Module 5 Anaesthesia and Analgesia Flashcards
Outline the pathophysiology of the pain pathways including the main neurotransmitters
Noxious stimulus activates cell membrane to open ion channels to cause depolarization. If action potential produced, it progresses to CNS via dorsal horn of spinal tract. Glutamate main excitatory transmitter in dorsal horn. Gaba main inhibitory. Glycine is inhibitory but also required for co-agonist with glutamate. There are opiod and alpha2 adrenoceptors pre and post synaptically.[Pre-synaptic opioid receptor activation is associated with decreased calcium influx and decreased release of neurotransmitters into the synapse. Post-synaptic opioid receptor activation is associated with hyperpolarization, which leads to decreased AP generation and inhibits second order neuronal activation.Local anaesthetics target spinal transmission via voltage gated sodium channel blockage, resulting in inhibition of nociceptive conduction.The spinothalamic tract transmits nociceptive information from the dorsal horn to the brain and can be modulated via GABA-mediated inhibition.
Recall the three types of pain
IINFLAMMATORY PAIN;from tissue injury. Normal response. Inflammatory mediators sensitize nociceptors during inflammation, and desensitises during healing.
NEUROPATHIC PAIN;peripheral nerve injury may result in changes to the nerve cell or surrounding glia which results in hypersensitivity to stimuli. This may result in allodynia.
PATHOLOGICAL PAIN; (or maladaptive pain). Pain has resulted in changes which have amplified and prolonged the pain beyond expected healing. Results in allodynia and hyperalgesia.
ALLODYNIA;normally non painful stimuli resulting in pain.
HYPERALGESIA;exaggerated response to a noxious stimuli.
Identify how to assess pain and use pain scales
Can use pain scales but is challenging. Ideally, looking for changes to behaviour/demeanour of individual. Signs of pain in dogs/cats include:
Increased heart rate
* Increased respiratory rate (often abnormal pattern also)
* Increased blood pressure
* Increased temperature
* Salivation
* Dilated pupils
Vocalization including purring in cats
* Restlessness or agitation
* Resentment of handling (particularly in painful area)
* Depression
* Reluctance to lie down or sleep
* Inappetance
* Aggression or timidity (out of normal character)
* Abnormal posturing or gait
* Failure to groom (cats), excessive licking or chewing at painful area
Define multimodal analgesia and pre-emptive analgesia
MULTI-MODAL ANALGESIA=addressing pain at multiple sites of the pain pathway, by using multiple drugs, such that doses and side effects are usually significantly reduced compared with single agent use.
PRE-EMPTIVE ANALGESIA; delivering pain relief prior to anticipated noxious stimulus.
Discuss basic pharmacology of commonly used analgesics, their advantages and
disadvantages
NSAIDS;Targets tissue around peripheral nociceptors. Great for inflamatory pain.
LOCAL ANAESTHETICS;reduces peripheral nociceptors’ ability to generate an action potential. Used systemically, also has effect on CNS.
OPIOIDS;act centrally to prevent transmission of nociceptor signal, potent pain relief for all types of pain.
GABAPENTANOIDS; act centrally to prevent transmission, particularly good for neuropathic pain.
ALPHA 2’S;(domitor, xylazine) act centrally by preventing central transmission from peripheral
nociceptor. Potent pain relief.
NMDA ANTAGONISTS;(ketamine) acts centrally, preventing transmission of nociceptive signal.
Exceptionally good for neuropathic pain.
TRAMADOL;acts centrally via production of metabolites, effectiveness of analgesia not proven.
PARACETAMOL; acts centrally with weak anti-inflammatory effects.
Select appropriate analgesic agents for different trauma situations
MILD soft tissue trauma
eg. Minor dog bite, cat bite
* Pain is not severe, therefore SQ route appropriate, as is oral.
* NSAIDs are an excellent option. Buprenorphine intravenously.
* Tramadol parenterally – not as a sole analgesic.
SEVERE soft tissue trauma
eg. Severe dog bite
* Pain is moderate to severe and already present. Intravenous route is recommended.
* Methadone IV or IM or fentanyl IV.
* NSAIDs following stabilization
FRACTURES/DISLOCATIONS
* Pain is severe and already present – IV or IM administration of methone/methadone.
Alternatively, fentanyl IV followed by CRI.
* NSAIDs following stabilization and can be continued orally for ongoing analgesia.
* Fentanyl patch is an excellent option for ongoing pain relief.
* Reduction of luxation and stabilization of fracture will provide significant pain relief.
NEUROLOGICAL PAIN
* Pain is often severe and difficult to control.
* Intravenous opioids. Ketamine is an excellent option but should not be used alone.
Consider Fentanyl Ketamine Lignocaine CRI (dogs only).
* Gabapentin is not of use in acute pain or as a sole analgesic but is a good option for
chronic neurologic pain.
ABDOMINAL SURGERIES
* Fentanyl administered as a CRI during surgery will dramatically reduce the required
inhalational agent as well as provide analgesia.
* Any GI surgery – avoid NSAIDs.
* Caesareans – avoid some opioids until foetuses have been removed.
* Avoid NSAIDs in severely debilitated patients – septic peritonitis, GDV
* Severely painful patients – Morphine, Lignocaine, Ketamine or Fentanyl, Lignocaine,
Ketamine CRIs are an excellent option. *cats are overly sensitive to the neurological
side effects of lignocaine.
Discuss the advantages and disadvantages of some commonly used sedative and
anaesthetic agents
ACEPROMAZINE;Good sedation, can do with any pain relief, reduces ga dose, moderate vasodilation (decreased bp), nowadays whether lowers seizure threshold or not is debatable. Brachys may be more susceptible to fainting/apnoea. Markedly reduce dose for hepatics/cardiacs/mdr1. Avoid in splenectomy as causes increased blood pooling in spleen (increased haemorrhage/blood loss).
BENZODIAZEPENES(DIAZEPAM,MIDAZOLAM);Minimal cardiac effect. can use with any painkiller. Diazepam may cause paradoxical hyperexcitation.
ALFAXALONE;minimal cardiovascular effect. can do i/v, i/m, cri. May have agitated wake up if not adequate premed.
KETAMINE;i/v/i/m, must use with a muscle relaxant (eg diazepam), increased muscle tome, eyes stay open, avoid in cardiac issues, increased bp.
PROPOFOL;cumulative in cats. Some adverse individual reactions. rare cause of heinz body anaemia in cats. Contra-indicated if at risk of seizures, but is used for valium unresponsive status epilepticus. Contra indicated if anaphylaxis history, if hyperlipidaemic or hypoproteinaemic.
MEDETOMIDINE;(domitor). alpha 2 adrenergic agonist. Avoid in heart/liver/renal dz/shock/very old/very young. Does provide some pain relief. the sedation is reversible but the profound cardiovascular suppression is not fully reversible.
ISOFLUORANE; can cause hypotension and increased intracranial pressure.
List considerations for anaesthetic drugs based on common emergency presentations
and surgeries
HEAD TRAUMA; increased intracranial pressure can reduce cerebral perfusion. (Cerebral Perfusion Pressure = MAP- ICP) Therefore super important to maintain blood pressure. (avoid acp, reduce isofluorane demand by coupling eg with fentanyl cri or consider i/v ga. Midazolam recommended premed.
CARDIOVASCULAR DZ; avoid fluid overload. maintain blood pressure (avoid hypertension)and avoid stress. pre oxygenate. Consider i/v ga or inhalational ga with cri fentanyl etc. avoid ketamine(esp if hypertrophic cardiomyopathy).
LIVER DZ; drug doses might vary (delayed clearance for hepatically metabolised or may have less uptake for protein boun in hypoalbuminaemia etc. Hypoalbuminaemia may result in hypotension. Avoid benzodiazepines. Nsaids very cautiously. Propofol ok.
RENAL DZ-drugs may have longer half life in renally cleared. azotaemia can make bbb more leaky to drugs. Avoid ketamine in cat renals. Propofol/alfaxan good. Benzodiazepines good. care with nsaids.
CAESAREAN recommend no premed, then once pups out intra-op premed and a single dose of nsaid at extubation.Nsaids will go into milk. Opiods will cross placenta but Buprenorphine crosses way less than does fentanyl. Can treat pups with naloxone of necessary (although ideally avoid situation by not giving to bitch until pups out).
GDV-avoid nsaids. Beware of cardiac and electrolyte imbalances.
HYPOA-monitor Na and K. Avoid hypotension. steroids as required.
DIABETES-disrupt insulin/feeding as little as possible. Minimize pain/stress.
MANAGE INTRAOPERATIVE HYPOTENSION;Avoid developing by using opiod cri and reducing inhalational gas. If develops, reduce anaesthesia, then do fluid bolus if required then if not responding consider dopamine cri.
