: up to 1 hour

    • little change with geriatric

  • zolazepam: appears to be species dependent

• Dosage:

  • Diazepam: 0.2-0.4 mg/kg, IV; 0. mg/kg, IV for seizure

  • Midazolam: 0.2-0.4 mg/kg, IM, SQ, or IV

• reversible with flumazenil

  • occasionally benzodiazepines can cause prolonged recoveries