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Anesthesia: non-depolarizing neuromuscular blockade

ISSN 2398-2942

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Introduction

  • Non-depolarizing neuromuscular blocking agents exert their effects on the neuromuscular junction primarily due to competition with acetylcholine for receptor sites on the postsynaptic membrane.
  • These drugs are also referred to as "competitive" neuromuscular blocking agents.
  • As the neuromuscular blocking agent is redistributed from the neuromuscular junction, more receptors become available for binding with acetylcholine and normal neuromuscular function is eventually restored.
  • If acetylcholine is allowed to build-up as the non-depolarizing neuromuscular blocking agent is being redistributed, it can compete for available postsynaptic receptors and neuromuscular function can be restored. This is the basis for reversal of neuromuscular blockade Anesthesia: reversal of neuromuscular blocking agents.

Uses

Advantages

  • Absolute, controllable, reversible relaxation.
  • Improved surgical conditions - less traction needed to separate tissues containing skeletal muscle; less tissue trauma; less post-operative pain; wound breakdown less likely; more efficient use of surgical time; faster recovery; reduced convalescent period; facilitated exposure of 'deep' viscera; smaller abdominal incisions required; lowered risk of adverse reflex autonomic nerve responses to traction; facilitates ophthalmic surgery.
  • Reduced anesthetic requirement.
  • Improved control of ventilation - facilitates PPV (increases thoracic compliance and lowers inflation pressures needed); reduces associated adverse hemodynamic effects (preserves venous return and cardiac output); reduces complications associated with spontaneous breathing superimposing on ventilator rhythm.
  • Neuromuscular blocking agents do not cross placental barrier and so represent no risk to the neonate.

Disadvantages

  • Awareness - neuromuscular blockers have little effect on consciousness but eliminate movement (the most common sign of inadequate anesthesia) and the motor component of cranial nerve and other reflexes (used to assess 'depth' of unconsciousness).
  • Hypoventilation /apnea - PPV equipment is essential as even low doses can depress ventilation.
  • Side-effects, eg tachycardia , hypertension Hypertension , hypotension, bronchospasm - less common with modern muscle relaxant drugs.
  • Absence of motor response to nerve stimulation - increased risk of inadvertent motor nerve damage during spinal or intracranial operations.
  • Response variation - potentially hazardous to rely on rigid dosing regimes - close monitoring essential.
  • Incomplete antagonism/'recurarization' - less common with modern drugs.

Technical problems

Alternative techniques

  • Depolarizing neuromuscular blockade:
    • Depolarizing neuromuscular blocking agents are rarely used in veterinary practice nowadays.
    • Drugs, such as succinylcholine, are structurally related to acetylcholine initially producing similar effects by binding to the postsynaptic receptors, but they are less rapidly removed from the motor endplate.
    • Reversal agents normally used for non-depolarizing neuromuscular blockade will actually cause prolongation of blockade and should not be used.
  • Deep general anesthesia - an undesirable anesthetic depth may be required for desired muscle relaxation.
  • Extradural/spinal anesthesia - some large diameter myelinated fibers remain unaffected.
  • Local anesthetics - some techniques difficult to perform.

Procedure

  • Drug/dose-dependent.

Decision taking

Criteria for choosing test

  • The disadvantages of the administration of a particular neuromuscular blocker must be weighed against the advantages for the particular surgical procedure. Factors should include risk, equipment for monitoring and experience.

Risk assessment

  • The anesthetist's experience and the ability to monitor neuromuscular transmission accurately must be assessed before embarking on blockade, particularly in 'high risk' patients where altered cardiac, renal or hepatic function may affect normal drug behavior.
  • More inexperienced anesthetists are better developing skills on more routine operations.

Requirements

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Preparation

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Technique

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Aftercare

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Outcomes

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Further Reading

Publications

Refereed papers

Other sources of information

  • BSAVA Manual of Small Animal Anaesthesia and Analgesia.(1999) Seymour C & Gleed R D (eds).