Practical Instrumentation and Avoiding Errors Material.pdf · Practical Instrumentation and...
Transcript of Practical Instrumentation and Avoiding Errors Material.pdf · Practical Instrumentation and...
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Practical Instrumentation and Avoiding Errors SuperEMG Plus
February 2011William S. Pease, M.D.The Ohio State UniversityCollege of Medicine
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Electricity and so much more!
Recording membrane potentials
Can only measure activityImplications made from absence of activity!
Know where things are!Settings on instrument prepared for what you might record.Vary settings when response not apparent
Small, late, etc?
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Describe these potentials.Muscle at rest.
Several different spikes?Duration?Regular firing?Amplitude?
Sweep = 10 ms/divGain = 200 µV/div
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hat is the size of the important nals here?
Ans= 20µV; so use a 50µV/cm Gain
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Several different spikes?Duration?Regular firing?Amplitude?
Describe these potentials.Muscle at rest.
Sweep = 10 ms/divGain = 100 µV/div
Irregular End-plate Potentials
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Nerve Action Potential Recording
The traveling action potential wave of SNAP is notsymmetricalDistant portions are recorded by bothelectrodesNegative peak recorded under E1
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Instrumentation
Be prepared to record the signals related to your differential diagnosis!Amplitude/GainFrequency & Duration/Sweep speedHear it/Audio volume
Keep it on during Nerve Conduction Testing
This will not harm the amplifier
Frequency/Analog to digital conversion rate
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What is error in settings?
Median sensory to D3Top-wrist stimBottom-palm stim
Cannot measure the 2nd sensory amplitude?
1 ms
20 µV
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Correct settings!Able to measure amplitude.What are amplitudes?
1 ms
20 µV
Now we see the 50% SNAP conduction block!
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NCV Vocabulary: Measurements
Distal Latency (A)Conduction Velocity
distance/time
Amplitude (B)Baseline-to-peak
Duration (C)Negative potential
Supra-maximal Stimulation = the stimulus that is 10% above that which produces the maximum response amplitude
AB
C
A
B
C
Motor
Sensory
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Case: Paraspinal Muscle at restDescribe the Findings
Regular, 1.3 Hz2 spikes, short durationPacemaker spikes
Slow sweep! 100ms/div
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Interference EMGl A: Fluorescent lights
+EMG
l B: Fluorescent lights
l C: Fluorescent lights + 60 Hz (power line)l (You can have
fluorescent lights fixed!)
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Interference EMG
D: 60 Hz, power line signalE: 60 Hz “notch “filter applied to D
But this causes loss of signal in this frequency band as well.
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Differential Amplifier
Powerline interference affects all electrodes equally.Subtraction at differential amplifier results in near Zero output.This is the preferred way to eliminate 60 Hz interference
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Filters-Electronic
The range of frequencies permitted to the output is the Bandwidth.The filter limits (“cut-off”) are at 50% power reduction. About 70% amplitude (V).
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FiltersPositive sharp waves
Left-Positive waves recorded with filters 20 Hz to 10kHz Right-Positive waves recorded with filters
100 Hz to 10kHz
Sharper peak on negative phaseLooks like positive waves seen in end-plate zones
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Low Freq. 3 Hz
Low Freq. 20 Hz
Low Freq. 100 Hz
CMAP: Effect of Low Frequency Filter
Median Motor Latency
100%85%55%
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Filters; Sensory NCV
High frequency filter change.10 kHz vs. 2 kHz (smooth)Use the correct reference values for your filter settings!
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The peak is the high frequency content.
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Electrode SeparationMedian Sensory Distal Latency (14cm)
Electrode separation
top = 4 cm
bottom = 2 cm
Lat=3.70Amp=27.6
Lat=3.65Amp=21.2
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Effect of Temperature on NCV
Sensory Nerve Action Potential (SNAP) recorded at 32°C and 27°C (lower)Slowing is approx. = 5%/°CAmplitude and duration will both increase!
Not seen in pathology!
27°C
32°C
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Nerve Conduction: Supramaximal Stimulation
Supramaximal stimulation is related to the size of the sensory or motor response!
Supramaximal stimulation is NOT the maximum shock that your stimulator can
deliver!
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Shock Artifact
Obscures initial response and baseline “0”Impedes measure of amplitude and duration
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Ground Electrode Placed Between the Stimulator and the Recording Electrodes
Ground Electrode
(It can also be placed on the dorsum of the hand.)
RecordingElectrodes
Stimulator pin
(The correct technique for many years!)
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Shock Artifact: Rotation of the Anode (about the Cathode)
Reduction of shock artifact can often be achieved by rotation of the anodeWatch your stimulator wires-stay away from recording wires ( )
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Rotation of the Anode(about the Cathode)
Variation in shock artifact with rotation of the anode about the cathode
Dumitru(Textbook)
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Rotation of the Anode(about the Cathode)Shock artifact occurs when E1 and G2 are on different lines of electrical potential in the stimulator’s electrical fieldsRotation moves these fields and can re-orient the electrical fields so that E1 and G2 are on an equi-potential line.
100V150V
300V500V
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Shock Artifact:Other Factors
Dry skin thoroughly between stim and recording elect.Increase distance between stimulator and recording elect. (if possible)Check or replace skin surface electrodesMove ground closer to recording electrode
Dry skin
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Shock Artifact: Troubleshooting
Minimize use of electrode gelAvoid excessive stimulusTry needle stimulusUse constant-current, rather than constant-voltage stimulus
Did patient use skin lotion?
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MotorSweep speed
2 ms/cmGain (Amplifier)
2-5 mVFilters
L=2 Hz, H=10 kHz
SensorySweep speed
1 ms/cmGain (Amplifier)
10-20 µVFilters
L=20 Hz, H=2 kHz
Nerve Conduction Set-up
N.B. “Most of the time”Adjust prn to see response clearly.
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Analog to Digital Conversionand Back
Sampling frequency of digital conversion must be more than two (2) times the highest frequency of interest in the study.
Can miss movementBetween the lines!
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Top= RadialBottom= Median
Data Should be Consistent!
Average amplitude of MedianSensory response at the thumb is4 times the radial.
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Mild Carpal Tunnel Syndrome
Top= Radial stimMiddle= Median stimBottom= Midpoint stim (Bactrian Sign) ( )
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1 2
3
1
2
3
1
2
3 4
L MEDIAN - CTS Screen
Troubleshooting ExampleData Inconsistent
Median stim, Dig 1 rec
Radial stim, Dig 1 rec
“Midpoint” stim, Dig 1 rec
Median stim, Dig 3 rec
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1 2
3
1
2
3
1
2
3 4
Troubleshooting ExampleData Inconsistent
Median stim, Dig 1 rec
Radial stim, Dig 1 rec
“Midpoint” stim, Dig 1 rec
Median stim, Dig 3 rec
“Missing” nerve causes us to increase stimulation intensity until we see something (anything)!In this case, partial activation of distant nerve.
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Common Errors
Missing severe diseaseAbsent response is replaced by alternate response (e.g. ulnar motor rather than median motor response in CTS, or radial rather than median sensory response at thumb)
Review data for inconsistencies, assume that inconsistent data contains technical errors!
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Sural Sensory Nerve Conduction
Sweep=1 ms/div, gain=10 µV/div.
A:14cm 1
2
3
R SURAL - Lat Malleolus
Calf
10ms 10µV
1E1
Pathologic SNAP with duration >1.5 and < 3.0 ms. If duration is > 3 ms, then it is not a sensory potential.
Ground ElectrodeRecordingElectrodes
Stimulation Site
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Not Sure It’s a Potential?
Markers appear on this signal.But there are other waves, too.
Can you reproduce it?
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(Pseudo-accessory peroneal)
Peroneal Motor NCVWhat is inconsistent?
Is it accessory peroneal?Popliteal stim is largest (P)Too little stim at ankle (A) and fibula (F)
F
P
A
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Median Motor NCV
E1 off of Motor point (but has negative initiation)
E1-G2 short circuit
Correct
All stimulations at the wrist
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Median Motor NCVInconsistency
Stimulate at:1. Palm (P)2. Wrist (W)3. Elbow (E)
P W E
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Median Motor NCV
Stimulation at the wrist (W1)
Elbow stim (E)
Wrist stimulation at correct (lower) intensity (just supramaximal) does not also stimulate ulnar
Excessive !
W1
E
W2
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Distance Measurement Problems
1. Excessive stimulation changes the point of activation
depth affects accuracy (needle)
2. Tape measure errors
Skin contours
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Motor Nerve RecordingERRORS - Altering E1 position
Motor Point
1 cm lateral
1 cm distal
Median Motor Nerve Latency
1. Wrist MP2. Wrist 1 Lat3. Wrist 1 dist
-20%
-47%
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Progessive WeaknessPossible AIDP (GBS)
No response is seen after a high intensity stimulation;What do you do next?
100.0mA
2 ms
5 mV
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Demyelinating Neuropathy
Anticipate the possible pathology of severe slowing and small amplitude!Change:
Sweep= 5 ms/cm (slower) &Gain= 1 mV/cm (greater amplification)
100.0mA
5 ms
1 mV
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Median Motor Conduction
Median motor conduction velocity?
A2 Latency wrist stim
A3 Elbow stimSweep 5 ms/divGain 5 mV/div
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Monopolar vs. Concentric:Are They Different?
Of Course They Are!
When Does It Matter?
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Purpose of StudyAcquire qualitative and quantitative information regarding the electrical property of muscle
Requirements:
Accurate
Efficient
Minimal risk
Needle Electromyography
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Recording Volume in the Muscle
Concentric has a hemispheric space.Monopolar has a spherical space.Monopolar needle may be able to record from more fibers, although closest fibers have greatest influence.
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AdvantagesLower impedanceLarger recording volumeLower CostLess PainfulLocation in muscle does not affect resultsNon-directional
DisadvantagesGreater noiseSurface reference electrode requiredMore wear when re-used
Monopolar Needles
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Recording Surfaces
Monopolar needle records only from its intramuscular tipReference electrode is on the skin
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Recording Surfaces
Concentric needle has two intra-muscular recording surfaces.Electrical potentials are recorded with both of them, and may be inverted.When deep in the muscle, the outer shaft averages most signals and reduces them.
E2E1
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Resting Activity
Sherman, Walker, Donofrio ’90Fibrillation Potentials more frequently seen with concentricsGain settings: 50 µV for concentrics, 100 µV for monopolarsSuggested that the concentric needle causes more trauma, hence more fibs.
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Resting Activity
Kohara, Kimura ’93Fibrillations counted were ≥20 µVMonopolar
15.3 / 0.5 sConcentric
7.3 / 0.5 s
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MUAP Amplitude Recordingin Needle Electromyography
Max amplitude1830 (±670)1460 (±570)Kohara ‘93
Simultaneous recording of MUPs
557 (±466)340 (±181)Dumitru ’97
Max amplitude1038 (±369)912 (±315)Pease ’88
Sharp rise time585 (±315) μV436 (±269) μVNandedkar‘91
CommentTechnique
MonopolarConcentric
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MUAP Duration Recordingin Needle Electromyography
Similar results in all of the comparative studiesConsidered best reflection of local fiber density
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Monopolar NeedleAll routine studies!First cases for a new traineeNerve stimulation in deep location
Concentric NeedleElectrically noisy locations (e.g. ICU)Allowing residents the comparative experience
When Does It Matter?
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Sweep speed10 ms/cm(= 0.1 sec screen)
Gain (Amplifier)50 µV for insertion and rest200-500 µV for MUPs
RecruitmentSweep speed10-50 ms/cmGain
500-1,000 µV
FiltersL=20 Hz H=10 kHz
Needle EMG Set-up
N.B. “Most of the time”Adjust prn to see response clearly.
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InfectiousUniversal Precautions – GLOVES with needlesNeedles Disposed after use (do not recap, or if you must, then do “one-handed recapping”)
ElectricalCurrent Leak and Ground Inspection by Engr.Avoid Extension CordsOne ground on patient (Only!)Common ground for other equipment (all on same circuit)
Safety in EDX Medicine:
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AL-SHEKHLEE, et al., Muscle Nerve 2003 ; 27:517
Leakage currentA greater riskWhen two devicesAre in play. Avoid The current path!
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Paraspinal muscle hematoma
AAEM guidelines in electrodiagnostic medicine. Risks in elec-trodiagnostic medicine. Muscle Nerve 1999;22:S53 .S58.Caress JB,Rutkove SB,Carlin M,Khoshbin S,Preston DC. Paraspinal muscle hematoma after electromyography. Neurology 1997;47:269.
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Needle EMG20 Hz10,000 Hz
SFEMG500 Hz30,000 Hz
Motor NCV2 Hz10,000 Hz
Sensory NCV20 Hz2,000 Hz
Filter Recommendations
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Troubleshooting Noise
Fluorescent Light NoiseTurn off, use incandescentsReplace starters with new, solid-state designs
60 Hz Power NoiseUn-plug other devices at wallRe-arrange power cord locations
Check all plug connections at amplifier and devices
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Troubleshooting Noise
Next Steps-Differential DiagnosisProper electrode positions
Electrode impedanceReplace all surface and needle electrodes sequentially, clean and/or abrade skin
Bad wires (damage may be hidden)Wiggle or tug wires to test stabilityReplace all wires and connectorsAvoid long lead wires
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Troubleshooting Noise
Amplifier or computer malfunctionReboot system*, possible software malfunction/instability.
*ALWAYS unplug electrodes and patient connections before shutting down computer or unplugging equipment (can cause capacitor discharge)!
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Troubleshooting Noise-Other Steps
Make skin-to-skin contact with patient“Ground yourself”
Move instrument and amplifier locations and orientation
Wires and housings act as “antennas”
Have instrument inspected for electrical problems and current leaks
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Analog to Digital Signal Transfer(ref: John Cadwell, in Aminoff text 1998)
Analog signal original
Digital sampling at 32 Hx A to D
Analog signal “reconstructed”
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Differential Amplifier(ref: Rogoff and Reiner, in Licht 1961)
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Differential Amplifier(ref: Rogoff and Reiner, in Licht 1961)
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Differential Amplifier(ref: Rogoff and Reiner, in Licht 1961)
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References
Nandedkar, S. Instrumentation. In Johnson’sPractical Electromyography, WS Pease, H Lew, EW Johnson (Eds), 2007Kimura J. Electrodiagnosis in Diseases of Nerve and Muslce. 3rd Ed, 2001Dumitru D. Instrumentation. In Dumitru D (Ed), Electrodiagnostic Medicine, 2nd Ed.Al-Shekehlee A, et al. IATROGENIC COMPLICATIONS AND RISKS OF NERVE CONDUCTION STUDIES AND NEEDLE ELECTROMYOGRAPHY. Muscle Nerve 27:517-526,2003