BT_RT 1.19 - Malignant Hyperthermia Flashcards

Question

Post-Tetanic Potentiation Description

Answer 1

Temporary increase in single twitch height after tetanic stimulation (e.g. 50Hz 3 seconds) (note may also account for TOF ratio >1) Occurs with non-depolarising drugs only Lasts for ~7 minutes

Answer 2

Inverse correlation of post-tetanic count with degree of blockade Useful during deep blockade e.g. brain surgery PTC 9 -> TOF count 1 PTC 5 -> TOF count 1 in 10 minutes PTC 2 -> TOC count 1 in 20 minutes

Answer 3

↑ICF [Ca2+] via L-Ca2+ at pre-synaptic terminal due to Repeated activation of the nerve Repeated activation of the VDCC Insufficient time for Ca2+ extrusion Reserve pool of ACh mobilized (larger, dispersed, tethered to cytoskeleton) by ↑ICF [Ca2+] Stronger contraction

Answer 4

Nerve stimulator Objective monitor (quantitative analysis) - Mechanomyography (MMG) - Electromyography (EMG) - Acceleromyography (AMG) Subjective assessment (qualitative analysis) - Visual - Tactile Clinical assessment Objective assessment is more accurate and reliable than manual/clinical assessment Order of preference (high to low) Nerve stimulator — objective monitor Nerve stimulator — subjective assessment Clinical assessment

Answer 5

Mechanomyography Electromyography Acceleromyography Kinemyography

Answer 6

Measures the strength of evoked isometric contraction of a muscle, most commonly adductor pollicis (AP) after stimulation of the ulnar nerve The hand is rigidly fixed in a stable position and a band is looped around the thumb, which is connected to a force-displacement transducer 200-300g of tension is applied to abduct the thumb (a preload) Ulnar nerve is stimulated and the evoked AP contraction strength is measured While MMG is regarded as the gold standard in measuring twitch response, it is not commercially available and is used mainly for research purposes

Answer 7

Measures the amplitude of the evoked compound action potential in a muscle after stimulation of the nerve When the ulnar nerve is monitored, electrodes are placed over the thenar eminence (over the belly of muscle and over the muscle insertion point) to record the electrical response in the muscle stimulated This mode is mainly used in research Pros Equipment easier to set up Response reflects only factors that affect NMJ transmission, while MMG response is also affected by factors that affect excitation-contraction coupling and muscle contraction itself Can be used to monitor sites where it is impractical to measure mechanical responses with MMG, eg larynx, diaphragm Cons Measured responses may be inaccurate if improper electrode placement, potentially secondary to direct muscle stimulation Sensitive to electrical interference, eg diathermy

Answer 8

Measures the strength of evoked isotonic contraction of a muscle Uses the principle that force = mass x acceleration Piezoelectrode is attached to the tip of the thumb Used to measure the force produced by a muscle after the motor nerve has been stimulated An accelerometer is affixed to the thumb (if ulnar nerve is being monitored) and measures the acceleration generated by AP adduction, which is proportional to the force generated A voltage is created when the muscle accelerates and that acceleration is proportional to the force of contraction Thumb must be free to move Isotonic - no change in muscle tension = joint must be free to move Pros Widely available commercial monitor for clinical use Does not require fixation of the hand, as long as the thumb can move freely Clinically, use has been shown to reduce the incidence of residual paralysis Cons Responses do not correlate very strongly with responses from MMG and EMG Control value of TOFR, ie without relaxant, is commonly 1.1–1.2 for unknown reasons, therefore is has been suggested that a TOFR ≥ 1 should be used to indicate adequate reversal, rather than TOFR ≥ 0.9

Answer 9

is based on a mechano-sensor strip that contains a piezoelectric polymer The strip placed between index finger and thumb When bent generates an electrical signal that is proportional to the magnitude of bending Measurements shown not to correlate with MMG limiting application

Answer 10

2 electrodes are placed over the nerve to be stimulated In theory, the negative (black) electrode should be placed distally over the nerve and the positive (red) electrode more proximally over the nerve or somewhere nearby, as this arrangement more readily stimulates the nerve and elicits a greater neuromuscular response

Answer 11

Negative electrode 1cm proximal to wrist crease and positive electrode 3–6cm proximally Elicits thumb adduction (adductor pollicis) and finger flexion In this arrangement, the polarity of the electrodes does not matter so much

Answer 12

Negative electrode proximal to wrist crease and positive electrode over ulnar groove at elbow Thumb adduction more pronounced due to additional flexor carpi ulnaris stimulation In this arrangement ensuring that the distal electrode is negative → maximal response

Answer 13

Innervation: Temporal branches of facial nerve (CN VII) Negative electrode over the temporal branch of facial nerve and positive electrode elsewhere over the forehead

Answer 14

The stimulus is delivered as a square wave current

Answer 15

Supramaximal pulse of duration 0.2msec delivered at frequency of once per second (1Hz) or once per 10 seconds (0.1Hz) Requires use of an objective monitor, eg MMG, to assess the strength of the evoked response A control response is measured before any muscle relaxant is administered to gain a baseline measurement The depth of block is assessed by depression of the twitch strength Twitch height begin to reduce at ~ 75% receptor occupancy with ND-NMBs The twitch strength will reduce from 100% to 0% as the receptor occupancy increases from 75% to 95% It cannot differentiate between depolarizing and nondepolarizing block, as for both types of block, the twitch response will be reduced with increasing depth of block Uses: assess development of block at induction It is the only mode that can assess the depth of a depolarising block Advantages: easy to perform Disadvantages: require knowledge of baseline twitch height, very crude assessment

Answer 16

Delivered by 4 identical supramaximal stimuli (60 – 80mA), each of 0.1 sec duration, at 2Hz (2 per second); the sequence may be repeated every 10 – 20 seconds if used ‘continuously’ Need supramaximal stimulus (typically >50 mA) to ensure all muscle fibres recruited Current of 25% above the maximal stimulus required

Answer 17

assess recovery from moderate to deep block with non-depolarising NMBs

Answer 18

Is control response required?

Answer 19

ratio of the strength of the fourth twitch to the first twitch (T4/T1), the train of four ratio (TOFR)

Answer 20

The number of twitch responses will reduce from four to zero The depth of block is assessed by the count of how many twitches are present, the train of four count (TOFC)

Answer 21

should not be repeated within a 10 secs interval, to avoid increased fade in the latter set

Answer 22

Qualitatively impossible to distinguish between a TOFR of 40% or more Quantitative acceleromyography does this reliably - determines safe reversal of TOFR > 0.9 Gold standard of neuromuscular monitoring is mechanomyography - measure force of muscle contraction Needs constant preload and difficult to set up

Answer 23

A TOFC 1 – 2 is generally a sufficient depth of surgical relaxation for most surgery types

Answer 24

supramaximal stimuli, less painful than tetanic stimuli for awake patients

Answer 25

TOF ratio requires objective measurement (e.g. quantitatively monitoring with accelerometery), can only qualitatively assess fade when TOFR < 0.4 Quantitative assessment of TOF is not useful in monitoring deep and profound block as the TOFC is 0

Answer 26

improve qualitative assessment of neuromuscular function (to detect even small degree of fade associated with shallow neuromuscular block)

Answer 27

Two mini-tetanic bursts — each consisting of 2 – 3 stimuli of 0.2msec duration at 50Hz — separated by 750msec 3 x 0.2 msec stimuli at 50 Hz --> 0.75sec pause --> 3 x 0.2 msec stimuli at 50 Hz Two alternative stimulation patterns used for DBS, depending on the manufacturer DBS3,3 Both bursts consists of 3 stimuli DBR corresponds closely to TOFR (I think this is the more commonly used variation) DBS3,2 First burst consists of 3 stimuli, the second consists of 2 stimuli Detection of fade is slightly improved over DBS3,3

Answer 28

the ratio of the strength of the second twitch to the first twitch (D2/D1), the double burst ratio (DBR)

Answer 29

DBS is superior to TOF in detecting fade subjectively, however an objective monitor is still required to reliably exclude residual paralysis

Answer 30

2 twitches

Answer 31

easy to perform Reliability of qualitative monitoring with DBS is improved over that of TOF monitoring Allows detection of fade when the second response is 60% of the first response Compared to 40% of the first response for TOF

Answer 32

more painful than TOF; no better than TOF when objective measurement is used

Answer 33

first apply tetanic stimulation (0.1 msec stimuli at 50 Hz maintained for 5 sec) --> 3 second pause --> repeated single twitch (0.1 msec) at 1 Hz --> subjectively measure number of single twitches Tetanic stimulation results in ACh synthesis and mobilisation --> immediate increase in available ACh store --> momentary increased response to subsequent supramaximal stimuli --> post-tetanic facilitation The number of detectable twitches is termed the post tetanic count The PTC is inversely proportional to the degree of paralysis

Answer 34

assess recovery from profound or deep blockade (when TOF number of zero observed) When profound neuromuscular block is required Retinal surgery, when movement or coughing could have devastating effects

Answer 35

During the tetanic phase, large amounts of ACh are released → may be enough to overcome the block of the post synaptic nACh receptors → initial muscle contraction However, ACh stores are rapidly depleted with ongoing stimulation → less and less ACh released → tetanic fade During the tetanic phase and subsequent pause, post tetanic facilitation/potentiation starts → ↑synthesis and packaging of ACh pre-synaptically During the ST 1Hz phase, due to the ↑ACh release, twitches may now reappear which weren’t present prior to the tetanic stimulation Fade will occur and the depth of block is assessed the count of twitches after tetanic stimulation, ie the PTC

Answer 36

sensitive in deep paralysis very painful

Answer 37

burst of rapid (50 ~ 200 Hz) stimuli (each lasting 0.1 msec), maintained for 5 seconds --> monitor strength of contraction for subtle fade Normal evoked stimulus – tetanic contraction maintained at maximal intensity However, at > 100 Hz, muscle fatigue can occur, leading to fade

Answer 38

detect subtle fade associated with superficial blocks by ND-NMBs aka 'residual curarisation'

Answer 39

Phase 1 depolarising NMB Tetanic contraction maintained without fatigue or fade, but of reduced intensity Non-depolarising NMB or Phase 2 depolarising NMB Gradual diminution of evoked muscle contraction responses is seen (fade)

Answer 40

Clinically, it is the most sensitive (and reliable) monitor of minor degrees of NMB (receptor occupancy of 50% when fade absent at 100 Hz; 70% at 50 Hz)

Answer 41

Limited by severe pain, not suitable for awake patients

BT_RT 1.19 - Malignant Hyperthermia Flashcards

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