Anesthesia: non-depolarizing neuromuscular blockade

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 .

Uses

• To achieve adequate surgical muscle relaxation without compromising recovery:
  • Long operations requiring absolute relaxation, eg intra-ocular procedures, microsurgery.
  • Procedures requiring specific muscle relaxation, eg laparotomy , Caesarean section , some orthopedic surgery (reduction of dislocations), diaphragm surgery .
  • To allow reduction of anesthetic dose, preservation of cardiopulmonary function and reduced operating times in 'high risk' patients.
  • Where positive pressure ventilation (PPV) required.
  • Where spinal reflexes need to be suppressed, eg ear canal surgery .

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 , 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.

Requirements

Materials required

Ideal equipment

• Peripheral nerve stimulator (desktop or hand held).
• Availability of equipment for PPV.

Sources

Publications

• Recent references from PubMed.