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Flashcards in Nerve Stimulator Deck (19)
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Nerve stimulators: criteria for nerve selection

Motor element present and close to the skin

Contraction in nerve-supplied muscle must be visible/accessible for evoked response monitoring

Pulse width should be <0.5 millisec to prevent impulse from extending beyond refractory period

Duration should be 0.1 sec


Indications for NMJ monitoring

Long interventions
No movement allowed during procedure
No reversal preferred during procedure
Anticipated electrolyte balance disturbance
Expected drug interactions
Changed pharmacokinetics/dynamics


Single twitch event: what does it reflect, what is it used for, and what is its frequency?

Reflects events at post synaptic membrane (events at the receptor)

Used for monitoring onset of block

Frequency = 1 Hz or 0.1 Hz

**Calibration required before relaxation (not suitable for day to day clinical practice as it means we need to calibrate it before MRs are in - meaning we have to shock the patients while they are awake)


ToF event: what does it reflect, what is it used for, and what is its pattern?

Reflects events at pre synaptic membrane

Used routinely to monitor onset, maintenance, and recovery of NMB

Pattern: four supra maximal stimuli (2 Hz or q 0.5 s); may be repeated q 15 sec


ToF ratio - what is it and what are its properties?

The ratio of the last/4th signal to the first signal (B/A)

Remains the same despite changes in absolute response and should tell you extent of block + what kind of block is occurring

[depolarizing block: ratio of 1; non-depolarizing block: ratio <1]

Smaller this ratio, the more block that is occurring; ratio of 1 for non-dep block is usually good indication for reversal


ToF signs and symptoms (0.70-0.75)

-Diplopia (misaligned eyes) and visual disturbances
-Decreased hand grip
-Inability to sit up without assistance
-Tongue depressor test (-)
-Severe facial weakness
-Speaking a major effort


ToF signs and symptoms (0.85-0.90)

Diplopia and visual disturbances
Generalized fatigue


Tetanus event - what is it used for, and what is its behavior

Used to gauge pt readiness for reversal (positive test means we can reverse)

5 second stimulation that fades with non-depolarizing block


Post-tetany effect

Sustained tetany floods the NMJ with ACh; gives a false ToF if it precedes another ToF series immediately (antagonism)

(Give time for ACh to diffuse away) --> Wait 12-15 sec before testing that muscle again


Post-tetanic count (PTC) - use and behavior

Used to assess intensity of deep blocks

Sustained tetany for 5 seconds, followed by single twitch stimulation at 1 Hz (1 per second) for 10 seconds ; PTC count is how many of these post tetanic twitches occur before complete fade

-ToF = 0 @ PTC of 5
-ToF = 1 @ PTC of ~8-10
-@ PTC of 10, patient can be reversed


Double burst stimulation - use and behavior

Similar use as ToF

Two short (0.2 ms) bursts of 50 Hz tetanic stimuli separated by 0.75 seconds
-Most commonly used with 3 impulses in each burst
-Ratio of 2nd response amplitude to the first is equivalent to ToF ratio (smaller the more reversible)


Why do we use the muscles near the eyes for nerve stim monitors?

Orbicularis oculi muscles have onset, duration, and sensitivity that are very similar to the respiratory muscles, esp diaphragm, which are the most resistant to NMB (so if those are blocked, its a good bet that most other nerves are blocked too - can work for reversal assessment as well)


What other muscle is commonly used to assess reversibility?

The adductor pollicis ("thumb muscle")) - has a larger margin of safety since it is a peripheral muscle and its movement is easily detectable + usually out of the way of surgical site

(peripheral muscles = last to recover from NMB so if its reversible, its a good bet that most other muscles are reversible as well)


Effect of magnesium on NMJ blockers

Magnesium by itself causes generalized weaknesses and relaxed muscles when injected at NMJs so it extends the effects of NMB


What can happen if we give too much reversal to patients?

Too much reversal can repotentiate effects of muscle relaxants and make patients weak post operatively


What should we consider when determining whether to reverse or not and how much reversal to use?

Half life of your MR and how many half lives its been since last administration as well as pre existing conditions in the patient that may affect their NMJs (electrolyte imbalance, residual anesthetics, etc..)


Clinical applications of nerve stim monitoring

Onset: Orbicularis oculi - ST or TOF
Surgical relaxation: 1 or 2 responses to TOF = sufficient block.
When intense block required - PTC
Recovery: TOF ratio, DBS
Reversal: When 2 or more TOF responses
Extubation: When TOF reaches 70 – 90 %


ToF NMB %s @ each response #

1: 95%
2: 90%
3: 85%
4: 75%


Limitations to NMJ monitoring

-Individual variability (NMJ responses may appear normal despite persistence of block)

-Some pts may exhibit weakness at ToF ratio as high as 0.8-0.9

-Increased skin impedence due to perioperative hypothermia may limit our interpretation of evoked responses