VT131 Anesthetic drugs P.P.

Anesthetic Drugs I Study Notes

Premedication

Premedication: Administration of drugs before anesthesia to prepare the patient.
Single Use Only: Emphasis on the importance of using anesthetic drugs for a single patient only.

Terminology

Anesthetic Agent: Any drug used to induce a loss of sensation with or without unconsciousness.
Adjunct Agent: A drug that is not a true anesthetic but is used during anesthesia to produce other desired effects.
Balanced Anesthesia: Utilizing a combination of drugs at lower doses to provide a safer anesthetic compared to single agents at higher doses.
Pharmacokinetics: The effect the body has on a drug (absorption, distribution, metabolism, excretion).
Pharmacodynamics: The effect the drug has on the body (biological effects, mechanisms of action).
Agonists: Drugs that bind to and stimulate tissue receptors.
Antagonists: Drugs that bind to receptors but do not stimulate them, often displacing the agonist; mainly used for reversals.
Partial Agonists: Bind to and partially stimulate receptors.
Agonist-Antagonists: Bind to more than one receptor type, simultaneously stimulating at least one receptor while blocking another.

Classification of Anesthetic Drugs

Based on Route of Administration: Can include oral, intravenous, intramuscular, etc.
Based on Time Period Used: Short-acting vs. long-acting anesthetics.
Based on Principal Effect: Many drugs exhibit multiple effects.
Based on Chemistry or Drug Class: Classification based on the chemical structure or categories of drugs.

Regulations for Controlled Substances

Drug Regulation: Anesthetic drugs at risk of theft and abuse are regulated by the DEA under the Controlled Substances Act (CSA).
Handling Requirements: Strict handling, recordkeeping, and storage are mandated for controlled substances.
Criminal Acts: Theft, illegal use, or diversion of these substances can lead to imprisonment and/or fines, impacting eligibility for professional licensure.

Drug Schedules under the Controlled Substances Act

Drugs are classified into five schedules according to their potential for abuse:
  - Schedule I: Highest abuse potential; none used in veterinary medicine.
  - Schedule II: High potential; examples include Fentanyl, Morphine, Hydromorphone.
  - Schedule III: Medium potential; examples include Ketamine, Telazol, Buprenorphine, Pentobarbital.
  - Schedule IV: Low potential; examples include Diazepam, Midazolam, Alfaxalone.
  - Schedule V: Lowest potential; example includes Lomotil.

Record-keeping Regulations

General Inventory: Required every 2 years for controlled drugs.
Daily Accounting: Done in controlled drug logs to ensure accurate use tracking.
Log Requirements: Controlled substance logs must be recorded in a bound booklet, numbered pages with indelible ink, and retained for a minimum of 2 years.
Separation of Schedule II Drugs: Logs for Schedule II drugs must be kept distinct from other records.

Storage Requirements for Controlled Substances

Security: Controlled substances must be securely locked in a strong cabinet.
Supervision: Containers must not be left unattended in public areas.
Reporting: Any unexplained loss or suspected theft must be reported to the DEA and local police within one business day; DEA Form 106 must be completed and submitted.

Preanesthetic Medications

Purpose of Premeds: Given alone or in combination to assist balanced anesthesia.
Common Premeds Include:
  - Anticholinergics
  - Phenothiazines
  - Benzodiazepines
  - Alpha2-adrenoceptor agonists (alpha2-agonists)
  - Opioids

Reasons for Use of Preanesthetic Medications

To calm or sedate an excited, frightened, or aggressive animal.
To minimize adverse effects from other concurrent drugs.
To reduce the required doses of other anesthetic agents.
To produce smoother anesthetic inductions and recoveries.
To alleviate pain and discomfort pre-operatively, intra-operatively, and post-operatively.
To induce muscle relaxation in the patient.

Administration of Preanesthetic Medications

Administered via intravenous (IV), intramuscular (IM), or subcutaneous (SQ) routes.
Patients should be monitored in a quiet area for 20-30 minutes while medications take effect.

Combining Drugs in a Syringe

Multiple anesthetic agents are often used together, generally mixed in a single syringe.
Most anesthetic drugs are water-soluble, allowing for safe combination except for diazepam, which is non-water-soluble; diazepam can only be mixed with ketamine.

Anticholinergics

Definition: Non-controlled drugs used to prevent and treat bradycardia and decrease salivary secretions.
Examples: Atropine, Glycopyrrolate.

Mode of Action

Mechanism: Anticholinergics act as blockers of the parasympathetic nervous system, termed parasympatholytics.
Vagus Nerve Influence: It is mainly innervated by the vagus nerve, which can be stimulated during intubation, causing bradycardia and bronchoconstriction. Anticholinergics are employed to mitigate these effects.

Administration

Routes: IV, IM, SC, or intratracheally (IT).
Atropine: Faster onset, shorter peak, shorter duration; preferred for emergencies.
Glycopyrrolate: Slower onset, longer peak, extended duration; minimally crosses the placenta; suited for cesarean sections.

Effects of Anticholinergics

CNS Effects: Anticholinergics do not function as sedatives and exhibit no central nervous system effects.
Cardiovascular Effects: They prevent bradycardia; caution is advised in animals with preexisting tachycardia or heart disease.
Respiratory Effects: Decrease and thicken respiratory and salivary secretions, effecting bronchodilation.
Additional Effects:.
- May cause mydriasis (pupil dilation) and reduce tear secretion.
- Decrease gastrointestinal motility.

Tranquilizers and Sedatives

Definitions:
- Tranquilizer: Reduces anxiety without necessarily decreasing wakefulness.
- Sedative: Causes a reduction in mental activity and induces sleepiness.
Often, the terms are accidentally interchanged due to overlapping effects.

Common Tranquilizer/Sedatives

Phenothiazines.
Benzodiazepines.
Alpha2-adrenoceptor agonists (alpha2-agonists).

Phenothiazines

Example: Acepromazine.
Uses: Decreases general anesthetic requirements, smooths induction/recovery, and provides sedation/tranquilization for minor procedures. Approved for use in dogs, cats, and horses; administered IV or IM.

Breed and Species Sensitivities

Responses to acepromazine vary by species and breed; caution is warranted.
Dosage Reduction: Recommended for certain breeds (Collies and Australian Shepherds) to avoid prolonged sedation. Giant breeds and sensitive breeds like greyhounds and boxers may experience adverse effects such as severe bradycardia and hypotension.
Cautions: Use carefully in older, sick, or weakened patients.

Effects of Phenothiazines

CNS Effects: Causes sedation, not providing pain control but reducing anxiety; may lower seizure threshold; occasional excitement or aggression may occur.
Cardiovascular Effects: Antiarrhythmic; causes hypotension and hypothermia.
Other Effects: Anti-emetic properties; may cause prolapse of the penis in horses; may decrease PCV due to heightened splenic uptake of RBCs; prevents histamine release and mitigates allergic reactions.

Benzodiazepines

Definition: Tranquilizers used as muscle relaxants and anticonvulsants, often combined with other drugs.
Examples:
  - Midazolam (administered IV or IM).
  - Zolazepam (administered IV or IM).
  - Diazepam (administered IV, irritating if given IM).
  - Flumazenil (reversal agent for benzodiazepines).
Storage: Diazepam and midazolam must be stored in brown glass vials due to light sensitivity; Zolazepam is available in combination with tiletamine.

Effects of Benzodiazepines

CNS Effects: Unreliable sedation in young dogs when used alone; possesses anticonvulsant activity; no analgesic effects.
Cardiovascular and Respiratory Effects: Minimal influence on cardiovascular and respiratory systems, which indicates a high safety margin. IV diazepam or midazolam are utilized for terminating active seizures.
Other Effects: Excellent muscle relaxants, decrease demands for general anesthetics, cross the placenta (potentially causing CNS depression in neonates delivered via cesarean section).

Alpha2-Adrenoceptor Agonists

Definition: Also known as alpha2-agonists; utilized for sedation, analgesia, and muscle relaxation, administered IV or IM.
Safety: Generally safe for young and healthy patients, but should be avoided in geriatric, diabetic, pregnant, pediatric, or ill patients.
Examples:
  - Xylazine
  - Dexmedetomidine
  - Detomidine

Effects of Alpha2-Adrenoceptor Agonists

CNS Effects: Potent sedatives that provide short-lived analgesia; potential for temporary behavioral changes; may induce muscle tremors in horses and cause cattle to lie down or collapse.
Cardiovascular Effects: Initial phase includes vasoconstriction causing hypertension and reflex bradycardia (pale mucous membranes); can cause dramatic bradycardia (30-50 bpm in dogs); may progress to hypotension and decreased cardiac output, potentially leading to arrhythmias.
Respiratory Effects: Respiratory depression, particularly in cattle.
Other Effects: Muscle relaxation, amplified effects of other anesthetics, potential for vomiting, transient hyperglycemia, hypothermia; xylazine may induce abortion in pregnant cattle, and horses may exhibit sweating.

Monitoring and Reversal

Importance of monitoring vital signs for patients receiving alpha2-agonists.
Anticholinergic Administration: Not effective for reducing bradycardia and may exacerbate cardiac workload.
Reversals:
- Yohimbine and tolazoline reverse xylazine.
- Atipamezole reverses dexmedetomidine; administered IM in cats, IM for dogs or slow IV in emergencies.

Opioids

Use: Employed for analgesia, sedation, and reduction of other anesthetic requirements; have a wide safety margin, even for debilitated patients.
Types:
  - Agonists:
    - Morphine
    - Hydromorphone
    - Oxymorphone
    - Fentanyl
    - Meperidine
  - Partial Agonist: Buprenorphine.
  - Agonist-Antagonists: Butorphanol and nalbuphine.
  - Antagonist (reversal): Naloxone.

Mode of Action and Pharmacology

Mechanism: Act on receptors in the brain and spinal cord.
Types of Opioid Receptors:
  - Mu (μ) Agonism: Leads to analgesia, bradycardia, hypothermia, hypoventilation, vomiting, and constipation.
  - Kappa (κ) Agonism: Induces minor analgesia, sedation, and dysphoria.
  - Delta Agonism: Causes hypoventilation, constipation, and urinary retention.

Effects of Opioids

CNS Effects: Potent analgesics; may cause CNS depression or excitement; caution about increased intracranial pressure in head trauma cases.
Cardiovascular Effects: Can induce vagus-induced bradycardia.
Respiratory Effects: Dose-dependent respiratory depression; may lead to panting in dogs.
Other Effects: May cause hypothermia in dogs and hyperthermia in cats; signs such as salivation, vomiting, diarrhea, GI stasis, constipation; sensitivity to noise; miosis in dogs and mydriasis in cats, ruminants, and horses; may induce sweating in horses; contribute to decreased urine production and urinary retention; reactions such as facial swelling and hypotension might arise from histamine release when using morphine or meperidine.

Neuroleptanalgesia

Definition: A state of sedation and analgesia induced by the simultaneous administration of an opioid and a tranquilizer; ideal for minor procedures.
Example: Dexdomitor and butorphanol.
Considerations: Prepare for possible intubation and ventilation as necessary during neuroleptanalgesia procedures.

Opioid Antagonists (Reversals)

Definition: Opioids can be reversed with antagonists like naloxone, which will reverse both desirable and undesirable effects of the opioid.
Partial Reversals: Agonist-antagonists like butorphanol can also provide partial reversal of pure agonists’ effects.