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Assessing neuromuscular block

Created: 5/4/2004
Updated: 3/6/2019
 
In the anaesthetised patient, co-operation is unavailable, hence clinical assessment is limited to coughing, straining, patient movement, or changes in lung compliance, all of which only vaguely and non-quantifiably indicate inadequate paralysis. In ocular, micro- and neurosurgery, such movement may be hazardous for the patient and excessive doses of muscle relaxants may be given with subsequent inadequate or delayed reversal. Large inter-individual variations in pharmacokinetics and the pharmacodynamics of non-depolarising blockers make it hard to judge doses or top-up intervals or use predictive empirical models satisfactorily in these patients.

Assessing the degree of neuromuscular block depends on the characteristic patterns of response to various muscle relaxants.

  Depolarising agents Non-depolarising agents
Fasciculation Yes No
Tetany  No fade Fade
Post-tetanic potentiation No Yes
Anticholinesterases Increase block Decrease block
Additional non-depolariser Antagonism Potentiation
Repeated doses May induce Phase 2 block No change in character

Clinical assessment


The following clinical signs are listed in order of increasing block:

• Diplopia, weakness of extra-ocular muscles
• Perception of weakness or heaviness
• Difficulty swallowing
• Inability to sustain headlift, decreased peak inspiratory force
• Obvious weakness, poorly sustained grip
• Decreased vital capacity, flow rates
• Jerky movements, intercostal paralysis, respiratory failure
• Diaphragmatic paralysis

The use of muscle relaxants is mostly limited to conscious co-operative patients. Sustained headlift, tongue protrusion and grip indicate that the patient is awake, that tetanic fade is minimal and that virtually complete neuromuscular junction (NMJ) )recovery has occurred. Peak inspiratory pressure correlates fairly well with TO4 ratio. Adequate tidal volume and fairly normal blood gases while breathing spontaneously on an endotracheal tube are not sufficient criteria for extubation because at the same time laryngeal and intercostal muscle groups may be very weak.

Extubation requires integrity of each component in the respiratory system, so clinical assessment of the anticipated adequacy of ventilation following extubation is essential, and involves consideration of many factors beside NMJ function, i.e. lung function and mechanics, level of consciousness, likelihood of regurgitation, adequacy of local airway reflexes, etc.


ArticleDate:20040405
SiteSection: Article
 
   
    
                                            
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