2636.cvr:2636.cvr 8/22/2007 7:56 PM Page 1 Pocket Guide.pdf · 2 With progressive drug-induced sedation, BIS numbers decline, and BIS values should be interpreted with this continuum

2636.cvr:2636.cvr 8/22/2007 7:56 PM Page 1

MonitoringConsciousness

Using the Bispectral IndexTM

During Anesthesia

A Pocket Guide for Clinicians

SECOND EDITION

Scott D. Kelley, M. D.Medical Director

Aspect Medical Systems

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Learning Objectives

After reading this guide, the anesthesia clinician will be ableto:

• Describe the link between anesthetic effect, EEGsignals and the BIS Index

• Integrate BIS information during induction,maintenance and emergence

• Identify special situations which can influence BISmonitoring

• Formulate responses to sudden BIS changes occurringduring anesthesia

• Summarize the evidence-based impact of utilizing BISmonitoring during anesthesia care

• Recommend a role for BIS monitoring in a strategy toreduce the risk of awareness

• List resources and pathways to access additionalclinical support for BIS monitoring

This resource is intended for educational purposes only. It is notintended to provide comprehensive or patient-specific clinical practicerecommendations for BIS monitoring technology. The clinical choicesdiscussed in this text may or may not be consistent with your ownpatient requirements, your clinical practice approaches, or guidelinesfor practice that are endorsed by your institution or practice group. Itis the responsibility of each clinician to make his/her own determinationregarding clinical practice decisions that are in the best interest ofpatients. Readers are advised to review the current productinformation including the Indications for Use currently provided by themanufacturer. Neither the publisher, author, nor Aspect MedicalSystems, Inc. assumes any responsibility for any injury and or damage topersons or property resulting from information provided in this text.

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Table of Contents

Executive Overview & Key Points . . . . . . . . . . . . . . . . . . . . . . . . .1

The BIS Index – A Clinically-Validated Processed EEG Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

BIS Monitoring During Typical General Anesthesia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Special Issues Impacting BIS Monitoring . . . . . . . . . . . . . . . . . .18

Clinical Management: Responding to Sudden BIS Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

Clinical Impact of BIS Monitoring . . . . . . . . . . . . . . . . . . . . . . .24

Using BIS Monitoring to Reduce Intraoperative Awareness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

The Evolving Role of Brain Function Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

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1

Executive Overview and Key Points

Executive Overview & Key Points Bispectral Index (BIS) monitoring systems allow anesthesiaprofessionals the ability to access processed EEG information as ameasure of the effect of certain anesthetics during the care ofpatients they select to monitor. The clinical impact of BIS monitoringhas been demonstrated in a variety of randomized controlled trialsthat reveal the potential for BIS monitoring to facilitate improvements– including patient safety – in anesthesia care.

Because BIS monitoring may be new to some anesthesiaprofessionals, it is important to recognize the fundamentalelements of BIS technology and appreciate the linkages betweenthe BIS monitoring information and the clinical status of thepatient. Prior to using BIS monitoring information as an adjunctto guide anesthesia care, it is also important to review importantsituations and limitations that can influence the BIS number.

A more in-depth discussion of the following key points can befound in this guide:

• BIS Index: A Processed EEG Parameter with ClinicalValidation (See Page 5)– The BIS Index is the output from advanced EEG signal

analysis developed by Aspect Medical Systems. Duringsignal analysis, multiple characteristics of the EEG aredetermined. The BIS algorithm was developed to quantifythe changes in these EEG features that best correlate withdrug-induced changes in clinical state.

• BIS Clinical Range: A Continuum Concept (See Page 7)– The BIS Index is a dimensionless number scaled to

clinical endpoints as well as specific EEG features. Awake,unsedated individuals typically have BIS values >97.

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2

With progressive drug-induced sedation, BIS numbersdecline, and BIS values should be interpreted with thiscontinuum in mind. A BIS value of 60 has a highsensitivity for identifying drug-induced unconsciousness.However, in some settings and with some combinations ofsedatives and analgesics, unconscious individuals mayhave BIS values >60. BIS values <30 signify increasingamounts of EEG suppression. A BIS value of 0 representsan isoelectric EEG signal.

• Using BIS During General Anesthesia (See Page 11)– Administration of general anesthesia involves using

anesthetic medications to induce and maintainunconsciousness, and then reducing and/or discontinuingthe anesthetics to permit emergence and return ofconsciousness. Anesthesia professionals shouldappreciate that in the majority of clinical investigationsusing BIS to help guide anesthetic agent dosing, theprimary anesthetics were adjusted to maintain BIS valuesless than 60 during surgery.

– Consideration of BIS information may be useful in variousclinical situations that develop during anesthesia care.Similarly, clinicians should also be prepared to assess andrespond to unexpected changes in the BIS values. Ideally,BIS information should be integrated with other availablemonitoring information and patient assessment.

• Special Issues Impacting BIS Monitoring (See Page 18)– It is important to understand that several clinical

situations can influence the accuracy of the BIS value asan indicator of anesthetic hypnotic effect. Four key areasinclude: the influence of muscle tone (EMG) from theforehead muscles; electrical and mechanical artifacts


3

Executive Overview and Key Points

from medical devices; abnormal EEG states; and certainanesthetic agents and adjuvants – which can all lead toelevated BIS values. Serious clinical conditions – whichmay require prompt response – have been associated withthe sudden appearance of low BIS values.

• Clinical Impact of BIS Monitoring (See Page 24)– A substantial number of randomized controlled trials

demonstrate the impact of BIS-guided anesthesia care onpatient outcomes. Compared with standard clinicalpractice, adjusting primary anesthetic dosing to maintainBIS values within a target range (typically BIS values of 45 to 60 during maintenance) has, with certain anestheticagents, reduced anesthetic dosing, emergence andrecovery times. Use of BIS monitoring to help guideanesthetic administration may also be associated with thereduction of the incidence of awareness with recall inadults during general anesthesia and sedation.

• BIS Monitoring & Reducing Awareness (See Page 25)– Unintended intraoperative awareness may occur in

0.1 to 0.2% of adult patients undergoing generalanesthesia. Because of the potential for psychologicalinjury, numerous organizations are supporting efforts toreduce the incidence of awareness. The effectiveness ofBIS monitoring has been demonstrated in two prospectivetrials, and clinicians may wish to consider this evidence indeveloping patient-specific strategies to avoid awareness.

More recent information and additional clinical, educational, andtraining resources can be accessed at www.BISeducation.com. Ifyou require clinical information on the use of BIS, please contactAspect Medical at 800-442-8655 or [email protected].


4

Important Information About Using BISMonitoring

BIS monitoring systems are intended for use by healthcarepersonnel trained in their proper use. They are intendedfor use on adult and pediatric patients to monitor thestate of the brain by data acquisition of EEG signals.

The BIS may be used as an aid in monitoring theeffects of certain anesthetic agents; and its usagewith certain anesthetic agents may be associatedwith a reduction in primary anesthetic use and areduction in emergence and recovery time. Use of BIS monitoring to help guide anestheticadministration may be associated with thereduction of the incidence of awareness with recallin adults during general anesthesia and sedation.

BIS is a complex monitoring technology intended for useas an adjunct to clinical judgment and training. Clinicaljudgment should always be used when interpreting the BISin conjunction with other available clinical signs. Relianceon the BIS alone for intraoperative anestheticmanagement is not recommended. As with anymonitored parameter, artifacts and poor signal quality maylead to inappropriate BIS values. Potential artifacts may becaused by poor skin contact (high impedance), muscleactivity or rigidity, head and body motion, sustained eyemovements, improper sensor placement and unusual orexcessive electrical interference. BIS values should also be interpreted cautiously with certain anestheticcombinations, such as those relying primarily on eitherketamine or nitrous oxide/narcotics to produceunconsciousness. Due to limited clinical experience in thefollowing applications, BIS values should be interpretedcautiously in patients with known neurological disordersand those taking other psychoactive medications.


The BIS Index – A Clinically-Validated Processed EEG ParameterThe BIS Index is a processed EEG parameter with extensivevalidation and demonstrated clinical utility. It is derivedutilizing a composite of measures from EEG signal processingtechniques including bispectral analysis, power spectralanalysis, and time domain analysis. These measures werecombined via an algorithm to optimize the correlationbetween the EEG and the clinical effects of anesthesia, andquantified using the BIS Index range.

In 1996, the U.S. Food and Drug Administration cleared theBIS Index as an aid in monitoring the effects of certainanesthetic agents. In 2003, the Food and Drug Administrationcleared an additional indication which states: “Use of BISmonitoring to help guide anesthetic administration may beassociated with the reduction of the incidence of awarenesswith recall in adults during general anesthesia and sedation.”The use of BIS monitoring to guide anesthetic administrationand monitor patient status is a clinical decision. It is theresponsibility of each clinician to make clinical practicedecisions that are in the best interest of the patient.

Today, the BIS Index remains the most validated form ofconsciousness or brain function monitoring used within theclinical context of anesthesia and sedation care. BIS Indexvalues are the result of two particular innovations: bispectralanalysis and the BIS algorithm.

Bispectral analysis is a signal processing methodology thatassesses relationships among signal components and capturessynchronization within signals like the EEG. By quantifying

5

The BIS Index


6

Figure 1: The BIS algorithm, developed through statisticalmodeling, combines the contribution of each of the key EEGfeatures to generate the scaled BIS Index.

the correlation between all the frequencies within the signal,bispectral analysis yields an additional EEG facet of brainactivity.1

The BIS algorithm was developed to combine the EEGfeatures (bispectral and others) which were highly correlatedwith sedation/hypnosis in the EEGs from more than 5,000adult subjects. The four key EEG features that characterizedthe full spectrum of anesthetic-induced changes were thedegree of high frequency (14 to 30 Hz) activation, theamount of low frequency synchronization, the presence ofnearly suppressed periods within the EEG, and the presenceof fully suppressed (i.e. isoelectric, “flat line”) periods withinthe EEG.2 The algorithm enables the optimum combination ofthese EEG features to provide a reliable processed EEGparameter of anesthetic and sedative effect – the BIS Index(Figure 1).


7

The BIS Index

The BIS Index: A Continuum

The BIS Index is a number between 0 and 100 scaled tocorrelate with important clinical endpoints and EEG statesduring administration of anesthetic agents (Figure 2). BIS values near 100 represent an “awake” clinical statewhile 0 denotes the maximal EEG effect possible (i.e., anisoelectric EEG).

Figure 2: The BIS Index is scaled to correlate with importantclinical endpoints during administration of anesthetic agent.


8

It should be noted that the BIS index range represents acontinuum that correlates to the clinical state and expectedresponses (Figure 3).

100

80

60

40

20

0

Awake• Responds to normal voice

Light/Moderate Sedation• May respond to loud commands or mild prodding/shaking

General Anesthesia• Low probability of explicit recall• Unresponsive to verbal stimulus

Deep Hypnotic State

• Burst Suppression

Flat Line EEG

BIS

Ind

ex R

ange

This chart reflects a general association between clinical

state and BIS values. Ranges are based on results from a

multi-center study of the BIS involving the administration

of specific anesthetic agents.8 BIS values and ranges

assume that the EEG is free of artifacts that can affect its

performance. Titration of anesthetics to BIS ranges

should be dependent upon the individual goals

established for each patient. These goals and associated

BIS ranges may vary over time and in the context of

patient status and treatment plan.

Figure 3: BIS Index Range: A Continuum of Clinical State andEEG Changes


9

The BIS Index

As BIS values decrease below 70, memory function ismarkedly impaired and the probability of explicit recalldecreases dramatically. During sedation care, BIS values>70 may be observed during apparently adequate levels ofsedation. At these levels, however, there may be a greaterprobability of consciousness and potential for recall.3

In volunteer studies, a threshold value of BIS <60 has ahigh sensitivity to reflect unconsciousness. As notedpreviously, the specificity of this threshold value may bequite dependent upon the anesthetic technique utilized –particularly with the combination of opioid analgesics.Although a continuum of responses may occur around a BISvalue of 60, prospective clinical trials have demonstratedthat maintaining BIS values in the range of 45 to 60 ensuresadequate hypnotic effect during balanced general anesthesiawhile improving the recovery process.4 Similarly, in twolarge prospective trials, maintaining BIS values less than 60was the clinical strategy associated with reducing theincidence of intraoperative awareness.5, 6

BIS Index values lower than 40 signify a greater effect of theanesthetic on the EEG. At very low BIS values, the degree ofEEG suppression is the primary determinant of the BISvalue.7 A BIS value of 0 occurs with detection of anisoelectric EEG signal.

BIS responses are similar when most, but not all, anestheticagents are administered in increasing amounts. Specifically,BIS responses to typical hypnotic agents (midazolam,propofol, thiopental, isoflurane) were similar.8, 9 However,


10

halothane has been found to have higher BIS values at anequipotent minimum alveolar concentration dose.10 Further,BIS responses to ketamine administration are atypical.11 Inaddition, BIS responses to administration of analgesic agents– including opioid analgesics and nitrous oxide – dependon the level of concomitant stimulation.

BIS Index values may reflect the reduced cerebral metabolicrate produced by most hypnotics. A significant correlationbetween BIS Index values and reduction in whole brainmetabolic activity due to increasing anesthetic effect wasmeasured using positron emission tomography (Figure 4).12

Importantly, however, factors other than drug administrationthat can influence brain metabolism (e.g., alterations intemperature or physiologic homeostasis) may also producechanges in the BIS Index.

PET

% BMRBIS

100

95

64

66

54

62

38

34

Figure 4: Significant correlation is seen between decreasing brainmetabolic rate (% BMR = percent of initial whole-brain glucosemetabolism measured from PE T scan) and increasing anestheticeffect (as measured by decreasing BIS value). (Adapted fromReference 12)

Finally, it is important to note that the BIS value provides ameasurement of brain status derived from the EEG, not theconcentration of a particular drug. For example, BIS valuesdecrease during natural sleep as well as duringadministration of an anesthetic agent.13


11

BIS Monitoring During Typical GA

BIS Monitoring During TypicalGeneral AnesthesiaBIS monitoring provides potentially useful information duringeach of three phases of a “typical” general anesthetic case:

• Induction of anesthesia (and typically airwaymanagement)

• Maintenance of anesthesia• Emergence from anesthesia

BIS systems display the BIS Index value as a single value,calculated from data gathered over the last 15 to 30 secondsof EEG recording and updated every second. Deriving theBIS Index value from several seconds of EEG data effectively“smooths” the data to prevent excessive fluctuations in BISvalues. It also allows a value to be determined even if theEEG signal is briefly interrupted. Most BIS systems allow theuser to change the smoothing rate to be appropriate to theclinical environment.

A BIS value, while extremely responsive, is not instantaneouslyaltered by changes in clinical status. When abrupt changesoccur in hypnotic state – for example, during induction orrapid emergence – the BIS value may lag behind the observedclinical change by approximately 5 to 10 seconds.

Most BIS systems also display a graphical trend – the BIStrend (Table 1) – which represents the ongoing calculationsof the BIS Index during the case. Table 1 uses the BIS trendto present the information available from BIS monitoringduring each of the three phases of a general anesthetic case.


12

Tabl

e 1:

B

IS m

onit

orin

gdu

ring

a g

ener

alan

esth

etic

cas

e.

BIS

Dur

ing

Indu

ctio

n

•BIS

mon

itorin

g m

ay b

e us

eful

to g

auge

resp

onse

to in

trave

nous

indu

ctio

n do

se.14

•BIS

resp

onse

s ar

e se

nsiti

ve to

var

ious

adju

vant

s th

at in

fluen

ce in

trave

nous

indu

ctio

n of

ane

sthe

sia.

15, 1

6

BIS

Dur

ing

Mai

nten

ance

•In

resp

onse

to n

oxio

us s

timul

atio

n, B

ISre

spon

ses

may

be

obse

rved

eith

er in

pa

ralle

l with

or i

ndep

ende

nt fr

omhe

mod

ynam

ic re

spon

ses.

22, 2

3, 1

4

BIS

Dur

ing

Em

erge

nce

•BIS

mon

itorin

g pe

rmits

redu

ctio

n in

anes

thes

ia d

osin

g in

tand

em w

ith th

ede

crea

se in

sur

gica

l stim

ulat

ion,

pro

mot

ing

a ra

pid

emer

genc

e th

at a

void

s pr

emat

ure

reco

very

of c

onsc

ious

ness

as

wel

l as

dela

yed

emer

genc

e fro

m a

nest

hesi

a.


13

BIS Monitoring During Typical GA•D

urin

g in

hala

tion

indu

ctio

n, B

IS m

onito

ring

reve

als

inte

rpat

ient

var

iabi

lity

of o

nset

tim

e,as

wel

l as

the

effe

ct o

f oth

er m

edic

atio

ns o

rst

rate

gies

.17, 1

8, 1

9

•BIS

mon

itorin

g ca

n fa

cilit

ate

diffe

rent

stra

tegi

es fo

r int

ubat

ion

or p

lace

men

t of

airw

ay d

evic

es (e

.g.,

LMA)

.20

•BIS

resp

onse

s du

ring

intu

batio

n ar

e al

soim

porta

nt.

His

tory

of a

nd a

ntic

ipat

eddi

fficu

lt in

tuba

tion

are

risk

fact

ors

for

intra

oper

ative

awa

rene

ss.21

Prol

onge

din

tuba

tion

atte

mpt

s m

ay re

sult

in d

ecre

ased

hypn

otic

effe

ct fr

om th

e in

duct

ion

agen

tw

ithou

t obv

ious

som

atic

mov

emen

t.

•Bec

ause

of t

hese

con

side

ratio

ns, a

goo

dst

rate

gy is

to im

plem

ent B

IS m

onito

ring

alon

g w

ith o

ther

sta

ndar

d pa

tient

mon

itors

(EC

G, b

lood

pre

ssur

e, S

pO2,

cap

nogr

aphy

)pr

ior t

o in

duct

ion

in o

rder

to in

divi

dual

izepa

tient

car

e du

ring

both

indu

ctio

n an

dai

rway

man

agem

ent.

•Clin

ical

tria

ls d

emon

stra

te th

atad

just

men

t of a

nest

hetic

dos

ing

tom

aint

ain

BIS

valu

es w

ithin

a ta

rget

rang

e of

45

to 6

0 du

ring

mai

nten

ance

resu

lts in

impr

oved

per

iope

rativ

ere

cove

ry p

atte

rns

as c

ompa

red

with

stan

dard

ane

sthe

sia

care

.4, 2

5

•BIS

resp

onse

s to

stim

ulat

ion

may

be

mar

kedl

y at

tenu

ated

in a

dos

e-de

pend

ent f

ashi

on w

ith o

pioi

dad

min

istra

tion,

e.g

., fe

ntan

yl o

rre

mife

ntan

il.26

•BIS

var

iabi

lity

– th

e cy

clic

osc

illatio

n in

BIS

dur

ing

surg

ery

– m

ay b

e us

eful

to o

bser

ve.

Both

sho

rt-te

rm B

ISva

riabi

lity

and

BIS-

deriv

ed E

MG

act

ivity

have

bee

n us

eful

in a

sses

sing

the

adeq

uacy

of a

nalg

esia

in s

urgi

cal

patie

nts.

In v

olun

teer

s, o

pioi

d an

alge

siare

duce

d BI

S va

riabi

lity.27

, 28,

29,

30

•Abr

upt,

unex

pect

ed c

hang

es in

the

BIS

trend

war

rant

add

ition

al a

sses

smen

tan

d cl

inic

al c

orre

latio

n. (

See

Tabl

es 4

and

5.)

•BIS

tren

d w

ill re

flect

the

decr

easi

ng h

ypno

ticef

fect

whe

n an

esth

etic

age

nt d

elive

ry is

redu

ced

or s

topp

ed a

t the

end

of s

urge

ry.

•BIS

val

ues

durin

g em

erge

nce

are

varia

ble:

-M

ay in

crea

se g

radu

ally

in re

spon

se to

are

duct

ion

in a

nest

hetic

dos

e (e

.g.,

end-

tidal

age

nt c

once

ntra

tion)

.

-M

ay in

crea

se ra

pidl

y to

val

ues

>60

prio

rto

retu

rn o

f con

scio

usne

ss, p

artic

ular

ly if

EMG

tone

incr

ease

s su

bsta

ntia

lly.

-Ar

e ty

pica

lly lo

wer

imm

edia

tely

afte

rem

erge

nce

than

at b

asel

ine,

con

sist

ent

with

resi

dual

dru

g ef

fect

.

•With

ade

quat

e an

alge

sia,

a p

atie

nt m

ayre

mai

n un

cons

ciou

s an

d di

spla

y BI

S va

lues

<60

desp

ite lo

w c

once

ntra

tion

of h

ypno

ticag

ent u

ntil

addi

tiona

l stim

ulat

ion

is p

rovi

ded

(e.g

., or

opha

ryng

eal s

uctio

ning

, pos

ition

ing)

.

•Hig

h BI

S va

lues

in a

n un

resp

onsi

ve p

atie

ntco

uld

resu

lt fro

m E

MG

arti

fact

or f

rom

resi

dual

NM

B ef

fect

.


14

Integrating BIS Information DuringAnesthesia Care

The integration of BIS monitoring with other traditionalmonitoring has created new paradigms for intraoperativepatient assessment and management.31, 32, 33 Table 2 outlinesconceptual management strategies based on integration ofclinical profile with BIS data for “balanced” anesthesiatechniques utilizing hypnotic and analgesic components.Using the BIS value in combination with hemodynamic dataand patient assessment can facilitate the rational selection ofsedatives, analgesics and autonomic blockers.

Although a BIS value of 40 to 60 is a typical target duringthe maintenance phase, the BIS value target range needs tobe tailored to the anesthetic technique. For example, duringbalanced anesthesia including opioid administration toprovide adequate analgesia, a target range of 45 to 60 maybe very appropriate. However, for anesthesia managementwhich utilizes little or no opioid or analgesic supplementation,increased dosing of the hypnotic agent – typically, a volatileanesthetic – to produce acceptable suppression of a noxiousstimulation response (e.g., movement) will result in lowerBIS values, commonly in the 25 to 35 range.

Since there is no single anesthetic technique that isappropriate for every patient for every clinical situation,optimum use of BIS monitoring to guide anesthesia care will depend upon the clinical goals of the anesthesiaprofessional. Based on this consideration and agent-specificBIS responses (discussed in greater detail earlier), it is


15


important to keep in mind that there is no single BIS valueor range that can be recommended as appropriate for allpatients, conditions, and anesthetic techniques.

It is important to emphasize that reliance on BISmonitoring alone for intraoperative anestheticmanagement is not recommended. Clinicaljudgment is crucial when interpreting BIS data.Patient assessment should include evaluation andcorrelation of BIS data with hemodynamic and othermonitoring data as well as observation of clinicalsigns. The BIS value is an additional piece ofinformation to be incorporated with otherinformation available for patient assessment.


16

• Hypertension• Tachycardia• Movement• Autonomic responses

"Light"

• Stable hemodynamics

• No movement/responses"Adequate"

• Hemodynamic instability• Hypotension• Arrhythmia

"Deep"

Physical Signs Clinical Profile

Table 2: Anesthesia management strategies using the BIS Index.


17


* Potential impact of artifact should be considered when interpreting BIS values.

High value

• Assess level of surgical stimulation• Confirm delivery of hypnotics/analgesics• Consider ↑ hypnotic/ ↑ analgesic dosing • Consider antihypertensive administration

Desired range(e.g., BIS 45 to 60)

Low value

• Assess level of surgical stimulation• Consider ↑ analgesic dosing • Consider antihypertensive administration

• Consider antihypertensive administration• Assess level of surgical stimulation• Consider ↓ hypnotic/ ↑ analgesic dosing

• Assess level of surgical stimulation• Consider ↑ hypnotic dosing • Consider ↑ analgesic dosing

High value


Low value

• Continue observation

• Consider ↓ hypnotic dosing • Consider ↓ analgesic dosing

• Consider blood pressure support• Assess for other etiologies• Consider ↑ hypnotic/ ↑ analgesic dosing

High value


Low value

• Assess for other etiologies• Consider blood pressure support

• Consider ↓ hypnotic/ ↓ analgesic dosing• Consider blood pressure support• Assess for other etiologies

BIS Value* Management Strategy


18

Special Issues Impacting BISMonitoring Numerous prospective trials demonstrate that despite thepotential for artifact and other issues, reliable BIS values canbe obtained throughout many types of clinical cases.5, 34

However, in certain circumstances, BIS values may not be anaccurate reflection of the hypnotic state. As noted, BISmonitoring is an adjunct to clinical judgment, not asubstitute for it.

The clinician should be prepared to identify and respond tosituations where the underlying EEG signals – and hence theBIS value – may not accurately reflect the clinical endpointsof sedation and hypnosis. For example, BIS values >60 mayoccur as the result of external artifacts, certain pharmacologicagents, or other unrelated causes rather than reflectinginadequate anesthetic effect and the potential forintraoperative awareness. Similarly, BIS values <40 maydevelop as a consequence of serious clinical conditions, andnot merely from additional anesthetic effect. As mentioned,alterations in physiologic status which reduce brainmetabolism may result in decreased BIS values.

A recent review paper provides a comprehensive discussionof the spectrum of possible artifact and clinical conditionswhich may impact the displayed BIS value.35 It is importantfor clinicians to consider these conditions when evaluatingunusual BIS values or trend responses. These conditionsare augmented with clinical examples in Table 3.


19

Special Issues Impacting BIS Monitoring

EMG Artifact and Neuromuscular Blocking Agents (NMB)

• Excessive muscle tone from forehead muscles may increase BIS values (“EMG artifact”).

• NMB agents reduce EMGactivity and may result inBIS decrease.

• During stable anesthesiawithout EMG artifact, NMBagents have little or noeffect on BIS.

Medical Devices

Electromechanical artifactmay, under certain conditions,increase BIS values:• Pacemakers • Forced-air warmers applied

over the head• Surgical navigation systems

(sinus surgery)• Endoscopic shaver devices

(shoulder, sinus surgery)• Electrocautery

Serious Clinical Conditions

The following have beenassociated with low BIS valuesduring the intraoperativeperiod, presumably because ofmarked reduction in cerebralmetabolism:• Cardiac arrest,

hypovolemia, hypotension• Cerebral ischemia/

hypoperfusion• Hypoglycemia, hypothermia

0

20

40

60

80

100

9:00 9:03 9:07 9:11 9:15

BIS

CardiopulmonaryBypass

HypovolemicCardiac Arrest

BIS Trend

Time

Table 3: Reported factors influencing BIS.


20

Abnormal EEG States

May be associated with lowBIS values:• Postictal state, dementia,

cerebral palsy, low voltageEEG

• Severe brain injury, brain death

• Paradoxical arousal orparadoxical delta

May be associated withincreased BIS values: • Epileptiform EEG activity

Certain Anesthetic Agents and Adjuvants

• Ketamine – may transiently increase BIS values due toEEG activation

• Etomidate – drug-inducedmyoclonus may transientlyincrease BIS values

• Halothane – results inhigher BIS values thanisoflurane or sevoflurane atequipotent MAC doses

• Isoflurane – transientparadoxical response toincreased dose has beenreported

• Nitrous oxide – may have minimal effect on BIS• Ephedrine, but not phenylephrine, may increase BIS

0

20

40

60

80

100

10:20 10:35 10:50

Ketamine.5 mg/kg

BIS elevation presumedsecondary to ketamine

BIS

Time

BIS Trend

Table 3: Reported factors influencing BIS (Cont’d).


21

Clinical Management

Clinical Management: Respondingto Sudden BIS Changes When BIS monitoring is used during anesthesia care,fluctuations in BIS values will likely be noted. Suchvariability, like a single fluctuation in blood pressure, is notnecessarily clinically significant. However, specificconsideration should be given to sudden BIS changes orsituations where BIS seems inappropriately high or low.

For example, changes in the hypnotic state due to changesin dose and/or patterns of agent delivery will producechanges in the BIS value. Normally, if the change inanesthetic dosing was incremental – e.g., slight adjustmentin the vaporizer setting – subsequent changes in BIS valueswould be gradual. In contrast, a sudden dramatic changewould be unexpected and additional assessment would beappropriate.

Tables 4 and 5 present an assessment process for suddenincreases or decreases in the BIS value.


22

Table 4: BIS increase/high value assessment.

Responding to a Sudden BIS Increase

Examine for thepresence of artifacts(EMG,electrocautery orhigh frequencysignals)

High frequency artifacts including thoselisted may contaminate the EEG signal andbias the BIS toward a higher value.

Ensure thatanesthetic deliverysystems areoperating properlyso that the intended dose ofanesthetic agent isreaching the patient

Changes in vaporizer setting, fresh-gas flowrates, intravenous infusion pump setting,and intravenous delivery routes mayaccount for a sudden change in level ofanesthetic effect and the resulting BISvalue.

Ensure that theanesthetic dose is sufficient

An abrupt change in the BIS may reflect anew cortical state relative to anestheticdosing and changes in surgical conditions.

Assess the currentlevel of surgicalstimulation

The BIS may show a transient increase inresponse to increases in noxiousstimulation.


23

Clinical Management

Responding to a Sudden BIS Decrease

Assess forpharmacologicchanges

Bolus administration of intravenousanesthetic, recent changes in inhalationanesthesia, administration of adjuvant agents(beta blockers, alpha2 agonists) can all resultin acute decreases in the BIS.

Assess the currentlevel of surgicalstimulation

The BIS may show a decrease in responseto decreases in noxious stimulation.

Consider decreaseas possibleresponse toadministration ofmuscle relaxants

In some situations, the BIS will decrease inresponse to administration of neuro-muscular blocking agent, especially ifexcessive EMG was present prior to giving it.

Assess for otherpotentialphysiologic changes

Profound hypotension, hypothermia,hypoglycemia, or anoxia can producedecreases in the brain state activity.

Assess foremergence fromanesthesia

Paradoxical emergence patterns have beendescribed with transient abrupt decreases inthe BIS prior to awakening during inhalationanesthesia. The clinical significance of suchchanges remains unknown.

Table 5: BIS decrease/low value assessment.


24

Clinical Impact of BIS MonitoringThere is a large and growing body of scientific literature onBIS monitoring that can be reviewed by the practitioner toascertain usage of BIS for a patient based on the type ofanesthetic agent, dosage and individual patient parameters.

To date, at least 25 prospective, randomized clinicalinvestigations have measured the influence of BIS-guidedanesthesia care compared with standard practice. In most ofthese studies, the primary anesthetic was adjusted to maintainBIS values in a “target zone,” typically either 40 to 60 or 45 to 60.

The range of benefits that have been observed in at least oneclinical trial with certain anesthetic agents include:

• Reduction in primary anesthetic use4

• Reduction in emergence and recovery time4

• Improved patient satisfaction36

• Decreased incidence of intraoperative awareness and recall5, 6

The first two benefits above were shown in a study by Gan andco-workers that utilized propofol/alfentanil/nitrous oxide andfound that 1) 13 to 23% less hypnotic drug was used; 2) 35 to40% faster wake up was obtained; 3) 16% faster eligibility forPACU discharge was achieved; and 4) more patients wererated as “excellent-fully oriented” on admission to the PACU(43% vs 23%).4

As noted, BIS monitoring is also being recognized as aneffective intervention to decrease the incidence ofintraoperative awareness – an issue that has taken on newimportance in the last several years.5, 6 Using BIS monitoringto reduce intraoperative awareness is discussed in detail in thefollowing section.


25

Using BIS Monitoring to Reduce Intraoperative Awareness

25

Using BIS Monitoring To ReduceIntraoperative AwarenessDespite best intentions, a small percentage of patientsundergoing general anesthesia regain consciousnessunexpectedly and are able to form sufficient memory torecall portions of their intraoperative experience. Thissection discusses the role that BIS monitoring can play indecreasing the incidence of this adverse event.

Intraoperative Awareness During Anesthesia

In several large-scale prospective investigations, the incidenceof intraoperative awareness has been measured to occurduring general anesthesia in 0.1 to 0.2% of patients.37, 38, 39

In 2004, the Joint Commission’s Sentinel Event Alert #32noted that each year, 20,000 to 40,000 patients may becomecognizant and have recall of events during surgery.40

An overview of various perioperative factors that put patients atincreased risk for awareness is presented in Table 6. Presenceof some of these risk factors has been reported to increase therelative risk for awareness to nearly 1% of patients.


26

Patient andAnestheticHistory

Previous episode of awarenessSubstance use or abuseChronic pain patients on high doses ofopioidsHistory of or anticipated difficultintubationASA physical status 4 to 5Limited hemodynamic reserve

SurgicalProcedures

Cardiac, trauma, emergency surgeryCesarean section

AnestheticManagement

Planned use of:• Muscle relaxants during maintenance

phase• Total intravenous anesthesia• Nitrous oxide–opioid anesthesia

Reduced anesthetic doses during paralysis

Table 6: Potential risk factors for awareness: an overview.


27


The presumed cause of intraoperative awareness is a periodof inadequate anesthetic effect resulting from an insufficientanesthetic dose, disruption of anesthetic delivery, orpotentially inherent anesthetic resistance.41 For example, insome clinical situations, administration of very lowanesthetic doses may be appropriate in light ofhemodynamic compromise or other clinical goals. Thesedoses, however, are associated with a higher frequency ofintraoperative awareness.

Patient reports include frightening descriptions ofintraoperative awareness, highlighting the potentialhorrendous sensations and emotions that may occur ifanesthetic effect is inadequate.42 Patients who experienceintraoperative awareness may develop a spectrum ofpsychological injury ranging from mild, transient symptoms to severe, disabling symptoms consistent withpost-traumatic stress disorder.43


28

ASA Practice Advisory

The “Practice Advisory for Intraoperative Awareness andBrain Function Monitoring” published in 2006 describesusing multiple monitoring modalities – clinical techniques,conventional monitoring and brain function monitoring – toassess anesthetic depth and reduce the likelihood ofintraoperative awareness.21 The Practice Advisory consensusopinion was that “the decision to use a brain functionmonitor should be made on a case-by-case basis by theindividual practioner for selected patients.” It should benoted that the Practice Advisory also stated that brainmonitoring is not routinely indicated for all patientsundergoing general anesthesia, and that brain functionmonitoring currently has the same status as the many othermonitoring modalities used in selected situationsdetermined by individual clinicians.

The ASA Practice Advisory aims to help the anesthesiaprofessional develop a clinical strategy to minimize theoccurrence of awareness. Such a strategy involves elementsof care occurring throughout the perioperative period –preoperative assessment and preparation, intraoperativemonitoring and intervention, and postoperative follow-upactivities. An overview of the resulting clinical strategy ispresented in Table 7. Alternatively, clinicians may wish toimplement the algorithm approach presented in Figure 5 tominimize the risk for awareness.


29


Table 7: Clinical strategy to minimize awareness: an overview.

PreoperativePeriod

Assess risk:• Patient• Procedure• Anesthetic techniqueProvide informed consent in high-risk situations

IntraoperativePeriod

Consider premedication to provide amnesia

Use multiple modalities to assess anesthetic depth• Clinical signs

- Masked with use of muscle relaxant• Conventional monitoring

- BP, HR, end-tidal agent• Brain function monitoring (e.g., BIS)Consider amnestics for unintended consciousness

PostoperativePeriod

Assess patient reports of awareness

Provide patient with appropriate follow-up care

Report occurrence for quality assurance purposes

The Practice Advisory also alerted anesthesia clinicians torecognize that dosing anesthetic agents to achieve certainbrain function values in an attempt to prevent intraoperativeawareness may conflict with other medical concernsincluding vital organ function and existing co-morbidity.21

Similarly, the Joint Commission’s Sentinel Event Alert #32noted that anesthesia professionals must weigh thepsychological risks of anesthesia awareness against thephysiological risks of excessive anesthesia.40


30

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BIS Monitoring and Awareness: Evidence

To date, the addition of BIS monitoring is the only monitoringintervention shown by scientific evidence to reduceintraoperative awareness. Two large, prospective trials haveboth found an approximate 80% reduction in the incidence ofawareness when using BIS monitoring in addition to otherroutine monitors.5, 6 In addition to these two large trials,other reports of BIS monitoring and intraoperative awarenesshave appeared in the literature. These include a smallrandomized controlled trial,44 a large cohort observationalstudy,37 as well as several case reports.45, 46, 47, 48

The two large prospective trials provide a clinical managementframework for effectively using BIS monitoring. In oneinvestigation, the anesthesia staff was instructed to maintain BISvalues within a range of 40 to 60, and to avoid values greaterthan 60 during induction and maintenance.5 This managementresulted in significant benefit: only two patients in the 4,945treated patients reported awareness, representing a 77%reduction compared to the investigators' prior study (Figure 6).

32

Figure 6: BIS monitoring reduced awareness by 77% in routinepatients undergoing relaxant general anesthesia.5

0.2%

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0.0%Historical Control BIS

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14/ 7,826

Inci

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Aw

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33


In the other randomized trial involving patients at increasedrisk for awareness, BIS monitoring was initiated prior toinduction, and the delivery of anesthetics was titrated tomaintain BIS values between 40 to 60 from laryngoscopyuntil wound closure.6 This care resulted in an 82%reduction in the incidence of awareness (Figure 7).

Figure 7: BIS monitoring reduced awareness by 82% inpatients at increased risk for awareness.6

Inci

denc

e of

Aw

aren

ess 1.0%

0.8%

0.6%

0.4%

0.2%

0.0%Control BIS

2/ 1,225

11/ 1,238

In each study, two episodes of awareness were reported inpatients despite the use of BIS monitoring. All four cases ofawareness occurred during periods of significant noxiousstimulation (e.g., intubation, median sternotomy) and wereassociated with BIS values near or greater than 60. Thesecases highlight the need for the anesthesia professional to beparticularly vigilant to BIS responses to noxious stimulationand to be prepared to intervene promptly when BIS valuesexceed 60 for some time.


34

Intraoperative Awareness & AnesthesiaPractice

In addition to the ASA, professional societies from aroundthe world have addressed the specific topic of brainmonitoring to prevent awareness. The American Associationof Nurse Anesthetists,49 the Royal College of Anaesthetists andthe Association of Anaesthetists of Great Britain andIreland,50 and the Australian and New Zealand College ofAnesthetists51 emphasize that brain function monitors beconsidered for use and/or available in clinical situations thatplace a patient at increased risk for awareness. Forexample, the AANA's 2006 Position Statement advised that“brain function monitoring, if available, should beconsidered particularly in situations where the risk ofintraoperative awareness is increased.”49 These statementsaugment the opinions of ASA members: 69% of ASA-members surveyed in the Practice Advisory agreed orstrongly agreed with the statement: “Brain function monitorsare valuable and should be used to reduce the risk ofintraoperative awareness for patients with conditions thatmay place them at risk for intraoperative awareness.”

It should be appreciated that other patients, withoutrecognized risk factors, will experience awareness due tounanticipated or unrecognized intraoperative events. It isimportant to remember the clinical evidence demonstratingthe efficacy of BIS monitoring in this situation as well.5


35

The Evolving Role of Brain Function Monitoring

The Evolving Role of BrainFunction MonitoringDespite remarkable improvements in the assessment of thecardiovascular and respiratory systems during anesthesia,determination of the effect of anesthetic agents on the centralnervous system had remained a challenge. Now, technologiesthat permit routine neurophysiologic monitoring of thecentral nervous system provide a direct measure of anestheticeffect during anesthesia.52 Combining brain functionmonitoring with traditional monitoring and assessment ofclinical signs, can provide the anesthesia professional a morecomplete approach to optimizing the selection and/or dosingof anesthetic and adjuvant agents for each patient.

Concerns regarding the consequences of both inadequateand excessive anesthetic effect have increased in the last fewyears. As noted previously, inadequate anesthetic effect isthe primary etiology of unintentional intraoperativeawareness.41 This adverse event was discussed in detail inthe previous section.

Excessive anesthetic effect also has consequences. In somesituations, excessive anesthetic effect may result incardiovascular depression, and very rarely, cardiac arrest.53

More recently, new concerns about other consequences ofexcessive anesthetic effect have appeared. Exposure to highdoses of volatile anesthetic is a risk for acute transientepileptiform changes in the EEG.54 In addition, excessiveanesthetic effect has been associated with adverse long-termoutcome.55, 56


The ability of brain function monitoring to allow theanesthesia professional to monitor patient-specificanesthetic effect is important. Avoidance of excessiveanesthetic effect reduces the occurrence of prolongedrecovery and delayed orientation.4, 36, 57

As future investigations and clinical experience establish thepotential short-term and long-term risks of excessiveanesthetic effect, it may become important for anesthesiaprofessionals to better modulate patient exposure toanesthesia. Given the increasing recognition ofconsequences of excessive – as well as inadequate –anesthetic effect, it is likely that more anesthesia clinicianswill integrate brain function monitoring into overallanesthesia management.

36


SummaryThis pocket guide discussed how BIS brain functionmonitoring can be used most effectively during the differentphases of anesthesia care. It is important for anesthesiaprofessionals to fully appreciate the applications, limitationsand special considerations for use of BIS monitoring. During the past decade, BIS monitoring has been utilized inthe care of more than 18 million patients with a welldocumented safety and efficacy record. As a result, BISmonitoring is well established as a useful device within theanesthesia professional’s realm. Evidence in the literature documents patient benefits in thearea of safety and in the quality of anesthesia care resultingfrom the use of BIS monitoring. These clinical investigationsprovide an evidence-based rationale for incorporation of BISmonitoring as a tool to facilitate intraoperative managementwith certain anesthetic agents. Depending upon the specific patient characteristics, surgicalprocedure and planned anesthetic technique, utilization ofBIS monitoring may be a very appropriate decision. However,the decision to use BIS monitoring should be made on a case-by-case basis by the individual practitioner. As clinical experience and investigation continue, anesthesiaclinicians are encouraged to stay current with availableliterature regarding the use, benefits, and limitations of BISmonitoring to guide patient care. Additional clinicalinformation and other educational resources can be accessedat www.BISeducation.com. Clinical suport is also available viatelephone (USA Toll Free: 800.442.8655; Outside USA:+1.617.559.7655) and email ([email protected]).

37

Summary


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4. Gan T, et al. Anesthesiology. 1997;87:808-15.

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20. Kodaka M, et al. Br J Anaesth. 2004;92:242-5.

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29. Mathews D, et al. Anesth Analg. 2003;96:1062-4.

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32. Mavoungou P, et al. Ann Fr Anesth et de Reanim. 2000;19:582-7.

33. Stanski D, et al. Measuring Depth of Anesthesia. In Miller's Anesthesia, 6thEdition. Edited by Miller RD. New York, Elsevier/Churchill Livingstone, 2005,1227-64.

34. Liu S. Anesthesiology. 2004;101:311-5.

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36. Luginbuhl M, et al. Acta Anaesthesiol Scand. 2003;47:165-73.

37. Sebel P, et al. Anesth Analg. 2004;99:833-9.

38. Sandin R, et al. Lancet. 2000;355:707-11.

39. Myles P, et al. Br J Anaesth. 2000;84:6-10.

40. Preventing, and managing the impact of, anesthesia awareness. SentinelEvent Alert. 2004;32:1-3. (Accessed July 20, 2007, athttp://www.jointcommission.org/SentinelEvent)

41. Ghoneim MM. Anesthesiology. 2000;92:597-602.

42. Rowan KJ. Anaesth Intensive Care. 2002;30:505-6.

43. Lennmarken C, et al. Acta Anaesthesiol Scand. 2002;46:229-31.

44. Puri G, et al. Eur J Anaesthesiol. 2003;20:451-6.


45. Mychaskiw G, et al. Anesth Analg. 2001;92:808-9.

46. Luginbuhl M, et al. Anesthesiology. 2002;96:241-3.

47. Mathews D, et al. Br J Anaesth. 2005;95:193-6.

48. Rampersad S, et al. Anesth Analg. 2005;100:1363-4.

49. Position Statement 2.12: Unintended Awareness Under General Anesthesia.American Association of Nurse Anesthetists, 2006. (Accessed July 20, 2007,at http://www.aana.com/resources.aspx?ucNavMenu_TSMenuTargetID=51&ucNavMenu_TSMenuTargetType=4&ucNavMenu_TSMenuID=6&id=1747)

50. Loss of Consciousness Monitoring: A Joint Statement by the Royal College ofAnaesthetists and the Association of Anaesthetists of Great Britain andIreland. The Association of Anaesthetists of Great Britain and Ireland, 2006.(Accessed July 20, 2007, athttp://www.aagbi.org/release_lossofconsciousness.html)

51. Recommendations on Monitoring during Anaesthesia. Australian and NewZealand College of Anaesthetists, 2006. (Accessed July 20, 2007, athttp://www.medeserv.com.au/anzca/pdfdocs/PS18-2006.pdf)

52. Tonner PH, et al. Best Pract Res Clin Anaesthesiol. 2006;20:1-9.

53. Morray JP, et al. Anesthesiology. 2000;93:6-14.

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57. Recart A, et al. Anesth Analg. 2003;97:1667-74.

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41

Summary


Documents

2636.cvr:2636.cvr 8/22/2007 7:56 PM Page 1 Pocket Guide.pdf · 2 With progressive drug-induced sedation, BIS numbers decline, and BIS values should be interpreted with this continuum