Neuromuscular Blocking Drugs

Neuromuscular Blocking Drugs


Neuromuscular blocking drugs (NMBDs), referred to as peripheral muscle relaxants, in contrast to centrally acting muscle relaxants such as guaifenesin or diazepam, interfere with or block neuromuscular transmission at the motor end plate. They are useful adjuncts to general anesthesia because they provide short-term or reversible skeletal muscle relaxation. NMBDs do not provide analgesia, sedation, amnesia, or hypnosis, and they predispose the animal to hypothermia. Animals are unable to breathe, necessitating controlled ventilation and constant monitoring.

General Considerations

The primary pharmacologic effect of peripheral NMBDs is to produce skeletal muscle (SM) relaxation

II NMBDs are potentiated by many other drugs

III Other factors that may influence the effects of NMBDs

IV Potential mechanisms of SM relaxation

NMBDs are used adjunctively during anesthesia to produce controlled muscle relaxation

VI NMBDs produce muscle relaxation but do not produce analgesia or unconsciousness

VII NMBDs produce respiratory paralysis, which necessitates manual or mechanical ventilation support

VIII Hypothermia is an important secondary effect of prolonged SM relaxation

IX NMBDs are positively charged (ionized) and therefore do not pass the blood-brain barrier or cross the placenta in significant amounts

Various electrical stimulators and stimulation protocols can be used to determine the degree of neuromuscular blockade (Figs. 10-1 and 10-2)

Normal Neuromuscular Function

Acetylcholine (ACh) is released in small amounts, even in resting muscles

II Action–potential-dependent ACh release

III Combination of ACh with postjunctional nicotinic receptors (Nm receptors)

IV The duration of ACh activity at any cholinergic synapse is limited by the action of acetylcholinesterase (ACh esterase); ACh is metabolized to acetic acid plus choline at the synaptic cleft

Specific Neuromuscular Blocking Drugs (Tables 10-1 and 10-2)

TABLE 10-1

Dose Of Neuromuscular Blocking Drugs with Side Effects and Contraindications

Agent Species Dose Intravenously (mg/kg) Duration of Action (min) Side Effects Contraindications
Succinylcholine chloride Dog 0.3-0.4 1-38 Little cardiovascular effect; muscarinic effect is bradycardia; nicotinic effects are hypertension, increased intraocular pressure; hyperpyrexia Organophosphate anthelmintics, chronic liver disease, malnutrition, high-K+, glaucoma, penetrating eye injury
  Cat 3-5 (total) 2-6    
  Pig 0.75-2 1-3    
  Horse 0.1-0.33 1-10    
Pancuronium bromide Dog 0.02-0.06 15-108 Negligible Liver or kidney disease
Cat 0.02-0.06 14-15    
Pig 0.07-0.12 7-30    
Horse 0.08-0.14 16-35    
Cattle 0.1 30-40    
Calf 0.04 26    
Sheep 0.005 21    
Vecuronium Dog 0.01-0.2 10-42 Negligible  
Cat 0.02-0.04 5-9    
Pig 0.1-0.2 5-20    
Horse 0.1 20-40    
Sheep 0.004 14    
Atracurium Dog 0.1-0.4 10-30    
Cat 0.1-0.25 10-15 Negligible  
Pig 0.5-2.5 10-60    
Horse 0.07-0.09 8-24 Negligible  
Sheep 6 µg/kg/hr      
Llama 0.15 6    
Cisatracurium Dog 0.02-0.1 10-30 Negligible  
Rocuronium Dog 0.3-0.6 mg/kg 20-30 min Negligible  
Pipecuronium Dog 70-90 µg/kg 16-81 Occasional hypotension  
Cat 40-60 µg/kg 17-24    

Sep 6, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Neuromuscular Blocking Drugs
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