Premedication Flashcards
What is premedication?
Provision of sedation & anxiolysis before induction of anaesthesia
What are the benefits of premedication?
Reduction of dose of induction agent
Reduction of dose of maintenance agent
Provide pre-emptive analgesia & preventive analgesia
Ensure smooth recovery
Prevent side effects associated with other anaesthetic drugs
Can start at home in anxious animals
Define pre-emptive analgesia? What are the benefits?
Administration of analgesic drugs prior to onset of noxious stimulation
Benefits:
- Reduces immediate post-operative pain
- Minimises intensity & duration of subsequent pain.
- Easier pain control with analgesics post-surgery.
- Prevents peripheral & central sensitisation.
Define multimodal analgesia. What are the benefits?
Combines different analgesics acting via different mechanisms or sites in the nervous system.
Benefits:
- Additive & improved pain relief
- Reduced drug doses minimise side effects
Define preventive analgesia
Combines multimodal and pre-emptive analgesia
Continues into the early post-operative period to maintain effective pain control.
Describe Acepromazine (ACP)
A phenothiazine
widely used premed in dogs, cats & horses
Usually used with opioid (neuroleptanalgesia) because synergism occurs
Interferes with dopamine transmission to provide sedation
Can reduce blood pressure & oxygen delivery to tissues
- due to reduction in haemoglobin concentration caused by splenic sequestration of RBC
SC, IM, IV & oral
Define Neuroleptanalgesia
state of sedation & analgesia induced by combining neuroleptic (sedative) & opioid analgesic, commonly used for minor procedures & premedication
What are the most commonly used drugs to produce neuroleptanalgesia
Phenothiazine agents
α2-adrenoceptor agonists
Opioid analgesics
Administering these drugs together produces synergistic effects (greater sedation and analgesia than when given individually).
What a-2 agonists are used in small animals
Medetomidine & dexmedetomidine
What a-2 agonists are used in horses
Xylazine, detomidine & romifidine
What a-2 agonists are used in cattle
Xylazine & detomidine
Describe a-2 agonists
Profound cardiovascular effects (dose dependent) & reduction in cardiac output
increased urine output
Produces miosis (pupil constriction), decreased IOP (intraocular pressure) & decreased CBP (ciliary body perfusion) but vomiting possible
Atipamezole is the antagonist
Why are a-2 agonists (e.g. medetomidine & dexmedetomidine) a popular premed?
They provide reliable sedation, analgesia, and muscle relaxation.
Dexmedetomidine is often preferred over medetomidine as it excludes biologically inert levomedetomidine.
Why is xylazine not recommended for premed in dogs & cats?
Xylazine is associated with higher risk of mortality compared to other α-2 agonists & safer alternatives like dexmedetomidine are available
How do a-2 agonists cause sedation?
- Stimulate α-2 adrenoreceptors in CNS
- Inhibit adenylyl cyclase, reducing cAMP levels
- Cause K efflux & block CA channels –> neuron hyperpolarisation
- Reduce norepinephrine release in noradrenergic pathways
- Suppress ascending CNS activity, producing sedation & hypnosis
What cardiovascular effects do a-2 agonists cause?
Initial phase: Intense vasoconstriction, increased blood pressure & reflex bradycardia
Later phase (20–30 min): Vasoconstriction wanes, blood pressure normalizes, but heart rate remains low due to centrally mediated sympatholysis.
How do a-2 agonists provide analgesia
They inhibit nociceptive neurons in dorsal horn of spinal cord & reduce substance P release
Describe Benzodiazepines & give examples.
E.g. Midazolam (water soluble), diazepam, zolazepam, alprazolam (others are dissolved in propylene glycol)
Work on GABA
effects:
- anxiolytic
- muscle relaxation
- cardiovascular stability
No analgesia
Suitable for ASA III-V patients
Avoid in healthy patients (causes excitement)
Antagonist is flumazenil
Often combined with:
- opioids
- ketamine
- alfaxalone
Describe Alfaxalone
IV or IM induction agent
Combined with other drugs (e.g. opioids) for sedation/pre-med
Useful option instead of a-2 agonists
Minimal effects on cardiovascular system
Drawback is large volume (IM) in big animals (painful)
Describe opioids
Used to provide pre-emptive & preventive analgesia & synergism with sedatives
Mild sedation when given alone to ASA I-II but more profound with ASA III-V
Limited cardiovascular effects
What is the difference between opiates & opioids?
Opiates: Drugs derived directly from opium (e.g., morphine, codeine).
Opioids: Includes naturally occurring, semisynthetic & synthetic substances with morphine-like activity acting at opioid receptors
What are the main uses of opioid analgesics?
Management of acute trauma pain
Pain relief during surgical or diagnostic procedures.
Treatment of painful medical conditions.
Long-term analgesic therapy for chronic pain, such as cancer-related pain.
Why are opioids referred to as narcotic analgesics?
They provide analgesia but can also induce sedation, euphoria, or sleep-like state (narcosis)
What are the legal requirements for Schedule 2 opioids in the UK?
Majority of opioids
Require prescription for requisition
Stored in a lockable cupboard meeting specific criteria.
Prescription/dispensing recorded in bound controlled drug register kept for 2 years.
Destruction of stock witnessed by authorised person or RCVS member unaffiliated with the practice.
