Nerve Stimulators and temperature monitoring Flashcards
What are the indications for neuromuscular monitoring
- Whenever using neuromuscular blocking drugs
- When unexpected movement could be catastrophic: e.g. intra-ocular surgery
- To help diagnose the cause of post-operative apnoea
- To guide dose requirements during infusions of NMB
- In patients with neuromuscular disease (e.g. myasthenia gravis)
What other reasons for use of a nerve stimulator exist?
For regional local anaesthetic block - e.g. brachial plexus block
By the surgeon - e.g. finding the location of the facial nerve during parotid tumour surgery
Describe the function of a peripheral nerve stimulator
A peripheral nerve stimulator delivers an electrical current to a peripheral motor nerve and the response of the muscle innervated by this nerve is noted.
What four methods can be used to assess the muscle’s response to the electrical stimulation
Visual
Tactile
Mechanical
Electrical
What is a supramaximal stimulus and why is this required during peripheral nerve stimulation?
A supramaximal stimulus has current significantly above that required to activate all the nerve or muscle fibers in contact with the electrode; used when response of all the fibers is desired.
BY ENSURING THAT ALL NERVE FIBRES ARE DEPOLARIZED, CHANGES IN TWITCH HEIGHT ARE SOLELY DUE TO THE EFFECTS AT THE NMJ (Rather than a change in the number of fibres activated)
How is application of a supramaximal stimulus achieved?
- Increasing the electrical stimulus until there is no increase in the strength of muscle contraction
- Then increasing the intensity by a further 10%
What current is normally required to achieve a supramaximal stimulus?
60mA (40 - 80 mA) - at a level uncomfortable for the conscious patient.
How can the current required for a supramaximal stimulus be reduced?
By moving the positive (red electrode) more proximal along the path of the ulnar nerve
What are the most frequently used nerves?
Ulnar nerve (Adductor Pollicus) Facial nerve (Orbicularis oculi)
Uncommon:
Accessory (SCM and trapezius)
Posterior tibial (Big toe plantar flexion)
common peroneal (Foot dorsiflexion)
What are the limitations of visual and tactile assessment of the muscles response?
Difficult to quantify accurately
Describe two techniques of mechanical assessment of the muscles response to peripheral nerve stimulation
- Measurement of tension in the muscle using either a pressure transducer or a strain gauge
- Accelerometry: a piezoelectric crystal generating a voltage proportional to the acceleration of the muscle
Describe how electrical assessment of muscle response to peripheral nerve stimulation can be achieved.
Via skin or needle electrodes electrodes that measure the EMG of the muscle in question
Name the five common patterns of electrical stimulation that have developed over the years
- Train of four
- Tetany
- Double burst stimulation
- Single twitch
- Post Tetanic count
Describe the duration and frequency of the stimulus applied by a peripheral nerve stimulator for Single Twitch pattern of stimulation.
Duration: 0.2 ms
Frequency: 0.1 - 1 Hz (1 cycle per second to 1 cycle every 10 seconds)
How is single twitch stimulation interpreted?
A baseline twitch height (prior to NMB) is required.
When 20% of the twitch height is recovered, reversal of NMB can be commenced
What are the problems with single twitch stimulation?
- Forget to perform the control twitch prior to NMB
- Failure to record the control twitch height
- This is an insensitive test as > 75% of post-synaptic receptors have to be blocked before there is any fall in twitch height
Describe the duration and frequency of the stimuli required for train of four pattern of nerve stimulation
Duration: 0.2 ms
Frequency: 2 Hz (2 cycles per second?
(4 twitches given over 2 seconds)
Repeated as needed but not more than every 10 seconds
How do non-depolarizing muscle relaxants affect the muscle response to the train of four stimulation pattern?
Progressive decrease in response to successive stimuli within the train of four = FADE.
Explain the phenomenon of ‘fade’
Fade is the muscle response to a train of four pattern of stimulation by a peripheral nerve stimulator that occurs in the presence of a non-depolarizing muscle relaxant.
The mechanism by which fade occurs is thought to be due to depletion of pre-synaptic Ach molecules available for release.
What is the train of four ratio (TOFR)
The ratio of the response to the 4th stimulus against the 1st stimulus gives the train of four ratio and indicates the degree of neuromuscular blockade.
What train of four ratio is considered to show reliably adequate recovery from neuromuscular blockade?
0.9
What does the TOF look like in the absence of a NMB
All four twitches are full and equal in height
Describe the train of four count (TOFC)
The number of twitches present (the TOF count) correlates well with the % of receptors blocked
0 - 100%
1 - 90% (Tracheal intubation requires a TOFC of ≤ 1)
2 - 80% (Maintenance of relaxation can be achieved with a TOFC of 1 or 2
3 - 75% (Reversal of neuromuscular blockade should not be attempted until there is a TOFC of 3 as a minimum
4 - < 75%
What is the advantage of the TOFC/TOFR
Does not require a pre-block response to be known
More sensitive
Used to monitor the level of surgical relaxation
Used for assessment of suitability for reversal
Describe the Double Burst Stimulation pattern of peripheral nerve stimulation
Three 0.2 ms stimuli are applied 20 ms apart. This is repeated after 750 ms.
What advantage does double burst stimulation have over TOF
More sensitive in detecting fade and therefore DBS is useful in assessing adequacy of reversal of non-depolarizing block
Describe Tetanic stimulation pattern of peripheral nerve stimulation
Multiple 0.2 ms stimuli applied at 50 Hz (50 cycles per second) for five seconds. This can be repeated every 5 minutes. This is extremely painful in the awake patient
What can tetanic stimulation be used for?
To assess recovery from neuromuscular blockade. Sustained contraction for 5 s indicates adequate reversal. If there is residual NMB the strength of the muscle contraction fades.
What is a Post tetanic count
This is a pattern of stimulation involving a burst of tetany followed after 3 seconds by a series of 0.2 ms stimuli at a frequency of 1Hz (1 cycle/second). The number of twitches then produced is then counted
When and why is post tetanic count used?
Used when profound non-depolarizing block is present, as there may be a PTC even when there is no response to a TOF. This is because post tetanic potentiation occurs - caused by the increased synthesis and release of Ach at the NMJ following tetanic stimulation
How is Post tetanic count interpreted?
The greater the PTC, the sooner a response to a TOF is seen.
A PTC of 1 or 2 indicates deep paralysis suitable for surgery such as microsurgery on the diaphragm
Why is it important to combine Neuromuscular monitoring with clinical methods for assessing recovery from NMB?
Because different muscle groups exhibit different sensitivity to neuromuscular blockade.
Highly sensitive
- Adductor pollicus (can lead to overestimation of the degree of block)
Less sensitive
- Diaphragm
- Laryngeal adductors
- Rectus abdominus
Insensitive
- Orbicularis occuli (can lead to underestimation of the degree of block)
What are the clinical methods used that may also be helpful in assessing recovery from NMB
Sustained head lift > 5 s
Adequate cough
Strong hand grip
Inadequate recovery from neuromuscular blockade may also lead to an uncoordinated jerking pattern of muscle contraction
What are the important effects of hypothermia in the anaesthetic patient?
- Impaired coagulation
- Impaired drug metabolism/excretion (potentiating drugs such as neuromuscular blockers)
- Impaired respiratory and cardiac muscle function
Intraoperative hypothermia is also associated with shivering which increases O2 consumption considerably
What are the important effects of hyperthermia in the anaesthetic patient?
Increases O2 consumption and fluid loss
Life threatening hyperthermia occurs with anaesthetic emergencies of malignant hyperthermia and thyrotoxic storm
How can thermometers be classified?
Electronic
Non-electronic
On what principle do most non-electronic thermometers rely
Solids, liquid and gases increase their volume when warmed
Describe a mercury thermometer
Glass bulb filled with liquid mercury attached to a narrow capillary tube. As the liquid Hg is warmed, it expands and moves up the capillary tube. The tube is aligned with a scale and the meniscus of the mercury column can then be read against the temperature scale
Why are mercury thermometers being removed from clinical practice?
Environmental toxicity concerns
What is a tempaDot thermometer
Dot sensor matrix with an adjacent temperature scale. Each dot changes colour at a specific temperature and the temperature is read off the last blue dot that hasn’t changed colour.
What are the four commonly used types of electronic thermometers
Thermistors
Resistance thermometers
Thermocouples
Infrared
What are thermistors
Small beads of metal oxide semiconductors, used at the tip of a temperature probe.
Resistance of a thermistor FALLS exponentially with increasing temperature (although usually manufactured to be nearly linear in the body temperature range)
The change in resistance can be measured and the body temperature calculated.
What are resistance thermometers?
Resistance thermometers rely on the fact that an increase in temperature causes an increase in the electrical resistance of a platinum wire. By connecting this wire to a Wheatstone bridge circuit, small changes in electrical resistance can be detected.
Define the Seebeck effect
At any junction of two dissimilar metals a small voltage is produced, the magnitude of which depends on the temperature of the junction. This junction is called a thermocouple.
What is needed to form a thermocouple
Two dissimilar metals (Copper and constantan)
A measuring junction
A reference junction (to complete the electric circuit)
The potential measured at the measuring junction is proportional to the temperature.
How does an infrared thermometer work?
Energy emitted by any object changes according to the temperature of that object.
The energy emitted can be measured and from this the temperature of the radiating object calculated. - usual site: tympanic membrane
What are the Pros and Cons of the mercury thermometer
Pros: simple and cheap
Cons: breakable, mercury toxicity, cross contamination
What are the pros and cons of the TempaDot
Pros: Disposable, no risk of cross contamination
cons: Slow
What are the pros and cons of the thermistor
Pros: small and quick
cons: Needs recalibrating
What are the pros and cons of a resistance thermometer?
Pros: Accurate
Cons: Slow, fragile, large
What are the pros and cons of a thermocouple
Pros: Quick, small, accurate
Cons: needs signal amplification and processing
What are the pros and cons of an oesophageal thermometer
Pros: Accurate if placed in the lower 1/3 of the oesophagus
Cons: Invasive. Inaccurate if placed incorrectly. Can read artificially low due to the cooling effects of inspired gases in the adjacent trachea (also during thoracotomy)
What are the pros and cons of the nasopharyngeal thermometer
Pros: Easy to position in anaesthetized patient who has an airway device (LMA/ETT)
Cons: Risk of epistaxis. Used only in anaesthetized patients. Position dependent
What are the pros and cons of a thermometer in the urinary bladder
Pros: Dual purpose - usually part of urinary catheter
cons: Invasive. Low urine output states can give inaccurate readings
What are the pros and cons of a thermometer in the blood
Pros: most accurate measure of core temperature when used with a pulmonary artery flotation catheter
Cons: Very invasive
What are the pros and cons of a rectal thermometer
Pros: useful during head and neck surgery
Cons: inaccurate and slow because of insulating effects of faeces - risk of rectal perforation in babies
