Technical aspects of NCV

Technical aspects of NCV : 

Technical aspects of NCV Chaman Lal Physiotherapist College Of Physiotherapy JPMC Karachi. +92-333-3256772 Chaman_PT@yahoo.com

Introduction: : 

Introduction: The clinical electrodiagnosis involves the recording , display, measurement and interpretation of action potentials arising from CNS. A number of technical and instrument related variables can influence the action potential.

Biophysics: : 

Biophysics: Normal muscles and nerve cells membrane have stable resting potential. Due to uniform distribution of + charge ions and – charge ions inside the cell membrane. Potential difference b/w 2 points outside and inside of cell is zero. During depolarization the voltage difference b/w 2 points occur and current flow along the cell membrane. Action potential current occurs. The current flow is resisted by intervening tissue, impedance. voltage = current x impedence Action potential displayed is called waveforms.

Apparatus: : 

Apparatus: The basic component use for the nerve conductions are: Electrodes. Stimulator. Amplifier. Filters. Display screen. Recording mechanism.

Electrodes: : 

Electrodes: TYPES: 3 type of electrodes are used in electrodiagnosis tests: Active. Reference. Ground. The action potential is measured b/w active and reference. Ground electrode serve as a zero voltage reference point.

CONT: : 

CONT: Material: Made up of variety of metals and alloys such as stainless steel, platinum, silver chloride, nickel, chromium, silver and gold. Polarization: Metal electrode when interact with electrolytes have electrochemical reactions result in electrode polarization. So silver silver chloride is use due to stable polarization potential.

Cont: : 

Cont: Shape: The surface electrodes are in the form of disc, cup or ring. Placement: In motor nerve conduction, The active electrode is placed over the motor point. The reference electrode is place on tendon. An interelectrode distance of 4 cm has been recommended. Electrodes are placed with the help of electrode paste and cleaning of skin is important prior to reduce the impedence.

Types of electrodes : 

Types of electrodes

Filters: : 

Filters: Settings for motor recording: LFF = 2Hz HFF = 10KHz Settings for sensory recording: LFF = 20Hz HFF= 3000Hz

Amplifier: : 

Amplifier: As biological signal are very small so variable degree of is needed before display. The action potential generated in nerve must flow through electrode and finally into amplifier.

Averager: : 

Averager: Averaging is a mechanisam to extract small signals which are burried in large noise. By averaging the signals become prominent and the noise which is randomly occurring is cancelled out.

Display: : 

Display: 2 techniques of waveforms display are commonly used in electrodiagnosis: The analogue oscilloscope display: In this signals are directly displayed following amplification and filtering. Signals are displayed on a cathode –ray tube The analogue to digital cover: In this a continous varying signal is sampled at discrete time interval and the amplitude of a signal is converted to a number following amplification and filtering.

Stimulator: : 

Stimulator: “Surface stimulating electrodes are usually two metals or felt pad electrodes place 1.5 to 3.0 cm a part.” It generates the square wave electrical pulse in milli amperes that depolarize the nerve under the cathode and hyperpolarize beneath the anode. Generates the action potential. As it is synchronize with the oscilloscope sweep, thus enabling the potential to be recorded at a distance by recording electrode.

Terminologies for type of stimulus: : 

Terminologies for type of stimulus: Sub- maximal stimulation: A stimulus intensity just capable of producing an observable response is called threshold stimulation or Sub Maximal response Supra-maximal stimulation: Maximal stimuli are those that do not produce any further increase in the response and a stimulus delivered at approximately 20-30% above maximal intensity is supra-maximal threshold

Cont: : 

Cont: Antidromic stimulation: Propagation of an impulse in the direction opposite to physiologic conduction (conduction along motor nerve fibers away from muscle and conduction along sensory fibers away from spinal cord. Orthodromic stimulation: The propagation of an impulse in the direction the same as physiologic conduction, that is conduction along motor fibers towards muscle and conduction along sensory fibers towards spinal cord.

Cont: : 

Cont: The control on stimulator are used for adjusting the following: Stimulus duration.(0.05-1.0 msec) Stimulus intensity.( increase in milliamperes in a constant current stimulator) Rate of stimulation. (0.25 to 50 per seconds) Longer duration and high intensity cause: Increment of stimulus artifacts. Pain and uncomfortable to patient. May stimulate adjacent nerve.

Stimulus artifact: : 

Stimulus artifact: “The stimulus artifact is the deflection from the baseline resulting from direct conduction of the stimulus.” At time the stimulus artifact may be so large that it mask the onset of response and artificially prolonged the latency. Result due to excessive cutaneous spread of stimulating current to recording electrode.

Cont: : 

Cont: It can be reduce by: Placing ground b/w stimulating and recording electrode. Adequate spread of stimulating, ground and recording wires. Ensuring equal impedence in active and reference electrode. Cleaning any perspiration or conducting gel b/w electrodes. Using the lowest stimulus intensity and duration to get supra max response. Avoiding short distance b/w stimulating and recording electrode.

Motor conduction studies: : 

Motor conduction studies: Motor nerve is recorded by stimulating nerve and recording from the Belly of the muscle supplied by that nerve Active recording electrode G1: Motor point or the belly of the muscle under investigation Reference recording electrode G2: Placed over the tendon, relatively inactive area Ground electrode: In between recording and stimulating electrode

Cont: : 

Cont: With this arrangement the recorded responses is a biphasic potential with initial larger upward (negative deflection) followed by smaller downward (positive) deflection. The evoked motor response is called a compound motor response: “sum of action potentials of individual muscle fibers”

Cont. : 

Cont. If initial positivity seen then the following should be considered: The active recording electrode is not on motor point. Active and reference electrode is transposed. Stimulating of neighboring nerves. Measurement of CMAP includes: Latency. Amplitude. Duration.

Latency: : 

Latency: “ Time interval b/w the onset of the stimulus and the initial deflection of the motor response.” Measured in milliseconds. For motor response distal latency is noted.

Cont: : 

Cont: To avoid error in calculating the latency following technical details should be considered Standard distance b/w the most distal stimulating site and active recording electrode and compared with respective normative data. Correction of latency: latency correction= measured-standard distance conduction velocity e.g median nerve: standard distance = 70mm distance used = 85mm CV = 50m/sec latency = 4.2 msec latency correction = 85 -70 = 0.3m sec 50 so, 4.2msec - 0.3 msec = 3.9 msec

Cont: : 

Cont: 2. The stimulus intensity should always be supramaximal to ensure all the nerve fascicles. Because there is a progressive decrease in latency with an increase in stimulus intensity from sub to supramaximal stimulus. This is decrease is due to the spread of stimulus distal to the point of stimulation. 3. With lower gain baseline deflection is not sharp with tends to longer latency to be reported. 4. Temperature = low temperature increase the latency.

Conduction velocity: : 

Conduction velocity: Speed of propagation of an action potential along a nerve is called conduction velocity. For two side stimulation: CV = distance b/w proximal and distal site proximal latency – distal latency

Amplitude, duration and area: : 

Amplitude, duration and area: Amplitude: It is measured from baseline to peak. Duration: The amount of the time between the CMAP onset latency and insertion with base line. Or from onset of baseline to its final return to bas2line. Area: It is the area under the negative spike of the CMAP.

Slide 29: 

Area Duration

Sensory nerve conduction: : 

Sensory nerve conduction: Sensory response: A compound nerve action potential CNAP produced produce by the electrical stimulation of the afferent nerve may be recorded over peripheral sensory nerve in a number of areas. The response obtained is called SNAP.

Cont: : 

Cont: SNAP can be recorded by placing: 1. Active recording electrode: - Distal segment of the nerve 2. Reference recording electrode: - 3-5 cm distal to active recording electrode 3. Ground electrode: - In between active recording electrode and cathode of stimulating electrode

Parameters measured in SNC: : 

Parameters measured in SNC: Latency measurement: Peak latency Amplitude: Base line to peak is preferable Conduction velocity: CV = distance b/w stimulus and recording electrode / onset latency