Visual P2–N2 Complex and Arousal at the Time of Encoding Predict the Time Domain

14 Pages·2010·2.48 MB·English

PERIOPERATIVE MEDICINE
Anesthesiology2010;113:000–000
Copyright©2010,theAmericanSocietyofAnesthesiologists,Inc.LippincottWilliams&Wilkins
Visual P2–N2 Complex and Arousal at the Time of
Encoding Predict the Time Domain Characteristics of
Amnesia for Multiple Intravenous Anesthetic Drugs in
Humans
Kane O. Pryor, M.D.,* Ruth A. Reinsel, Ph.D.,† Meghana Mehta, M.S.,‡ Yuelin Li, Ph.D.,§
John T. Wixted, Ph.D.,(cid:1) Robert A. Veselis, M.D.#
ABSTRACT domainusingatwo-parameter,first-ordernegativepowerfunc-
Background: Intravenous anesthetics have marked effects tion. The parietal event-related P2–N2 complex was derived
on memory function, even at subclinical concentrations. fromelectroencephalography,andarousalwasrepeatedlysam-
Fundamental questions remain in characterizing anesthetic pled.Eachdrugwasevaluatedattwoconcentrations.
amnesiaandidentifyingaffectedsystem-levelprocesses.The Results:Thenegativepowerfunctionconsistentlydescribed
authorsappliedamathematicalmodeltoevaluatetime-do- the course of amnesia (mean R2 (cid:1) 0.854), but there were
maincomponentsofanestheticamnesiainhumansubjects. markeddifferencesbetweendrugsinthemodulationofindivid-
Methods:Sixty-onevolunteerswererandomizedtoreceive ualcomponents(P(cid:2)0.0001).Initialmemorystrengthwasa
propofol (n (cid:1) 12), thiopental (n (cid:1) 13), midazolam (n (cid:1) functionofarousal(P(cid:1)0.005),whereassubsequentdecaywas
12),dexmedetomidine(n(cid:1)12),orplacebo(n(cid:1)12).With relatedtothereactiontime(P(cid:2)0.0001)andtheP2–N2com-
drugpresent,subjectsencodedpicturesintomemoryusinga plex(P(cid:1)0.007/0.002fordiscretecomponents).
375-itemcontinuousrecognitiontask,withsubsequentrec- Conclusions:Inhumans,theamnesiacausedbymultiplein-
ognitionlaterprobedwithdrugabsent.Memoryfunctionwas travenous anesthetic drugs is characterized by arousal-related
sampledatupto163timepointsandmodeledoverthetime effects on initial trace strength, and a subsequent decay pre-
dictedbyattenuationoftheP2–N2complexatencoding.The
authorsproposethatthefailureofnormalmemoryconsolida-
*Assistant Professor, Department of Anesthesiology, Weill Cor- tion follows drug-induced disruption of interregional syn-
nellMedicalCollege,NewYork,NewYork,andAssistantProfessor,
chronycriticalforneuronalplasticityanddiscusstheirfindings
Department of Anesthesiology and Critical Care, Memorial Sloan–
KetteringCancerCenter,NewYork,NewYork.†AssistantAttend- intheframeworkofmemorysystemstheory.
ingPsychologist,‡Programmer-Analyst,DepartmentofAnesthesi-
ologyandCriticalCare,§AssistantAttendingBehavioralScientist,
Department of Epidemiology and Biostatistics, Memorial Sloan– WhatWeAlreadyKnowaboutThisTopic
Kettering Cancer Center. (cid:1) Professor and Chair, Department of
Psychology,UniversityofCaliforniaatSanDiego,LaJolla,Califor- ❖ Long-term memory depends on the strength of the original
nia. # Professor, Department of Anesthesiology and Critical Care memorytraceandonthenaturaldecayofthattraceovertime
Medicine,MemorialSloan–KetteringCancerCenter,andAssociate ❖ The effects of anesthetics on these processes are not
Professor of Clinical Anesthesiology, Department of Anesthesiol- established
ogy,WeillCornellMedicalCollege.
ReceivedfromtheDepartmentofAnesthesiology,WeillCornell WhatThisArticleTellsUsThatIsNew
Medical College, New York, New York, and the Department of
AnesthesiologyandCriticalCare,MemorialSloan–KetteringCancer ❖ In61volunteers,memorytracestrengthwasrelatedtostateof
Center, New York, New York. Submitted for publication October arousalanddecayofthattracetoreactiontimeandtheP2–N2
14,2009.AcceptedforpublicationMarch17,2010.Supportedbya complexoftheposteriorparietalevent-relatedpotential
ResearchStarterGrantfromtheFoundationforAnesthesiaEduca- ❖ Dexmedetomidine affected memory primarily by affecting
tion and Research, Rochester, Minnesota (to Dr. Pryor); Award memory trace strength, whereas propofol and midazolam
K08GM083213 from the National Institutes of Health, Bethesda, markedaffecteddecayofthattrace
Maryland(toDr.Pryor);andinternalfundingfromtheDepartment
ofAnesthesiology,WeillCornellMedicalCollege,andtheDepart-
ment of Anesthesia and Critical Care, Memorial Sloan-Kettering INTRAVENOUS anesthetic drugs are able to cause an-
CancerCenter.
terograde amnesia at brain concentrations well below
AddresscorrespondencetoDr.Pryor:DepartmentofAnesthesi-
those required to induce unconsciousness.1,2 This implies
ology, 525 East 68th Street, M-329, New York, New York 10065.
[email protected] thatcertainneuralprocessescriticaltonormalmemoryfunc-
be found at www.anesthesiology.org or on the masthead page at tionaremoresusceptibletotheseagentsthanarethosenec-
thebeginningofthisissue.ANESTHESIOLOGY’sarticlesaremadefreely
essaryforconsciousness.Invitrostudieshaveidentifiedsev-
accessibletoallreaders,forpersonaluseonly,6monthsfromthe
coverdateoftheissue. eraldrugactionsthatcouldplausiblybeinvolved,including
Anesthesiology,V113(cid:127)No2(cid:127)August2010 1
PERIOPERATIVE MEDICINE
theinhibitionoflong-termpotentiation(LTP),3–7interfer- way represents a direct measure of (cid:3)synchrony, it is an
encewithextracellularsignal-regulatedproteinkinasefunc- informedstartingpointforinvestigatingtherelationship
tion,8,9 modulation of hippocampal protein expression,10 between a drug’s amnestic potential and the system-level
and suppression of arousal systems.11,12 However, funda- effectsonconnectivity.
mentalquestionsremainastowhatsignificanceanyofthese We studied drugs from three distinct classes of (cid:4)-ami-
effects has on human memory in vivo, and the degree to nobutyric acid (GABA)ergic agonist: thiopental (barbitu-
which this diverse group of drugs converges on a common rate), propofol (alkylphenol), and midazolam (benzodiaz-
amnesticpathway. epine). Although these drugs share the GABA subtype-A
Oneapproachtoevaluatememoryfunctioninhumansis receptorasaprincipaltarget,humanneuroimagingandelec-
to mathematically model what happens to memories over trophysiology studies have revealed marked differences be-
time.Innormalmemoryfunction,atraceisestablishedrap- tween them in regional activation patterns during memory
idly and then progressively decays. Previous studies show tasks34–36 and also in amnestic potency when the sedative
that this temporal course can be described by a negative effectofthedrugisheldconstant.2Wealsostudieddexme-
power function,13–16 approximated under typical assump- detomidine,whichbindsto(cid:5) -adrenoreceptorsinthelocus
2A
tionsbythesimpleequation coeruleusanddecreasesactivityinawidespreadnetworkof
cortical and subcortical noradrenergic pathways associated
m (cid:1)(cid:1)t(cid:3)(cid:2),
t withtonicarousalandvigilance,37,38butitisnotknown
where m is the memory strength at time t, (cid:1)describes the tohaveaspecificamnesticeffectunrelatedtosedation.All
strength of the initial memory trace, and (cid:2)describes the four active drugs were dosed to target a common level of
subsequentrateofdecayofthattrace(appendix).Itfollows end-amnesia (i.e., all subjects receiving an active drug
thatanyinterventionthataltersmemoryfunction,suchasan would ultimately forget the same amount of material).
amnesticanestheticdrug,willmodulatetheseparameters. Twoamnesticlevelswereassessedbyhavingeachsubject
Ourstudyiscenteredontheprinciplethatthepatternof perform the experimental procedure at two sequential
mathematical modulation is a function of the underlying steady-stateconcentrations.
mechanism(s)ofamnesia.MuchasFouriertransformationis
used to separate a complex waveform into component fre- Materials and Methods
quencies,thisprincipleenablesnonlinearmemorydecayto
be quantitatively dissociated into simpler subcomponents. Subjects
Therefore, our first objective was to experimentally deter- Sixty-sevenhealthy,right-handedvolunteerswererecruited
mine and compare how intravenous anesthetics modulate fromthegeneralcommunityandremuneratedfortheirpar-
themathematicalformofmemorydecay.Thenullhypoth- ticipation.Subjectsprovidedwrittenconsent,andthestudy
esiswasthatthereisnodifferencebetweendrugsinmodu- was approved and monitored by the Institutional Review
lationof(cid:1)and(cid:2)atequivalentabsolutelevelsofmemoryloss. Board of Memorial Sloan–Kettering Cancer Center (New
Oursecondobjectivewastodeterminewhethercomponents York, New York). Subjects were fluent in English, had a
ofthemathematicalmodelcouldbelinkedtoneurophysio- minimumofahighschooleducation,hadnocurrentmedi-
logicorneurobehavioraldrugeffects.Specifically,weinves- calorpsychiatriccomorbidity,andhadnohistoryofrecre-
tigatedthreevariablesthatallrequirethefunctionalintegra- ationaldrugabuse,headinjury,orpsychiatricdisorder.Sub-
tionandconnectivityofdistributedneuralsystems:(1)the jects were excluded if they had relevant allergies, a family
leveloftonicarousal,whichisknowntomodulateselective history of porphyria, or a body mass index exceeding 30
and sustained attention and attentional effort; (2) reaction kg/m2.Pregnantpatientswereexcludedthroughassessment
time,whichisassociatedwithinterregionalphasecoherence of plasma (cid:6)-human chorionic gonadotropin. Several days
at multiple oscillatory frequencies17,18; and (3) the P2 and before the study, subjects attended an orientation session,
N2componentsoftheposteriorparietalevent-relatedpoten- during which they practiced abbreviated versions of the
tial (ERP). Although several early components of the ERP memoryexperiments.
arebelievedtoemergefromaresetofoscillatoryactivitybya
visualevent,theP2–N2complexwaschosenbecauseofits Drugs and Drug Delivery
associationwith(cid:3)(3–8Hz)synchrony,19–22withtheadvan- Subjects were randomly allocated using computerized ran-
tageofbeingfarmorereliablyobtainedthandirectmeasures dom number generation (Excel software, Microsoft Corp.,
ofphasecoherence.Theimportanceof(cid:3)phaseappearsre- Redmond, WA) to receive propofol (n (cid:1) 13), thiopental
peatedly in the memory literature and has been linked to (n(cid:1)14),midazolam(n(cid:1)13),dexmedetomidine(n(cid:1)15),
successfulmemoryfunction,23–26memorytracedecay,27and or a placebo (n (cid:1) 12) mimicking one of the active drugs.
memory-relatedhippocampo-corticalfeedbackloops.28–31Itis Bothsubjectandinvestigatorwereblindedtothedrugalloca-
criticaltotheinductionofLTP,32aneuroplasticprocesswidely tion.Todeterminethetargetdrugconcentrations,datafrom
regarded as a key underlying mechanism for memory previousstudies1,2wereusedtoprovideestimatesforthecon-
function and is also modulated by halogenated volatile centrationsrequiredtobracketa50%retentionrateforvisual
anestheticsinvivo.33AlthoughtheP2–N2complexinno stimuliafter4h(withoutcorrectionforfalsealarms):thelow
2 Anesthesiology,V113(cid:127)No2(cid:127)August2010 Pryoretal.
P2–N2 and Arousal Predict Anesthetic Amnesia
Fig.1.Studyprofile.Themastertimelinefortheexperimentisshownatthebaseofthefigure,withaprofileofthespecifictasks
expandedintheupperportion.ThecentralelementsareasequenceofEncodingTasks(purplebars)performedwiththedrug
equilibratedatthetargetconcentration,followedbyaseriesofRecognitionTasks(bluebars)performedafterthedruginfusion
wasstopped.ItemsappearedtwiceduringtheEncodingTasksequenceandweredefinedtohavebeenencodediftheywere
correctlyrecognizedatthesecondpresentation.MemoryfortheitemwassubsequentlytestedwithinoneoftheRecognition
Tasks.Sequencingwithinthe12EncodingTask–RecognitionTaskpermutationspermittedtheprobingofmemoryperformance
at160timeintervals.EEG(cid:1)electroencephalography.
druglevelwastheestimatefor60%retention,andthehighdrug ages were assigned to one of three sets: 160 were used in
levelwastheestimatefor40%retention. Encoding Set 1, 160 in Encoding Set 2, and 320 used as
Drugsweredeliveredviaacatheterinsertedintoaveinonthe recognitionfoils.ImageswerepresentedusingSTIM2soft-
lefthand,with5%dextrose–0.45%NaClsolutionusedasa ware (Compumedics Neuroscan, Charlotte, NC) on a 17-
carrier.AHarvard22syringepump(HarvardApparatus,Inc., inch liquid crystal display monitor placed at eye level at a
Holliston,MA)wascontrolledbySTANPUMPpharmacoki- distanceof1.2m.Eachitemwaspresentedfor2,000msand
neticsoftware**installedonalaptopcomputer.Cognitiveex- followedbya1,000-msblankscreen,foratotalinterstimulus
periments commenced only after the pharmacokinetic model intervalof3,000ms.
predictedequilibrationattheeffectsite(brain),whichtookap-
proximately15min. Experimental Procedure
Theexperimentalsequenceisdepictedinfigure1.Afterar-
Exclusions and Stopping Point rival(8:00AM),goldcupelectroencephalographyleadswere
Sixsubjectswereexcludedfromanalysis.Foursubjects(one applied, the intravenous catheter was placed, and baseline
propofol, one thiopental, and two dexmedetomidine) were measurements were performed. Drug infusion was then
excludedbecauseatechnicalmalfunctionresultedintheloss commenced (10:00 AM), and after equilibration, the Low-
ofgreaterthan25%ofresponsedata.Onesubject(midazo- doseEncodingTaskwasperformedusingimagesfromoneof
lam) was excluded because the subject recognized less than theEncodingSetsdescribedearlier.Thetotalsequencewas
20%ofthepresentedmaterialatbothdruglevels.Onesub- 375itemslongandwasdividedintothreeblocksof125,each
ject (dexmedetomidine) failed to complete the study after separatedbya5-minrest.All160imagesintheEncodingSet
faintingduringtheinsertionoftheintravenouscatheter.Ex- were repeated during the sequence: (1) 70 were repeated
clusions were determined according to criteria established following one intervening item (Lag 1, 6-s probes); (2) 70
before commencing the study. Enrollment continued until wererepeatedfollowingeightinterveningitems(Lag8,27-s
atleast12nonexcludablesubjectswereineachgroup. probes);and(3)20imageswererepeatedfollowing20inter-
veningitems(Lag20,63-sprobes).Someadditionalimages
Materials wereusedtofillgapsinthesequence.Thesubject’staskwas
AllstimuliwerecolorphotographstakenfromtheInterna- to indicate using a mouse whether the item was being pre-
tional Affective Picture System.39 For the experiment, im- sentedforthefirsttime(new)orwasarepeateditem(old).
Oncethissequencewascompleted,thedrugconcentration
**Freely available from the WorldSIVA Open TCI Initiative at wasincreasedtothehigherlevel(11:00AM),andtheHigh-
http://opentci.org/doku.php.AccessedMarch2,2010. doseEncodingTaskwasperformed.Thesequencingforthis
Pryoretal. Anesthesiology, V 113 (cid:127) No 2 (cid:127) August 2010 3
PERIOPERATIVE MEDICINE
taskwasidenticaltothelow-drugtask,butitusedtheimages variability and accommodates the repeated measures study
from the other Encoding Set. The drug infusion was then design.Here,theinitialdecayfunctionisextendedtoform
stopped(12:00PM).FourRecognitionTaskswereperformed thefollowingequation:
exactly15,45,105,and225minafterstoppingdrug.Each
m (cid:1)[(cid:1) (cid:4)(cid:7)](cid:5)t(cid:3)((cid:2)i(cid:4)(cid:8)ij),
Recognition Task contained 160 stimuli: (1) 40 from the tij i ij
Low-dose Encoding Task; (2) 40 from the High-dose En- where subscript i indicates the treatment group, and sub-
coding Task; and (3) 80 novel images. Subjects were pro- script j the individual subject. Thus, the terms (cid:7) and (cid:8)
ij ij
videdalightlunchduringthecourseoftheafternoon,but characterizetheindividualvariationininitialsignalstrength
theywerenotpermittedtohavecaffeinatedbeverages. and rate of forgetting, respectively. This model was calcu-
To sample memory performance at approximately 10 latedusingthenlmepackageinthestatisticalcomputerlan-
min,whichwasshorterthanthatcouldbeachievedwiththe guageR.††Druganddruglevelwereincorporatedasfixed
firstRecognitionTask,theLag20imagesweretreateddif- factors,andtheindividualsubjectwasincorporatedasaran-
ferently.AllLag20imageshadtheirfirsttwopresentations domfactor.Allotherstatisticalcalculationswereperformed
(theinitialnewandoldpair)placedwithinthefirstblockofthe usingSPSSversion13.0(SPSSInc.,Chicago,IL).Hypoth-
Encoding Task. A third presentation was then placed in the esis testing was two-tailed in all cases, and the results were
second block of the Encoding Task, and this functioned as quotedassignificantforPvalueslessthan0.05.
therecognitionprobeforthatimage.Theseimageswereagain
presentedinboththethirdblockoftheEncodingTaskand Assessment of Tonic Arousal
intheRecognitionTask,butthesesubsequentpresentations SubjectscompletedthePositiveandNegativeAffectSched-
functionedasfillersandwerenotpartoftheanalysis. ule41 at baseline and at the beginning and end of each En-
codingTask.Affectivedimensionswereratedonascaleof1
(“veryslightlyornotatall”)through5(“extremely”).Self-
Calculation and Statistical Analysis of the Memory
reported sedation–arousal scores were calculated from the
Decay Function
averagedresponsesforthealert,attentive,andactiveitems,
Animagewasincludedinthecalculationofthedecayfunc-
usingbothschedulescompletedateachdruglevel.
tion only if it was demonstrably attended to and encoded.
Wedefinedencodingtohaveoccurredifanimagewasap-
Event-related Potentials
propriatelyidentifiedasoldwhenpresentedforthesecond
Theelectroencephalogramwasobtainedin55ofthe61sub-
timeintheEncodingTask.Thesubsequentrecognitionfor
jectsincludedinthebehavioralanalysis.Theelectroenceph-
allsuchitemswastraced,andtheexacttimeintervalbetween
alogramwasnotrecordedintheremainingsixsubjects(one
encoding and recognition (recognition interval) was calcu-
propofol,onethiopental,onemidazolam,andthreedexme-
latedusingthecomputertimestamp.Thevalueforeachitem
detomidine)becauseoftechnicalproblems.Nineteenchan-
was adjusted by subtracting the false-positive rate for the
nels were sampled at 1 kHz using Synamps amplifiers and
specificRecognitionTaskinwhichtheitemwaspresented.
Scansoftware(CompumedicsNeuroscan).Datawererefer-
Thedatawerenextclusteredinto13points.Twelveclusters
enced to the left mastoid during acquisition but re-refer-
wereestablishedbyaverageweightingallLag1andLag8items
encedtolinkedmastoidsforanalysis.Offline,theelectroen-
thatbelongedtoaspecificEncodingTask—RecognitionTask
cephalogram was epoched to 2,500 ms, with a 199-ms
permutation.Thethirteenthpointwasderivedfromthethird
prestimulus baseline. Electrooculography artifacts were at-
presentationoftheLag20items,whichwasembeddedwithin
tenuatedusingtheNeuroscanregressionalgorithm,andre-
the Encoding Task as described earlier. Subjectwise clusters
sidual artifacts were removed through automated rejection
wereaveragedtogeneratethegroupwisevalues.
followed by visual inspection. ERPs were obtained from
Twostatisticalmethodswerethenappliedtothememory
baselinecorrectionandaveragingaccordingtoitemcriteria.
decay function. First, the Marquardt–Levenberg nonlinear
TheamplitudeandlatencyoftheP2andN2components
regressionalgorithmwasusedtoderivediscretevaluesfor(cid:1)
oftheERPatthePzelectrodeweredetermined.Becausethe
and(cid:2)ineachdruganddosecombination.Thisisthecon-
time interval between first and second presentations is
ventional approach for modeling memory decay func-
knowntoinfluence(cid:3)synchrony,27whichwascriticaltoour
tions,13,15 and provided the point values required for the
interestintheP2–N2complex,weevaluatedonlythesecond
subsequent correlational analyses. However, this model is
presentation of the Lag 8 (27 s) probes. After filtering, the
compromisedforstatisticalcomparisonbetweenthegroups.
identificationoftheP2andN2peakswasperformedatthe
Instead, to provide the strongest statistical comparison of
leveloftheindividualsubjectthroughtwoindependentvi-
drug effects on (cid:1)and (cid:2), we used Kristensen’s nonlinear
sualinspections,andthegroupvaluewasderivedfromsub-
mixed-effects model,40 which incorporates subject-specific
jectaveraging.
††RDevelopmentCoreTeam:R:Alanguageandenvironmentfor Visual Verbal Learning Test
statistical computing. Vienna, Austria, R Foundation for Statistical
Itisnotpossibletoencodematerialinthepresenceofdrugand
Computing,2008.Availableat:http://www.r-project.org.Accessed
March2,2010. thenstudyearlyrecognitionwithoutsomeresidualdrugbeing
4 Anesthesiology,V113(cid:127)No2(cid:127)August2010 Pryoretal.
P2–N2 and Arousal Predict Anesthetic Amnesia
Table1.SubjectCharacteristics
Drug IncludedSubjects Age,yr Gender(M/F) BMI,kg/m2
Propofol 12 26.0(6.6) 8/4 23.1(2.8)
Thiopental 13 29.7(8.3) 10/3 24.9(2.9)
Midazolam 12 27.4(8.0) 7/5 22.9(3.1)
Dexmedetomidine 12 31.8(10.7) 7/5 23.2(3.2)
Placebo 12 30.1(5.5) 7/5 23.3(2.3)
Therewasnosignificantdifferencebetweengroupswithrespecttoage,genderdistribution,orbodymassindex(BMI).Onlysubjects
ultimatelyincludedintheanalysisarelisted.Valuesarepresentedasmean(SD).
present. Thus, a test was required to determine whether the Results
anestheticdrugshadanyretrogradeeffectontherecognitionof
Subject Characteristics
materiallearnedbeforethedrugisadministered.
There was no significant difference between the treatment
A visual modification of the Rey Auditory Verbal
groupsinsubjectdemographiccharacteristics(table1).The
Learning Test42 was developed. The learning phase oc-
averageageforthe61subjectsincludedintheanalysiswas
curredjustbeforetheinitialdrugadministration.Subjects
29.0 yr (SD 8.0; range, 18–50 yr). Male:female ratio was
wereshownaprimarylistof16emotionallyneutralwords
39:22. The average body mass index was 23.5 kg/m2 (SD
onacomputerscreen.Eachwordwaspresentedfor1,500
2.9;range,17.2–29.8kg/m2).
ms and followed by a blank screen for 500 ms using the
STIM2software.Immediatelyafterthepresentation,sub-
jects were given 30 s to name any words that they could Task Integrity
recall. This procedure was performed four consecutive Toassesstheabilityofsubjectstoadequatelyattendtoand
times. Subjects were then shown a secondary list of 16 perform the Encoding Task, we evaluated performance ac-
words and again given 30 s to name any words that they curacyfortheinitialpresentationsoftheLag1(6s)probes
could recall. This list was shown only once. Thus, it was (table2).Overallaccuracyinidentifyingtheinitialpresen-
expected that the primary list would be more strongly tationasnewwas91.6%(SD7.8),whereasaccuracyiniden-
learned than the secondary list. To counterbalance any tifyingthesecondpresentationasoldwas90.2%(SD9.9).
effect of individual words, three versions of the test were Alloftheactivedrugscauseddiminishedaccuracyatthehigh
createdandassignedrandomly. drug level, resulting in a statistically significant drug:level
Recognitionoccurredduringthedruginfusion.Subjects interactionforbothnew(F (cid:1)4.98,P(cid:1)0.002)andold
4,55
performedaforced-choicetaskinwhichtheywerepresented (F (cid:1) 3.404, P (cid:1) 0.015) items. There was no overall
4,55
wordpairsfor2,500msfollowedbyablankscreenfor500 accuracybiastowardeitherneworolditems(t (cid:1)(cid:3)1.853,
120
ms. One word was taken from either the primary or the P(cid:1)0.066).Further,accuracyforanitem’sfirst(new)pre-
secondarylist(old),andtheotherwasanovelfoil(new).The sentationdidnotinfluencetheaccuracyforitssecond(old)
task was to identify the old word. At each of the two drug presentation(t (cid:1)0.350,P(cid:1)0.727),suggestingthatmis-
114
levels,eightwordsfromeachoftheprimaryandsecondary categorizationhadnosignificanteffectonlearningfromitem
listswereprobed. exposure.
Table2.TaskPerformanceMeasuresandKeyDecayModelParameters
TaskPerformanceMeasures(6-sProbes) PowerDecayModel
New Old New Old tat
Pharmacokinetic Target Accuracy Accuracy Reaction Reaction R2 m (cid:1)1
t
Drug Model Concentration (%) (%) Time(ms) Time(ms) (cid:1) (cid:2) Value (min)
Propofol Schnider 0.45(cid:7)g/ml 95.4(3.4) 95.4(4.3) 955(232) 802(158) 1.401 0.179 0.763 6.6
0.90(cid:7)g/ml 86.3(8.7) 88.1(11.5) 1098(192) 900(159) 1.586 0.326 0.890 4.1
Thiopental Shafer 1.5(cid:7)g/ml 94.7(5.7) 93.4(7.9) 935(133) 778(146) 1.499 0.195 0.926 8.0
3.0(cid:7)g/ml 89.0(8.6) 85.9(14.1) 1016(123) 821(153) 1.285 0.221 0.870 3.1
Midazolam Greenblatt 20ng/ml 91.9(5.5) 87.4(11.4) 981(187) 889(192) 1.502 0.258 0.881 4.8
35ng/ml 85.1(12.8) 78.7(10.2) 1046(153) 946(166) 1.170 0.296 0.801 1.7
Dexmedetomidine Markku 0.20ng/ml 95.1(2.6) 93.3(5.0) 906(108) 779(88) 1.492 0.203 0.875 7.2
0.40ng/ml 85.7(7.4) 87.2(7.9) 979(147) 810(144) 1.132 0.200 0.830 1.9
Placebo — — 95.7(1.8) 95.5(4.5) 850(178) 784(194) 1.346 0.143 0.838 8.0
— 96.4(3.0) 96.4(3.3) 874(96) 752(128) 1.226 0.113 0.868 6.1
Newandoldaccuracymeasuresrepresentperformanceincorrectlycategorizingthefirst(new)andsecond(old)presentationswithin
the Encoding Task for those probes separated by 6 s. t at m (cid:1) 1 is the predicted time at which memory decay would become
t
behaviorallydetectable.Dataarepresentedasmean(SD).
Pryoretal. Anesthesiology, V 113 (cid:127) No 2 (cid:127) August 2010 5
PERIOPERATIVE MEDICINE
Althoughallactivedrugscausedanunavoidabledegree ficientlyequivalenttoenabledirectcomparisonbetweenthe
ofgloballyimpairedtaskperformanceatthehigherdose, drugsfortheprincipallyqualitativepurposesofthestudy.
overall performance remained generally high and unbi-
ased. The exclusion of miscategorized items from fur-
Comparison of (cid:1)and (cid:2)across Individual Drug and
ther analysis provided control against dynamics in task
Dose Conditions
integrity. Coefficient(cid:1).Therewasastrongindependenteffectofdrug
(F (cid:1)14.006,P(cid:2)0.0001)andalsoastrongindepen-
4,1493
Applicability of the Negative Power Function in dent effect of drug level (F (cid:1) 33.212, P (cid:2) 0.0001).
1,1493
Describing Drug-induced Amnesia Significant fixed effects were identified for the midazolam:
UsingtheMarquardt–Levenbergnonlinearregressionalgo- levelinteraction((cid:3)0.513,t (cid:1)(cid:3)2.465,P(cid:1)0.014)and
1493
rithm,therangeofR2valuesforthenegativepowerfunction forthedexmedetomidine:levelinteraction((cid:3)0.476,t (cid:1)
1493
modelm (cid:1)(cid:1)t(cid:3)(cid:2)was0.763–0.926(mean0.854;SD0.047; (cid:3)2.638,P(cid:1)0.008),with(cid:1)significantlydecreasingatthe
t
table 2). The model satisfied the axiomatic condition that higherdrugconcentration.Therewasalsoasignificantfixed
behavioralmemoryperformancewasintactintheinitialstate effect for the propofol:level interaction (0.889, t (cid:1)
1493
(i.e., m (cid:9) 1 as t 3 0), which was required because the 3.489,P(cid:1)0.001),whichisnotablebecauseofaparadoxical
t
analysisincludedonlyimagesthathadbeencorrectlyidenti- increasein(cid:1)atthehigherdrugconcentration.
fiedas“old”intheEncodingTask.Themodelpredictedthat Coefficient(cid:2).Therewasasignificantindependenteffectof
memory decay would become behaviorally detectable (i.e., drug(F (cid:1)2.759,P(cid:1)0.027)andastronginteractive
4,1493
m (cid:2)1)intherangeof1.7–8.0minafterencoding,depend- effectofdruganddruglevel(F (cid:1)13.910,P(cid:2)0.0001).
t 4,1493
ingonthedruganddosecondition(seeappendixforadis- However, there was no independent effect of drug level
cussion of the limitations of the two-parameter model in (F (cid:1)1.900,P(cid:1)0.168).Significantfixedeffectswere
1,1493
predictingthisvalue). identified for the independent effect of midazolam (0.110,
Wealsoattemptedtofitthedatatomultipleexponential, t (cid:1)2.648,P(cid:1)0.008)andfortheinteractiveeffectof
1493
logarithmic, and multiphased linear decay models. When propofol with increasing level (0.258, t (cid:1) 6.597, P (cid:2)
1493
constrainedtosatisfytheaxiomaticconditiondescribedear- 0.0001).
lier(m (cid:9)1),wefoundthatallalternatemodelseither Thus,werejectthenullhypothesisthatthereisnodiffer-
failedotr3ha0dmarkedlyincreasedunexplainedvariancerela- encebetweendrugsinmodulationof(cid:1)and(cid:2)atequivalent
tivetothepowermodel.Thenegativepowerfunction,thus, levelsofmemoryloss.Thedecaycurvesandcoefficientsfor
seems to robustly describe both the nondrug state and the eachdruganddoseconditionareshowninfigure2.
pharmacologicmodulationofmemoryfordrug–dosecom-
binationsstudied.
Neurobehavioral and Event-related Potential Correlates
of (cid:1)and (cid:2)
Assessment of Equiamnestic Dose Targeting Wenextexaminedwhetherthemodulationof(cid:1)or(cid:2)could
To determine whether the target drug concentrations had becorrelatedwithneurobehavioral(fig.3)andneurophysi-
resulted in approximately equal levels of end-amnesia, we ologic(fig.4)eventsatthetimeofencoding.Fortheseanal-
analyzedthethreeterminalclustersineachdecayfunction. yses,weusedthegroupwisecoefficientslistedintable2and
Forsubjectsreceivinganactivedrug,themeanrecognition evaluatedtherelationshipsusingthegenerallinearmodel.43
rate(correctedforfalsealarms)atthelowdrugconcentration Here,inaccommodatingtherepeated-measuresdesign,tand
was0.429(SEM0.020).ByrepeatedmeasuresANOVA(re- rstatisticsrepresentpartialcoefficientsafterthevariancebe-
peatedclustermeasurementspersubject),thedifferencesbe- causethepairedgroupinghasbeenremoved.
tween drugs was not quite statistically significant (F (cid:1) Self-reportedArousal.Overall,therewasastrongrelation-
2.745, P (cid:1) 0.054).‡‡ At the high drug concentratio3n,4,5the ship between self-reported arousal and (cid:1)(r (cid:1) 0.832, t7 (cid:1)
meanrecognitionratewas0.304(SEM0.020),withnosig- 3.965, P (cid:1) 0.005; fig. 3A). The within-group relationship
nificant difference between drugs (F (cid:1) 1.670, P (cid:1) was highly conserved for placebo, thiopental, midazolam,
3,44
0.187). By comparison, the mean recognition rate in the anddexmedetomidine,butnotforpropofol.Therewasno
placebogroupatthelowdruglevelwas0.592(SEM0.037) relationshipbetweenarousaland(cid:2)(t7(cid:1)(cid:3)0.060,P(cid:1)0.953
andatthehighdruglevelwas0.629(SEM0.036),withno fig.3B).
significantdifferencebetweenthetwolevels(F (cid:1)0.577, ReactionTime.Reactiontimewasdefinedasthetimefrom
1,11
P (cid:1) 0.463). We concluded that at each of the two drug stimulusonsettobuttonresponse.Therewasaverystrong
levels,thetargetdrugconcentrationshadresultedindegrees relationship between increasing average reaction time and
ofend-amnesiathat,althoughnotperfectlyequal,weresuf- higherlevelsof(cid:2)(r(cid:1)0.949,t7(cid:1)7.946,P(cid:2)0.0001;fig.
3D).Whentheresponsesforthefirst(new)andsecond(old)
stimuluspresentationsweretreatedseparately,therelationship
ma‡n‡cTehiisnntheearf-osilglonwifiicnagntorrdeesru:ltthdioeprievnestalfr(cid:6)omprroepcoogfonliti(cid:1)ondpeexrmfoer-- remainedrobustforbothnewimages(r(cid:1)0.908,t7(cid:1)5.734,
detomidine(cid:6)midazolam. P(cid:1)0.001;fig.3F)andoldimages(r(cid:1)0.941,t (cid:1)7.345,P(cid:1)
7
6 Anesthesiology,V113(cid:127)No2(cid:127)August2010 Pryoretal.
P2–N2 and Arousal Predict Anesthetic Amnesia
Fig.2.Memorydecaycurvesandestimatesfor(cid:1)and(cid:2)bydruganddosecondition.(A–F)Thepowerdecaycurvesforeach
of the drug-dose combinations. The abscissa represents the duration from the final encoding exposure to the subsequent
recognitionexposure(therecognitioninterval).Foreachcurve,recognitiondatawerecorrectedforfalsepositives,clustered
into 13 time points, and then fit to the equation m (cid:1) (cid:1)t(cid:3)(cid:2)using Marquardt–Levenberg nonlinear regression. The specific
t
equationsdescribingeachcurveareshownintheupperandlowerrightcornersofpanelsA–F.(GandH)Thevaluesfor(cid:1)and
(cid:2)areshown,witherrorbarsrepresentingtheSEMfortheestimatederivedfromthenonlinearregression.However,statistical
comparisonofthedecayfunctionswasperformedusingamorerobustnonlinearmixedeffectsmodel.40For(cid:1),independent
effectswereestablishedfordrug(P(cid:2)0.0001)andlevel(P(cid:2)0.0001),andsignificantfixedeffectsseenforthemidazolam:level
interaction(P(cid:1)0.014),thedexmedetomidine:levelinteraction(P(cid:1)0.008),andthepropofol:levelinteraction(P(cid:1)0.008).For
(cid:2),independenteffectswereestablishedfordrug(P(cid:1)0.027)anddruglevelinteraction(P(cid:2)0.0001),whereasfixedeffectswere
established for midazolam (P (cid:1) 0.008) and propofol:level interaction (P (cid:2) 0.0001). DEX (cid:1) dexmedetomidine; MIDAZ (cid:1)
midazolam;PLAC(cid:1)placebo;PROP(cid:1)propofol;THP(cid:1)thiopental.
0.0002; fig. 3F). There was no relationship between average was7.74(SD0.55,F (cid:1)0.368,P(cid:1)0.830)fortheprimary
4,56
reactiontimeand(cid:1)(t (cid:1)0.136,P(cid:1)0.895;fig.3C).Notably, listand6.56(SD1.10,F (cid:1)1.47,P(cid:1)0.225)forthesec-
7 4,56
althoughreactiontimeisoftenassociatedwithsedation,inthis ondarylist.Atthehighdruglevel,meancorrectrecognitionwas
study,therewasnosignificantrelationshipbetweentheaverage 7.51(SD0.68,F (cid:1)0.396,P(cid:1)0.811)fortheprimarylist
4,56
reactiontimeandthearousalscore(t (cid:1)(cid:3)0.919,P(cid:1)0.389; and5.95(SD1.30,F (cid:1)2.176,P(cid:1)0.085)forthesecondary
7 4,56
fig.3E). list.
ERPCorrelates.Therewasastronginverserelationshipbe- Memory Performance in the First Minute after Encoding.
tween the amplitude of the P2 component and (cid:2) (r (cid:1) Becausetheprobepairsforeachofthethreeintervals(6,27,
(cid:3)0.817, t7 (cid:1) (cid:3)3.743, P (cid:1) 0.007; fig. 4B). There was a and63s)wereinterleavedandbalancedthroughouttheEn-
positiverelationshipbetweenthelatencyoftheP2compo- codingTask,performanceconfounderssuchassedation,fa-
nentand(cid:2),butthisdidnotquiteachievestatisticalsignifi- tigue, false response bias, interference, or sequence effects
cance(r(cid:1)0.661,t7(cid:1)2.331,P(cid:1)0.053).Therewasastrong were distributed equally. Thus, any difference in perfor-
relationshipbetweenthelatencyoftheN2componentand(cid:2)
mance for probes separated by 27 or 63 s relative to those
(r (cid:1) 0.867, t (cid:1) 4.610, P (cid:1) 0.002; fig. 4C). The inverse
7 separatedby6swasafunctionofthetimeintervalandhence
relationship between N2 amplitude and (cid:2)did not achieve
amemoryeffect.Recognitionofthesecond(old)presenta-
significance (r (cid:1) (cid:3)0.547, t (cid:1) (cid:3)1.728, P (cid:1) 0.128). No
7 tionwascalculatedasaratioofrecognitionat6s.Forthe27-s
relationshipcouldbeestablishedbetweenanyofthecompo-
probes, two-way repeated measures ANOVA with factors of
nentsevaluatedandthecoefficient(cid:1). drug and level found no significant effect of drug (F (cid:1)
4,120
0.990, P (cid:1) 0.416), level (F (cid:1) 0.0475, P (cid:1) 0.828), or
1,120
Secondary Analyses drug:levelinteraction(F (cid:1)0.785,P(cid:1)0.537).Similarly,
4,120
Drug Effect on Recognition of Previously Encoded Mate- forthe63-sprobes,therewasnosignificanteffectofdrug(F
4,120
rial. We found no evidence that any of the drugs had any (cid:1)0.241,P(cid:1)0.915),level(F (cid:1)0.0116,P(cid:1)0.914),or
1,120
retrogradeorrecognitionprocesseffectwithinthetimewindow drug:levelinteraction(F (cid:1)0.544,P(cid:1)0.704).Resultsare
4,120
studied(fig.5).Atthelowdruglevel,meancorrectrecognition showninfigure6.
Pryoretal. Anesthesiology, V 113 (cid:127) No 2 (cid:127) August 2010 7
PERIOPERATIVE MEDICINE
Fig.3.Correlationof(cid:1)and(cid:2)withmeasuresofarousalandreactiontime.(A–F)Thepointvaluesforthecoefficients(cid:1)or(cid:2)(taken
from fig. 2) are compared with measures of arousal and reaction time. In each case, the gray lines link the two drug levels,
directedfromlowtohighdruglevel.Therewasastrongrelationshipbetweenarousaland(cid:1)(P(cid:1)0.005,A),butnotwith(cid:2)(P(cid:1)
0.953, B). Reaction time was instead strongly linked to (cid:2)(P (cid:2) 0.0001, D), but not to (cid:1)(P (cid:1) 0.895, C). When new and old
presentationsoftheimageswereconsideredseparately,the(cid:2)correlationremainedstrongforboth(P(cid:1)0.001andP(cid:1)0.0002,
F).Therewasnosignificantrelationshipbetweenreactiontimeandarousal(P(cid:1)0.389,E).Drugsarelabeledasfollows:D(cid:1)
dexmedetomidine;M(cid:1)midazolam;P(cid:1)propofol;T(cid:1)thiopental;X(cid:1)placebo.
Discussion sion:thestrengthoftheinitialmemorytraceisafunction
oftonicarousal,whereastherateofdecayofanestablished
Ourobjectivewastoapplyamathematicalmodeltochar-
acterizehowseveralintravenousanestheticsmodulatethe memoryisacceleratedwiththelossoftheP2–N2complex
establishment and decay of episodic memory for visual at encoding and a prolongation of reaction time. Al-
stimuli. We found that although the basic mathematical though anesthetic amnesia is evidently a heterogeneous
form of memory decay—the negative power function phenomenon, the underlying mechanisms may converge
m (cid:1) (cid:1)t(cid:3)(cid:2)—is consistently preserved, there are marked on only a very limited number of system-level functions.
t
differences between the drugs in the way they modulate Although it may take several minutes for the amnesia to
thefunction.Weconcurrentlyestablishedthateachmath- become behaviorally detectible, it seems that the fate of
ematical parameter could be correlated with neurophysi- thememorytraceislargelydeterminedatthemomentof
ologicandneurobehavioraleventswithremarkablepreci- exposure.
8 Anesthesiology,V113(cid:127)No2(cid:127)August2010 Pryoretal.
P2–N2 and Arousal Predict Anesthetic Amnesia
Fig.4.Correlationof(cid:1)and(cid:2)withtheparietalevent-relatedP2–N2complex.(A)Thegrandaveragewaveformsobtainedatthe
Pz electrode for the placebo group at the second level and for the propofol group at both drug levels, demonstrating the
progressivechangesinP2andN2amplitudeandlatencywithincreasingdruglevelareshown.(B,C)Therelationshipbetween
thepointvaluesfor(cid:2)(takenfromfig.2)andgroup(notgrand)averagecomponents.Relationshipswereevaluatedusingthe
generallinearmodel.43Therewasastrongrelationshipbetween(cid:2)andP2amplitude(P(cid:1)0.007,B)andN2latency(P(cid:1)0.002,
C).Noevent-relatedpotentialrelationshipscouldbeestablishedwith(cid:1).Graylineslinkthetwodruglevels,directedfromthe
lowtohighdruglevel.Drugsarelabeledasfollows:D(cid:1)dexmedetomidine;M(cid:1)midazolam;P(cid:1)propofol;T(cid:1)thiopental;X(cid:1)
placebo.
Dexmedetomidine closely approximates an archetypal thisresultstandsoutasthesoleexceptiontothearousal3
sedative-only drug. It causes a dose-dependent decrease in encoding(cid:2)P2–N23decayrule,itshouldnotbedismissed
the strength of the initial memory trace, which parallels a (ifitwereartifact,the(cid:2)valueshouldreciprocallyshiftand
decrease in tonic arousal. If anything, it has only minimal failtoconformtotherule,whichwedidnotobserve).Propo-
effect on the rate of decay, which does not seem to change fol,likemanyotheranesthetics,cancauseparadoxicalexci-
withdoseandwhichcorrespondstoamodestlossofP2–N2 tationphenomenaatlowdoses.Mathematicalnetworkmod-
and increased reaction time. Although dexmedetomidine eling shows that this can result from a membrane-level
willcausememoryimpairmentbecauseofweakenedencod- interactionofGABAsubtype-Acurrentandintrinsicmem-
ing, those memories that are established seem to subse- braneslowpotassiumcurrent,withtheconsequencebeinga
quentlybehaveinalargelynormalfashion—anobservation largescalelossofinterneuronalsynchrony,46whereaswork
thatwouldsuggestcautioninitsuseasasolesedativeagentin inpatientswithpathologicdisordersofconsciousnesswhose
clinicalsettingswhereamnesiaisrequired. cognitivefunctionisimprovedbyGABAergicdrugssuggests
In contrast, the amnestic effect of propofol is character- thatthemechanisminvolvessuppressionofinhibitorycon-
izedbyrobustencodingfollowedbyamarkedaccelerationin trolsignalsregulatingthalamocorticalfunction.47Whatever
decayoftheestablishedmemory.Theeffectisdose-depen- thespecificmechanism,ourresultssuggestthattheperceived
dentandisassociatedwithanequallymarkedlossofP2–N2 loss of arousal produced by propofol possesses features not
and increased reaction time. Although subjects receiving sharedbytheotherdrugs.
propofoldidreportsedation,thisdidnottranslateintoweak- The profile of midazolam is that of a mixed sedative-
enedestablishmentofthememorytrace.Thesefindingsare amnestic, with clear effects on both the establishment of
consistentwithourpreviousstudiesusingpropofol,inwhich memoryanditssubsequentdecaythatadherecloselytothe
we demonstrated intact acquisition of material into long- arousal3encoding(cid:2)P2–N23decayrule.Wefoundthat
term memory44 and normal encoding-related activation of theaccelerateddecaywasthedominanteffectatthelowdose,
leftinferiorprefrontalcortex.45However,thedesignofthe whereasincreasingtheconcentrationtothehighdoseacted
currentstudyallowedustoobserveaparadoxicalaugmenta- principally by causing a marked decrease in arousal, with
tion in initial trace strength at the doses studied. Although relativelylittleadditionaleffectondecay.Thisdose-depen-
Pryoretal. Anesthesiology, V 113 (cid:127) No 2 (cid:127) August 2010 9
PERIOPERATIVE MEDICINE
Fig.6.Memoryperformanceinthefirstminuteafterencod-
ing.IntheEncodingTask,imagepairswerepresented6,27,
or63sapart.Recognitionofthesecond(old)presentationof
the27and63sprobesisshownasaratioofrecognitionat
6 s. Random confounders such as fatigue or sequence ef-
fectsareequallydistributed,andthusdifferenceswouldre-
flectatruememoryeffect.At27s,therewasnosignificant
effect of drug (P (cid:1) 0.416), level (P (cid:1) 0.828), or drug:level
interaction(P(cid:1)0.537).Similarly,at63s,therewasnoeffect
ofdrug(P(cid:1)0.915),level(P(cid:1)0.914)ordrug:levelinteraction
(P(cid:1)0.704).Althoughourotherresultsimplythattheevents
leading to memory decay were established at the time of
encoding,noeffectonperformancewasdetectibleinthefirst
minute.ErrorbarsrepresentSEM.
cautioneddiscussionoftheobservedtrend—ourdecisionto
study multiple drugs at two levels incurs a strong multiple
comparisons paradox, and the results for thiopental would
haveachievedsignificancehadweinvestigatedfewerdrugs.
The profile of thiopental most closely resembles that of
dexmedetomidine, with the dominant effect being an
arousal-related decreaseinthestrengthoftheinitialmem-
orytrace.IncontrasttotheothertwoGABAergicdrugs,only
Fig.5.Drugeffectontherecognitionofpreviouslyencoded
amodesteffectontherateofdecayandP2–N2wasobserved.
material. Subjects learned two lists of 16 words before re-
This finding confirms previous studies demonstrating that
ceivingdrug:aprimarylistwasstronglylearnedthroughfour
repetitions,andasecondarylistmoreweaklylearnedthrough thememoryeffectofthiopentalispredominantlyrelatedto
onlyasinglerepetition.Later,duringthedruginfusion,sub- sedation,2,36,44andthemarkeddifferencebetweenthiopen-
jectsperformedforced-choicetaskstotestrecognitionofthe talandpropofolissupportedbyneuroimagingshowingclear
learned words. (A) The results for probed recognition of the differences in the regional cerebral blood flow patterns in-
primary list, and (B) the results for the secondary list. One-
ducedbythesetwodrugsatdosessimilartothehigherdrug
way analysis of variance was performed on each of the levelstudiedhere.35
four-wordlist:druglevelcombinationstoevaluatetheeffect
The measures of reaction time and the parietal P2–N2
of drug. There was no significant effect of any drug on the
recognitionofeitherstronglyorweaklylearnedwords.Error complex can offer only limited mechanistic information.
barsrepresentSEM.DEX(cid:1)dexmedetomidine;MIDAZ(cid:1)mida- However, the dramatic consistency in the relationship be-
zolam;PLAC(cid:1)placebo;PROP(cid:1)propofol;THP(cid:1)thiopental. tweenthesemeasuresandtherateofmemorydecay—valid
across all studied drugs—does strongly imply that acceler-
dent selectivity supports the picture that the two effects are atedtracedecayistheproductofaspecificlossoffunction
mechanistically dissociated. Further, the tapering effect of in- thatleadstothefailureofamemorytoconsolidate.Asrecent
creasingdoseondecaysuggeststhattheremaybeaceilingphe- primingexperimentshavedemonstratedthatreactiontimeis
nomenonwithrespecttoadrug’sabilitytoacceleratedecay. relatedtointerregionalsynchrony,18whatfunctionallycon-
We were not able to establish a statistically significant nectsthesetwomeasuresisthattheybothrepresentalossof
characterization of thiopental, but this should not negate a synchronouscooperativityacrossdistributednetworks.
10 Anesthesiology,V113(cid:127)No2(cid:127)August2010 Pryoretal.