TOF

Question 1

TOF

Answer 1

It is an Objective measure to assess Neuromuscular transmission in patients- often used to assess depth of paralysis esp in patients receiving the continuous NMB infusion

Question 2

Principle

Answer 2

NMBAs block the nicotinic receptors at the NMJ
->Force of contraction of the muscle depends upon the number of receptors activated

Answer 3

The four electrical stimuli- delivered in a rapid succession- over about 1.5sec(gap of 0.5 sec inbetween the successive stimuli), each stimulus lasts 0.2mili sec

Question 4

Important points

Answer 4

Allows assessment of different muscle groups
Not all muscle groups are affected equally by neuromuscular blockade.
Typically smaller muscle groups are more sensitive
The positive (red) lead is placed proximal

Question 5

Recommended sites

Answer 5

1). Ulnar nerve- first choice-
- target muscle stimulated- Adductor policis muscle-because it is only innervated by the ulnar nerve.
- Stimulation causes movement of thumb towards fingers.
- This site also avoids direct muscle stimulation
2). Other nerves- Facial nerve,esp temporal branch
-Muscle stimulated- corrugator supercilli musc- eyebrow moves downwards
3). Posterior Tibial nerve-
-**Flexor hallucis brevis **muscle is stimulated- causes plantar flexion of great toe

distal lead is always negative lead because it produces depolarization instead of hyperpolarization of the nerve, hence less stimulation is required to produce an effect.

Question 6

Avoid sites with

Answer 6

1). Oedema
2)Arterial / iv lines
3)unstable fractures
4) Clear of hair
5) Avoid ⬇️K, ⬇️Ca, ⬆️Mg

Question 7

Nomenclature

Answer 7

➡️Ideally, a supramaximal stimulus is also determined prior to paralysis where any further ⬆️ in stimulus is not going to cause an increase in response.
➡️Threshold- minimum current required to evoke a muscle contraction
➡️ Submaximal stimulus is a stimulus that is not strong enough to elicit a full muscle contraction.
➡️ Maximal stimulus- causes full muscle contraction
➡️ Supramaximal stimulus - 20% increase in current over the maximal stimulus to provide a buffer.

Start with the smallest and go up in small increments of 5-10mili amp.

Question 8

Assessing Response

Answer 8

Which each stimulus delivered, response is assessed at the target muscle group.
->No and strength of twitches depends upon the depth of NMB
->No block= 4 equally strong twitches
-> If <60-70%of receptors blocked–> 4 twitches will still be seen
-> Blockage of 65-70% receptors–> Fade response i.e decreasing strength of twitch with each response.
->75%block–> 4th twitch lost, fading present
-> 80%block–>3rd twitch lost
->90%block–>2nd twitch lost
->95%block–>no muscle twitch observed

Question 9

Assessment and Maintainence of the block

Answer 9

->2/4 twitches is generally a good level of block for the patients to maintain paralysis

->Reversal agents should not be given below a twitch count of 3

->The ratio of the amplitude of T1 to T4 (ToF ratio) can also be used as a measure of blockade:
->ToF ratio > 90% is adequate for extubation
ToF ratio > 70% suggests adequate respiratory function

->Observed Hourly initially and then every 15mins following a change in rate of infusion/ a bolus dose.

->Once a stable state achieved, monitoring increased to 4hrly
->If suddenly response drops unexpectedly to no twitch-
check the confounding factors first–batteries in device, oedema, electrolytes, change site