Neuromuscular junction blockers (NMBs) like atracurium are occasionally used in veterinary anesthesia for dogs and cats, particularly during specialized surgeries requiring complete muscle relaxation such as ophthalmic procedures, thoracotomy, or certain orthopedic surgeries. Proper monitoring and reversal of these agents is crucial for successful recovery in these companion animals.
Mechanism of Action of NMB Reversal Agents in Small Animals #
In veterinary medicine, neostigmine and edrophonium function similarly to their use in human medicine. They inhibit acetylcholinesterase at the neuromuscular junction, increasing acetylcholine concentration and allowing it to compete with non-depolarizing muscle relaxants for receptor binding sites.
For dogs, neostigmine forms a carbamylated complex with acetylcholinesterase that lasts approximately 30-40 minutes. In cats, this effect may persist slightly longer due to differences in metabolism. Edrophonium creates a weaker bond, resulting in shorter duration.
Monitoring of Neuromuscular Block in Veterinary Patients #
When monitoring neuromuscular blockade in dogs and cats, peripheral nerve stimulation is applied to the peroneal nerve with observation of the dorsiflexion of the paw, or to the facial nerve with observation of facial twitching.
Train-of-four (TOF) stimulation, which remains the most practical method in veterinary practice. In cats, a TOF ratio <0.7 correlates with significant residual paralysis, similar to findings in dogs.
Double-burst stimulation can be used in larger dogs but is less commonly employed in cats due to their smaller muscle mass making visual assessment difficult.
Tetanic stimulation at 50 Hz for 5 seconds is used cautiously in small animals, as higher frequencies may cause discomfort upon recovery.
Specific Dosing and Timing for NMB Reversal in Dogs and Cats #
Neostigmine dosing in dogs is typically 0.02 mg/kg IV. The onset of action occurs within 5-10 minutes.
Edrophonium can be administered at 0.1 mg/kg IV.
Timing is critical – in both species, attempting reversal when TOF shows zero twitches often results in incomplete recovery. For moderate blockade (1-2 twitches), dogs typically require 8-12 minutes for adequate reversal, while cats may need 10-15 minutes due to differences in neuromuscular physiology.
Monitoring of Reversal in Companion Animals #
In veterinary practice, quantitative monitoring devices are less commonly available, making qualitative assessment more prevalent. Visual observation of TOF responses remains the standard, with disappearance of fade between the first and fourth twitch suggesting adequate reversal.
Clinical indicators of sufficient reversal in dogs include return of spontaneous ventilation with normal tidal volume (10-20 ml/kg), return of palpebral and swallowing reflexes, and maintenance of head lift when manually positioned. Cats show similar signs but may also display characteristic ear twitching and whisker movement when adequately reversed.
Side Effects of Reversal Agents in Dogs and Cats #
Anticholinesterase drugs produce more pronounced muscarinic side effects in small animals than in humans, necessitating careful anticholinergic pretreatment.
In dogs, neostigmine without anticholinergic pretreatment can cause bradycardia (heart rates below 60 bpm), hypersalivation, vomiting, defecation, and bronchorrhea. Cats may experience even more profound bradycardia (heart rates below 100 bpm, compared to normal 120-200 bpm), bronchospasm, and miosis.
To counteract these effects, atropine is administered at 0.02-0.04 mg/kg IV in dogs and cats approximately 5 minutes before neostigmine. Alternatively, glycopyrrolate can be used at 0.01 mg/kg IV in dogs and cats.
