ISSN 2398-2977      

Therapeutics: musculoskeletal system

pequis
Contributor(s):

David Moll

Graham Munroe


Drugs used in the diagnosis of musculoskeletal disease in the horse

  • Local anesthetics.
  • Sedatives.
  • Tranquillizers.
  • General anesthetics.

Local anesthetic agent

  • Very commonly used   Anesthesia: local - overview  .
  • Mechanism of action:
    • Prevent voltage-dependent increase in sodium ion conductance.
    • Block the initiation and propagation of action potentials.
    • Interrupt transmission between peripheral nociceptors and the cerebral cortex.
  • Regional blocks of specific peripheral nerves.
  • Local infiltration of tissues may block nerve endings, fibers and specific nerves.
  • Intrasynovial blocks:
    • Joints: pain receptors in synovium, joint capsule and subchondral bone.
    • Sheaths/bursa: where pain is perceived in these structures is less clear.
  • Not selective to pain fibers:
    • Block   →   suppressed pain, cold/warmth, touch/pressure.

Uses

Reactions

  • Local tissue reactions:
    • Inflammation if severe can   →    necrosis: more common and potentailly more severe with epinephrine (adrenaline)   Epinephrine  .
    • Allergic reactions: more common and potentailly more severe with esters, eg procaine   Procaine hydrochloride  .
    • Neurotoxicity: rare but increases with 200 ml or more used locally in short period.
    • Ataxia after more proximal regional blocks in limb?
  • Systemic reactions:
    • Inadvertent intravenous injections.
    • Overdosage and systemic absorption.
    • CNS toxicity is dose dependent: depression/sedation    →   excitement, muscle twitches and convulsions.
    • Cardiovascular reactions:
      • Bradycardia    →   and/or conduction disturbances.
      • Myocardial depression, peripheral vasodilation    →   hypotension.
      • Severe cases    →   cardiovascular collapse

Onset, elimination and duration

  • Determined by the physiochemical properties of agent:
    • Rate of onset of action is determined by the acidic dissociation constant (pKa).
    • The agents lipid solubility determines its potency (higher lipid solubility    →   increased potency).
    • The degree of protein binding determines the duration of action (higher longer).
  • Elimination:
    • Ester-linked agents, eg procaine hydrolysed in blood: half-life in minutes.
    • Amides, eg lidocaine and rest undergo hepatic metabolism: half-life in hours.
    • LA and metabolites detected in equine urine for prolonged periods.
  • Class 2 foreign substances by ARCI.

Specific drugs

  • Lidocaine (lignocaine)  Lidocaine  :
    • 2% solution + epinephrine   Epinephrine   (11-12.5 mg/ml).
    • pKa 7.8; rapid onset (4 min); intermediate potency; rapid tissue penetration; medium duration of action (60-150 min).
    • Rapid systemic absorption (peak 20-30 min).
    • Peak urine concentration: 60 min.
    • Half-life: 48 min.
    • Duration of action prolonged by adding epinephrine (vasoconstrictor):
      • Decreases rate of absorption.
      • Do not use in synovial structures.
      • Side effects: irritant, tissue necrosis, cardiac arrhythmias.
      • Lower limb blocks may    →   digital ischemia and local reactions.
      • Maximum use in one clinical exam period 200 ml.
  • Mepivacaine  Mepivacaine  :
    • 2% solution.
    • pKa 7.7; rapid onset (5-10 min); intermediate potency; rapid tissue penetration; medium duration of action (68-210 min).
    • Does not cause vasodilation and epinephrine is not required.
    • Diffuses rapidly into the systemic circulation (peak 60 min).
    • More highly protein bound than lidocaine (slightly increased duration of action).
    • Peak urine concentration (2 h); mepivacaine and metabolite (50 and 33 h).
    • Less tissue irritant and recommended for joints.
    • More expensive.
    • No effect on bone scintigraphy.
  • Bupivacaine  Bupivacaine hydrochloride  :
    • 0.5% solution.
    • pKa 8.16; moderate onset; high potency; moderate tissue penetration.
    • Longer duration of action (4-8 h):
      • Be careful about accumulated doses.
      • No new block until after previous one worn off.
    • Peak urine concentrations: 
      • 2 h post-intra-articular injection.
      • 4 h post-subcutaneous injection.
    • No effect on bone scintigraphy.
    • Maximum dose should not exceed 2 mg/kg.
    • Therapeutic pain relief ?diagnostic use.
  • Ropivacaine:
    • pKa 8.16; moderate onset; high potency; moderate tissue penetration; long duration of action.
    • S-isomer of bupivacaine.
    • Reduced cardiotoxic effects.
    • Similar duration of action.
  • Procaine  Procaine hydrochloride  :
    • pKa 8.9; moderate onset; low potency; slow tissue penetration; short duration of action.
  • Prilocaine:
    • Similar to lidocaine but less toxicity.

Sedatives

Tranquillizers

  • To allow safe examination in excitable/nervous horses: acetylpromazine   Cyclosporine  0.010.02 mg/kg IV.

General anesthesia

Drugs used in the treatment of musculoskeletal disease in the horse

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Hallowell G (2007) Equine pain managementVet Review 124, 23-28.

Other sources of information

  • Bertone J J & Horspool L J I (2004) Equine Clinical Pharmacology. Saunders, UK. ISBN: 0702024848.

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