NEUROMUSCULARMONITORING Moderator: Dr.Dara Singh Negi Presented by: Dr. Arun K Sharma
Objectives of NM Monitoring • Onset of NM Blockade. • To determine level of muscle relaxation during surgery. • Assessing patients recovery from blockade to minimize risk of residual paralysis.
Why do we Monitor? Residual post-op NM Blockade • Functional impairment of pharyngeal and upper esophageal muscles • Impaired ability to maintain the airway • Increased risk for post-op pulmonary complications • Difficult to exclude clinically significant residual curarization by clinical evaluation
Who should be Monitored ? • Patients with severe renal, liver disease • Neuromuscular disorders like myasthenia gravis, myopathies, UMN and LMN lesions • Patients with severe pulmonary disease or marked obesity • Continuous infusion of NMBs or long acting NMBs • Long surgeries or surgeries requiring elimination of sudden movement
Principles of Peripheral Nerve Stimulation • Each muscle fiber to a stimulus follows an all-or-none pattern • Response of the whole muscle depends on the number of muscle fibers activated • Response of the muscle decreases in parallel with the numbers of fibers blocked • Reduction in response during constant stimulation reflects degree of NM Blockade • For this reason stimulus is supramaximal
Essential features of equipment: Square-wave impulse, 0.1- 0.5 msec duration. • Constant current variable voltage Battery powered. • Multiple patterns of stimulation (single twitch,train-of-four, double-burst, post-tetanic count).
Key features of exogenous nerve stimulation: • Nerve stimulator: A battery powered device that delivers depolarizing current via the electrodes. • Stimulus strength: It is the depolarizing intensityof stimulating current. It depends on duration (pulse width) of the stimulus and on the current intensity that reaches the current nerve fibers. • Pulse width: It is the duration of the individual impulse delivered by the nerve stimulator . The impulse should be <0.5 msec³ and 0.1 msec in duration to elicit nerve firing at a readily attainable current. Pulse width >0.5 msec extends beyond the refractory period of the nerve resulting in repetitive firing. The stimulus should produce mono-phasic and rectangular waveform. • Current intensity : It is the amperage (mA) of the current delivered by the nerve stimulator(0-80 mA). The intensity reaching the nerve is determined by the voltage generated by the stimulator and resistance and impedance of the electrodes, skin and underlying tissues. • Nerve stimulators are constant current and variable voltage delivery devices. • Reduction of temperature increases the tissue resistance (increased impedance) and may cause reduction in the current delivered to fall below the supramaximal level
Threshold current : It is the lowest current required to depolarize the most sensitive fibres in a given nerve bundle to elicit a detectable muscle response. • Supramaximal current : It is approximately10-20% higher intensity than the current required to depolarize all fibres in a particular nerve bundle. This is generally attained at current intensity 2-3 times higher than threshold current. • Submaximal current : A current intensity that induces firing of only a fraction fibres in a given nerve bundle. A potential advantage of submaximal current is that it is less painful than supramaximal current. • Stimulus frequency : The rate (Hz) at which each impulse is repeated in cycles per second (Hz).
Electrodes • Surface electrodes • Pregelled silver chloride surface electrodes for transmission of impulses to the nerves through the skin • Transcutaneous impedance reduced by rubbing • Conducting area should be small(7-11mm) • Needle electrodes • Subcutaneous needles deliver impulse near the nerve
METAL BALL ELECTRODES • Two metal balls or plates spaced about 1 inch apart, which attach directly to the stimulator • convenient to use but no good contact • Burns
POLARITY • Stimulators produce a direct current by using one negative and one positive electrode • Should be indicated on the stimulator • Maximal effect is achieved when the negative electrode is placed directly over the most superficial part of the nerve being stimulated • The positive electrode should be placed along the course of the nerve, usually proximally to avoid direct muscle stimulation
Electrode placement: • Ulnar nerve: place negative electrode (black) on wrist in line with the smallest digit 1-2cm below skin crease • positive electrode (red) 2-3cms proximal to the negative electrode • • Response: Adductor pollicis muscle – thumb adduction
Facial nerve: place negative electrode (black) by ear lobe and the positive (red) 2cms from the eyebrow (along facial nerve inferior and lateral to eye) • • Response: Orbicularis occuli muscle – eyelid twitching
Posterior tibial nerve: place the negative electrode (black) over inferolateral aspect of medial malleolus (palpate posterior tibial pulse and place electrode there) and positive electrode (red) 2-3cm proximal to the negative electrode • • Response: Fexor hallucis brevis muscle – planter flexion of big toe
Patterns of Stimulation • Single-Twitch Stimulation • Train-of-Four Stimulation • Tetanic Stimulation • Post-Tetanic Count Stimulation • Double-Burst Stimulation
Single-Twitch Stimulation • Single supramaximal stimuli applied to a nerve at frequencies from 1.0Hz-0.1Hz • Height of response depends on the number of unblocked junctions • Prerelaxant control value is needed • Does not detect receptor block of <70% • Used to assess potency of drugs • Stimulation dependent onset time
Train-of-Four Stimulation • Four supramaximal stimuli are given every 0.5 sec • “Fade” in the response provides the basis for evaluation • The ratio of the height of the 4th response(T4) to the 1st response(T1) is TOF ratio • In partial non- depolarizing block T4/T1 ratio is inversely proportional to degree of blockade • In Depolarizing block, no fade occurs in TOF ratio • Fade, in depolarizing block signifies the development of phase II block
Tetanic Stimulation • Tetanic Stimulation is 50-Hz stimulation given for 5 sec • During tetanus, progressive depletionof acetylcholine output is balanced by increased synthesis and transfer of transmitter from it’s mobilization stores. • NDMR reduces the margin of safety by reducing the number of free cholinergic receptors and also by impairing the mobilization of acetylcholine within the nerve terminal there by contributing to the fade in the response to tetanic and TOF stimulation. • A frequency of 50Hz is physiological as it is similar to that generated during maximal voluntary effort. • During normal NM transmission and pure depolarizing block the response is sustained • During non- depolarizing block & phase II block the response fades • During partial non- depolarizing block, tetanic stimulation is followed by post-tetanic facilitation
Post-Tetanic Count Stimulation • Mobilization and enhanced synthesis of acetylcholine continue during and after cessation of tetanic stimulation. • Used to assess degree of NM Blockade when there is no reaction single-twitch or TOF • Number of post-tetanic twitch correlates inversely with time for spontaneous recovery • Tetanic stimulation(50Hz for 5sec.) and observing post-tetanic response to single twitch stimulation at 1Hz,3sec after end of tetanic stimulation • Used during surgery where sudden movement must be eliminated(e.g., ophthalmic surgery) • Return of 1st response to TOF related to PTC
Double-Burst Stimulation • DBS consist of two train of three impulses at 50Hz tetanic stimulation separated by 750msec • Duration of each impulse is 0.2msec • DBS allow manual detection of residual blockade under clinical conditions • Tactile evaluation of fade in DBS 3,3 is superior to TOF as human senses DBS fade better. • However, absence of fade by tactile evaluation to DBS does not exclude residual NM Blockade
Non-depolarizing blockade • Intense NM Blockade • This phase is called “Period of no response” • Deep NM Blockade • Deep block characterized by absence of TOF response but presence of post-tetanic twitches • Surgical blockade • Begins when the 1st response to TOF stimulation appears • Presence of 1 or 2 responses to TOF indicates sufficient relaxation
Contd… • Recovery • Return of 4th response to TOF heralds recovery phase • presence of spontaneous respiration is not a sign of • adequate neuromuscular recovery. • T4/T1 ratio > 0.9 exclude clinically important residual NM Blockade • Antagonism of NM Blockade should not be initiated before at least two TOF responses are observed
Depolarizing NM Blockade • Phase I block • Response to TOF or tetanic stimulation does not fade, and no post-tetanic facilitation • Phase II block • “Fade” in response to TOF in depolarizing NM Blockade indicates phase II block • Occurs in pts with abnormal cholinesterase activity and prolonged infusion of succinylcholine
Visual or tactile: Not sensitive enough to excludepossibility of residual neuromuscular blockade. Fade is usually undetected until TOF ratio values are <0.5.
Recording devices for measuring NM Function • Compound muscle action potential: It is the cumulative electrical signal generated by the individual action potentials of the individual muscle fibres.
Electromyogram (EMG) • It records the compound MAP via recording electrodes placed near the mid portion or motor point of the muscle and a slightly remote indifferent side. • The latency of the compound MAP is the interval between stimulus artifact and evolved muscle response. • The amplitude of the compound MAP is proportional to the number of muscle units that generate a MAP within the designated time interval (epoch) and this correlates with the evoked mechanical responses. • For experimental studies • The best signal is usually obtained by placing the active receiving electrode over the belly of the muscle with the reference electrode over the tendon insertion site • The ground electrode is placed between the stimulating and recording electrodes.
Mechanomyographic device(isometric)(Adductor pollicis force translation monitor) • Quantifies the force of isometric contraction • The force electrical signal pressure monitor and recorded. • Key features : • Alignment of the direction of thumb movement with that of the pressure transducer. • Application of consistent amount of baseline muscle tension (preload 200-300 gms) • Transducer and monitor with adequate monitoring range and zeroing of the monitor before stimulation. DISADV: • These devices are difficult to set up for stable and accurate measurements • Proper transducer orientation, isometric conditions, and application of a stable preload are required • Maintenance of muscle temperature within limits is important
Accelerography(non isometric) • This technique uses a miniature piezoelectric transducer to determine the rate of angular acceleration. • Newton’s second law, F=m*a • Muscle must be able to move freely. • The piezoelectric crystal is distorted by the movement of the crystal inlaid transducer which is applied to the finger and an electric current is produced with an output voltage proportional to the deformation of the crystal. • This is a non-isometric measurement and there are less stringent requirements for immobilization of arm, fingers and thumb and also no preload is necessary. • TOFguard, • TOF–watch (OrganonTeknika), • Para Graph Neuromuscular Blockade Monitor (Vital signs), • Part of Datex AS/3 monitoring system (M-NMT)
Limitations of NM Monitoring • Neuromuscular responses may appear normal despite persistence of receptor occupancy by NMBs. • T4:T1 ratios is one even when 40-50% receptors are occupied • Patients may have weakness even at TOF ratio as high as 0.8 to 0.9 • Adequate recovery do not guarantee ventilatory function or airway protection • Hypothermia limits interpretation of responses