CNIM: Guidelines 9A, 11A, 9D, 11B (SSEPs)

purpose of amplifier

input signals with peak-to-peak amplitudes of 5-50 uV to equal the full range of the analog-to-digital A-D converter

analog-to-digital (A-D) converter

takes physiological signals and transfers them to a numeral value

amplifier bandpass frequency

0.1-5,000 Hz

amplifier bandpass decibel

-3 dB points

common mode rejection decibel

80 dB (10,000:1)

differential input impedance of amplifier

100 megOhm

noise level of the amplifier must NOT exceed

2 uV rms with the inputs connected to neutral and with a bandpass of 0.1-5,000 Hz

time (horizontal) resolution of averager

20 us/data point or less

LEAST amount of averaging trials

4,000

what range of the A-D converter should the artifact reject % be set to

90-98%

adequate bits of amp resolution at the the A-D converter

8 bits = adequate

preferred bits of amp resolution at the the A-D converter

12

how is excluding artifact-contaminated trials most common achieved

rejecting those trials that exceed the limits of the A-D converter or some adjustable %

minimum # of channels for SSEPs

4

minimum # of channels for BAEPs

2

Gaussian convolution

takes out raw data and creates average

chassis leakage current

current that flows from the equipment to the ground; can flow to the patient if NOT properly grounded

chassis current is present anytime __

the equipment is plugged into an outlet

can you use extension cords/power strips in the OR

no b/c it can increase leakage current

is a direct connect from the pt "ground" to chassis or power line ground allowed to be present

NO

what should be incorporated in order to actively prevent current flow to the pt

isolation amplifier or solid-state current limiter

chassis leakage should be less than __ with ground open (clinical)

300 uA rms

max chassis leakage with ground open (intraoperative)

100 uA rms

max leakage current through patient leads should be __ to ground with 120 VAC applied (power ON)

10 uA rms

max leakage current should be ___ at the patient end of the table when the power line ground is disconnected (power OFF)

50 uA rms

RMS

root mean square

purpose of biomed check before cases

to check leakage current & other safety parameters of equipment

when should chassis leakage check be done

twice a year

biannually

semi-annually = BEST CHOICE

purpose of an isolation amplifier/solid-state current limiter

prevent current from flowing to the patient

isolation transformer

allows an AC signal to be taken from one device and fed into another WITHOUT connecting the two circuits; blocks interference caused by ground loops

grounding system ensure that no __ > than __ can be measured across an impedance of __ between combination of conductive surfaces or grounding conductors

voltage; 20 mV rms; 1000 Ohms

what to do if data is not reproducible

do another set of averages/trials until replication

replication

two or more temporally independent averages

how is replication demonstrated

by consistency of latency and amp measures of EP components recording in:

latency: 1% of total sweep time

amp: 15% of the peak-to-peak amp

purpose of replication

to demonstrate that clinical EP are repeatable and are of neural --> NOT artifactual

ALL Evoked Potential record should have all the following info:

1) pt name, ID number, age, gender

2) date of exam and procedure

3) tech name or initials

4) recording channels

IMPORTANT items to document

- store averaged waveforms

- time of surgical events & procedures

- alerts issued

- anesthetics and drugs, and significant changes in dose

- significant changes in physiological parameters - BP and temp

- maintain documentation along with stored waveforms

- final report filed in pt's chart

normative data

data obtained from a reference population

if normative data is going to used from another lab (9A), what must be done?

- ALL collection parameters must be replicated from the reference data (identical if using someone else's)

- MINIMUM of 20 patients must be run to make sure they fall within the normal distribution

is normative data relied on intraoperatively (11A)

NO

does the pt serve as their own control (11A)

yes

when must baseline values for IOM data be established

following induction of anesthesia but BEFORE any surgical intervention (incision)

may need to be re-established if changes in anesthetic medications or other physiological parameters occur during the case (check with IP first)

when should monitoring begin

before any surgical manipulation of the nervous system

when should monitoring be discontinued

when the surgical procedure is terminated

purpose of SSEPs intraoperaitvely

to assess function of somatosensory pathways during surgical procedures in which the SC, brainstem, or cerebrum are at risk and to localize the sensorimotor cortex

how are peripheral nerves stimulated

transcutaneously using electrodes placed on the skin over the selected nerve

should SSEPs be used continuously during cautery (bovie)

NO

what type of stimulation are SSEPs

cathodal

cathodal stimulation

- neg charged anions (-) flow from cathode --> tissue --> anode

placement of cathode

between anode and first recording site

cathode

neg charged

attracts anions

DEPOLARIZATION

proximal

anode

pos charge

attracts anions

HYPERPOLARIZATION

distal

what is the selection of nerve stimulated to obtain SSEP determined by?

segmental level of the surgical procedure

median nerve SSEP stimulation spinal cord surgery level is above __

C6 (C1-C5)

ulnar nerve SSEP stimulation spinal cord surgery level is above __

C8 (C6-C8)

PTN and PN SSEP stimulation spinal cord surgery level is below __

C8

upper extremity SSEP purpose for lumbar cases

detects peripheral nerve and brachial plexus for ischemia or compression

is UN or median nerve SSEP monitoring better for lumbar cases

ulnar nerve

where should the stimulation unit be isolated

from the main portion of the stimulation circuitry to avoid large current flow to the pt

stimulation type (9D/11B)

monophasic rectangular pulses using constant voltage or constant current (BEST)

is constant current or constant voltage recommended for intraoperative SSEPs

constant current

why should stim parameters be within safe limits

to prevent tissue damage

pulse width (9D/11B)

100-300 us or 0.1-0.3 ms

stimulation rate 9D and 11B

9D: 3-5 Hz

11B: MN = 2-8 Hz, PTN = 2-10 Hz

make sure NOTHING is divisible by 60 Hz

stimulation intensity 9D and 11B

9D: adequate to produce a muscle twitch or motor threshold (not tetany); equalizing peripheral potentials

11B: 30-40 mA; equalizing peripheral potentials

parameters that influence the degree of pain to the waking patient

1) duration of stimulus (100-300 us) --> 300 = MOST painful; 100 us = LEAST painful

2) current density/surface area of the stimulation electrode

3) intensity of stimulation --> decrease stim intensity to motor threshold or decrease # of averages

large stimulators (surface electrodes) are least/most painful?

least painful

larger surface area =?

smaller current density

small stimulators (needle electrodes) are least/most painful?

most painful

smaller surface =?

larger current density

should you decrease stim intensity BELOW motor threshold

NO NEVER

what to do if the patient is uncomfortable in a clinical setting

1) decrease the number of averages (reps)

2) use a larger size stimulator (less current density)

impedance of electrodes must be under __

5 kOhms to minimize discomfort

how many grounds should be placed to avoid a ground loop

1 ground

where is the ground placed

on the stimulated limb to reduce stim artifact; between the cathode of the stimulating electrode and the 1st recording electrode

where is the ground placed for the:

median nerve

PTN

peroneal nerve

MN = forearm

PTN = calf

PN = thigh

should electrodes be wrapped with tape around the limb

NO

stimulation type:

9D

11B

9D: unilateral limb

11B: interleaving (NEVER stimulate at the same time); bilateral asynchronous

bilateral synchronous stimulation

BOTH sides are being stimulated at the same time & could lead to a false negative

false negative

data unchanged but the patient wakes up with a deficit

anodal blocking DOES NOT cause

inverted waveforms

obligate peaks to appear earlier

response to travel distally to the muscles

MN:

cathode

anode

twitch

cathode = between the tends of the palmaris longus and flexor carpi radialis

anode = 2-3 cm distal to the cathode or on the dorsum of the wirst

twitch = abduction of the thumb (1st digit); thenar abductor pollicis brevis

UN:

cathode

anode

twitch

cathode = medial to the palmaris longus, 2 cm proximal to the wrist crease

anode = 2-3 cm distal to the cathode or on the dorsum of the wrist

twitch = flexion of the 4th & 5th digits; hypothenar abductor digiti minimi

PN:

cathode

anode

twitch

cathode = below fibular head

anode = 2-3 distal to the cathode

twitch = plantar eversion of the foot; activates peroneus longus & peroneus brevis

PTN:

cathode

anode

twitch

cathode = midway between the medial border of the achilles tendon and the posterior border or medial malleolus

anode = 2-3 cm distal to cathode

twitch = plantar flexion of the greater toe or cupping of the sole of the foot; activates adductor hallucis

anodal blocking

cathode & anode are switched

results in a lack of a response or reduced amplitude response

more pronounced in peripheral peaks

failure to stimulate is due to:

increase in impedance

salt bridge

limb edema

peripheral neuropathy

variant anatomy

pedal edema

thenar

APB

hypothenar

ADM

median nerve and ulnar nerve ground placement

on the forearm

alternate site for PTN for peripheral neuropathy & below the knee amputation

peroneal nerve

above the knee amputation alternative site

stimulate femoral nerve

PTN responses are less/more consistent & have less/more inter-subject variability?

more consistent; less inter-subject variability

PN responses are less/more consistent & have less/more inter-subject variability?

less consistent; more inter-subject variability

PTN and PN ground placement

stimulated limb

rep rate:

9D

11B

9D: 2-3 Hz for uppers and lowers

11B: uppers (2-3 Hz); lowers (2-10 Hz)

bandpass:

9D

11B

9D: 1-30-3,000 Hz (-6 dB/octave)

11B: 30-1,000 Hz (-3 dB)

analysis time:

9D

11B

9D:

MN = 40 ms

PTN = 60 ms

11B:

MN = 50 ms (40-50 ms)

PTN = 75-100 ms

number of averages:

9D

11B

9D: several hundred to several thousand

- low noise = 2 reps

- high noise = 2+ reps

11B: 250-1000 reps

number of channels (9D/11B)

MINIMUM of 4