In the physiology practicals we did pithing of the frogs and connect their gastrocnemius muscle to graphic display which showed muscle twitch pattern with electrical stimulation,at that point the most important concern for me was how to hold the frog as I felt it very slimy,much later during my PG times when I struggled with neuromuscular monitoring in operative cases that I used to remember these old times,so lets go on today with a staple question"What are the factors affecting neuromuscular blockage?Discuss various methods to monitor neuromascular blockage?
Potentiation by inhalational agents
Volatile anaesthetics decrease the requirement of neuromascular blocking drugs by at least 15%,the actual degree of this post synaptic augmentation depends upon both the inhalational drug and the muscle relaxant(desflurane>sevoflurane>isoflurane and enflurane>halothane>N2O/O2/narcotic) and (pancuronium>vecuronium and atracurium)
Temperature
Hypothermia prolongs the blockade by decreasing the metabolism(mivacurium,atracurium and cisatracurium) and by delaying the excretion (pancuronium and vecuronium ) of the muscle relaxants.
Acid base balance
Respiratory acidosis potentiates the blockade of most nondepolarising muscle relaxants and antagonises its reversal.
Electrolyte abnormalities
Hypokalaemia and hypocalcemia augment the nondepolarising block while hypermagnesemia potentiates the peripheral nerve blocks.
Age
Drug interaction
Drugs like dantrolene ,quinidine calcium channel blockers,streptomycin,aminoglycosides,kanamycin,
Neomycin,polymixin,clindamycin are known to potentiate the nondepolarising muscle relaxants.
Concurrent diseases
Cirrhotic liver disease and chronic renal failur often result in greater volume of distribution and a lower plasma concentration for the given water soluble muscle relaxant drug hence in these patients the initial dose is increased but due to prolonged excretion time the maintenance doses are lowered.
The onset and the intensity of neuromascular blockade vary among muscle groups depending upon
Potentiation by inhalational agents
Volatile anaesthetics decrease the requirement of neuromascular blocking drugs by at least 15%,the actual degree of this post synaptic augmentation depends upon both the inhalational drug and the muscle relaxant(desflurane>sevoflurane>isoflurane and enflurane>halothane>N2O/O2/narcotic) and (pancuronium>vecuronium and atracurium)
Temperature
Hypothermia prolongs the blockade by decreasing the metabolism(mivacurium,atracurium and cisatracurium) and by delaying the excretion (pancuronium and vecuronium ) of the muscle relaxants.
Acid base balance
Respiratory acidosis potentiates the blockade of most nondepolarising muscle relaxants and antagonises its reversal.
Electrolyte abnormalities
Hypokalaemia and hypocalcemia augment the nondepolarising block while hypermagnesemia potentiates the peripheral nerve blocks.
Age
Neonates have an increased sensitivity to nondepolarising relaxants because of immature
neuromascular junctions but it does not need dose to be decreased as they have greater volume of
distribution .neuromascular junctions but it does not need dose to be decreased as they have greater volume of
Drug interaction
Drugs like dantrolene ,quinidine calcium channel blockers,streptomycin,aminoglycosides,kanamycin,
Neomycin,polymixin,clindamycin are known to potentiate the nondepolarising muscle relaxants.
Concurrent diseases
Cirrhotic liver disease and chronic renal failur often result in greater volume of distribution and a lower plasma concentration for the given water soluble muscle relaxant drug hence in these patients the initial dose is increased but due to prolonged excretion time the maintenance doses are lowered.
The onset and the intensity of neuromascular blockade vary among muscle groups depending upon
- Difference in blood flow
- Distance from central circulation
- Different fibre type
The jaw,diaphragm,larynx and facial muscles like orbicularis oculi respond quicker to the recovery from muscle relaxation then the glottic muscles which are resistant to nondepolarising drugs.
Methods to monitor neuromascular blockage
Monitoring the neuromascular blockade magnitude is accomplished by delivering an electrical stimulus near a peripheral motor nerve and evaluating the evoked response of the muscle innervated by that nerve.
Stimulation patterns
1) Single twitch: A single pulse of 0.2 millisec in duration is delivered and a control strength is noted to compare with subsequent twitches.
2) Train of Four: This stimulation pattern consists of 4 single pulses of equal intensity delivered
at interval of 0.5 sec.(2hertz) each of 0.2 ms long.
at interval of 0.5 sec.(2hertz) each of 0.2 ms long.
In control response all 4 responses are the same and in partial depolarising muscle blockade their is equal depression of all the 4 response.
In nondepolarising muscle blockade there is progressive depression of height of the evoked response from the first to the 4th response,the ratio of the T4/T1 is the ratio of amplitude and is expressed as a fraction.
Double burst stimulation
This pattern of stimulation consists of two short sequences of tetanic stimuli which are nomenclatured as DBS 3,3 or DBS 2,3 depending upon 2 to 3 single high frequency stimuli of 50Hz and 0.2 millisec separated by a time gap of 750 millisec.
The occurance of fade or a gradual diminution of evoked response on prolonged or repeated nerve stimulation is indicative of nondepolarising block.Adequate clinical recovery correlates well with absense of fade and so fade which is more obvious in tetanic stimulation and double burst stimulation than in Train of Four or Single twitch makes the above two methods the preffered choice of monitoring neuromascular recovery in nondepolarising blocks.
Tetanus
This pattern consists of repeated stimulus in rapid way which in absence of neuromascular block results in sustained muscle contraction,the ability of tetanic stimulation during period of nondepolarising block to increase the evoked response to a subsequent twitch is termed as posttetanic potentiation.
All the patterns of stimulation does not produce any fade in depolaring block they are seen only in nondepolarising nerve blocks
Methods of evaluating evoked response
Double burst stimulation
This pattern of stimulation consists of two short sequences of tetanic stimuli which are nomenclatured as DBS 3,3 or DBS 2,3 depending upon 2 to 3 single high frequency stimuli of 50Hz and 0.2 millisec separated by a time gap of 750 millisec.
The occurance of fade or a gradual diminution of evoked response on prolonged or repeated nerve stimulation is indicative of nondepolarising block.Adequate clinical recovery correlates well with absense of fade and so fade which is more obvious in tetanic stimulation and double burst stimulation than in Train of Four or Single twitch makes the above two methods the preffered choice of monitoring neuromascular recovery in nondepolarising blocks.
Tetanus
This pattern consists of repeated stimulus in rapid way which in absence of neuromascular block results in sustained muscle contraction,the ability of tetanic stimulation during period of nondepolarising block to increase the evoked response to a subsequent twitch is termed as posttetanic potentiation.
All the patterns of stimulation does not produce any fade in depolaring block they are seen only in nondepolarising nerve blocks
Methods of evaluating evoked response
- Visual response
- Tactile response
- mechanography
- Accelerography
- Kinemyography
- Electromyography
- Phonomygraphy.
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