Warning or alarm functions in commercial monitoring equipment generate many false alarms, which restrict their clinical usefulness. Nevertheless, warning functions are indispensable whenever the possibility exists that an unexpected problem escapes our attention. ...

Zie: Summary

Ballast, Albertus,

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Warning systems in anesthesia. Human vigilance supported by clinically relevant warnings

Ballast, Albertus

University of Groningen Research Database

  
 University of Groningen
Warning systems in anesthesia. Human vigilance supported by clinically relevant warnings
Ballast, Albertus
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
1992
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Ballast, A. (1992). Warning systems in anesthesia. Human vigilance supported by clinically relevant
warnings s.n.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 11-02-2018
1 1 4
SIJl\ /{ IVIAR)'
Warn ing  or  a la rm func t ions  in  commerc ia l  mon i to r ing  equ ipment  genera te  many
f alse alarms, which restr ict their cl inical usef ulness. Nevertheless, warning
functions are indispensable whenever the possibi l i ty exists that an unexpected
problem escapes our attention.
Anesthesia can be considered as a process of continuous optimization, correcting
any disturbance in the desired physiological state. This process requires the
anesthetist 's continuous alertness. When an init ial  disturbance escapes our
a t ten t ion ,  an  op t ima l  s ta te  i s  no  longer  ma in ta ined.  When the  d is tu rbances  deve lop
fur ther ,  management  becomes increas ing ly  d i f f i cu l t  and eventua l l y  permanent
damage may result.
I t  i s  a  d i f f i cu l t  task  fo r  a  human be ing  to  keep a le r t  o r  v ig i lan t  con t inuous ly ,
espec ia l l y  when the  inc idence o f  unexpected  prob lems is  as  smal l  as  i t  i s  in
anesthesia.
Mon i to r ing  equ ipment  i s  usua l ly  p rov ided w i th  a la rm func t ions  to  suppor t
vigi lance. These conventional alarm systems generate an alarm whenever the value
of a measured variable exceeds a l imit that has been set previously. In this thesis
d is tu rbances  tha t  a re  de tec ted  w i th  such an  a la rm sys tem are  ca l led 'va lue  events ' .
The l im i ts  a re  usua l ly  se t  a t  the  phys io log ica l  l im i ts .
The cont inuous  op t im iza t ion  process  o f  anes thes ia  occurs  a lmost  en t i re ly  w i th in
the  phys io log ica l  l im i ts .  D is tu rbances  wh ich  requ i re  op t im iza t ion  cannot  be
detec ted  by  a  convent iona l  a la rm sys tem unt i l  abnormal  va lues  have occur red .  The
in i t ia t ion  o f  d is tu rbances  -wh ich  requ i res  cor rec t ion-  can on ly  be  de tec ted  as  a
change, or more exactly as a dif ference between present and previous
measurements ,  In  th is  thes is  such in i t ia l  d is tu rbances  are  ca l led  ' change-events ' .
A  convent iona l  a la rm sys tem,  be ing  su i tab le  fo r  de tec t ion  o f  va lue-events  on ly ,
cannot  suppor t  the  cont inuous  op t im iza t ion  process  o f  anes thes ia  e f fec t i ve ly .
There fore ,  there  is  a  need fo r  a  warn ing  sys tem tha t  de tec ts  change-events  long
before values outside the normal range are measured.
Several systems have been described for the detection of changes within the
normal  range.  They  compare  the  var iab le 's  p resent  va lue  w i th  a  runn ing  average,
an  average o f  a  p reced ing  per iod .  Other  methods  compare  two runn ing  averages
ca lcu la ted  over  a  recent  per iod  and an  ear l ie r  per iod .  The d isadvantages  o f  these
sys tems are  the i r  comple te  insens i t i v i t y  fo r  s low changes and the  pers is ten t
character of their alarm states.
I t  has  been dec ided to  deve lop  a  warn ing  sys tem fo r  the  de tec t ion  o f
change-events ,  g iv ing  a  shor t  bu t  c lear  warn ing  whenever  necessary .  An impor tan t
s tep  in  th is  deve lopment  i s  to  de termine wh ich  change-events  a re  c l in ica l l y
re levant  and wor th  a  warn ing .  Both  rap id  and s low changes have to  be  cons idered,
a l though they  requ i re  d i f fe ren t  de tec t ion  methods .  For  changes near  the
phys io log ica l  l im i ts  an  enhanced sens i t i v i t y  i s  des i red  s ince  they  are  cons idered
more relevant than those in the centre of the normal range. Furthermore, the
re levance o f  changes depends on  the i r  ampl i tude and on  pa t ien t -  o r
anes the t is t - re la ted  ind iv idua l  fac to rs .
A warning signal is required whenever the degree of change exceeds a certain
th resho ld .  The cho ice  o f  the  th resho ld  de termines  how many warn ings  are
genera ted .  A  low th resho ld  p roduces  a  h igh  sens i t i v i t y ,  genera t ing  many warn ings ,
Sunnary 1 1 5
of which a large proport ion is relat ively irrelevant. A high threshold results in a low
sensit ivi ty, with the chance that some cl inical ly relevant events pass without
generating a warning. Numerical ly comparable changes are cl inical ly sometimes
relevant and sometimes irrelevant depending on secondary factors. The choice of
the threshold for the prototype was based on init ial  experience with the system and
is supported by a study of the variabi l i ty of measured values and their relat ion to
therapeutical intervention.
The numerieal methods for the detection of change events are described in a
separate chapter. The normalized running average dif ference (NRAD) is
introduced as a qual i tat ive and quanti tat ive representation of the average change
occur r ing  in  a  t ime w indow def ined by  two t ime-cons tan ts .  The NRAD is
calculatedas the product of a correction factor and the dif ference of two running
averages characterized by the two time constants.
Very slow changes can be detected by the Minimax method. I t  was introduced to
detec t  changes tha t  occur  ou ts ide  the  t ime w indow def ined fo r  the  NRAD.  The
Minimax method compares the present value of a variable with the very largest and
the very smallest value that has occurred during the whole observation period.
Fur thermore  a  change- ind ica tor  has  been in t roduced.  Th is  i s  the  ra t io  o f  the
NRAD and the appropriate warning threshold. I t  may be used in the future for
mathematical processing of coincidental changes occurring in dif ferent variables.
A rese t -mechan ism has  a lso  been in t roduced.  Th is  p revents  sus ta ined warn ing
states. As soon as a warning is given -because of a change- a new observation
period is started, using the value measured after the change as a new reference
value. There is no need for a sustained warning state provided the change occurred
ins ide  the  normal  range.  The resu l t  o f  th is  rese t -mechan ism is  tha t  t ime is  d iv ided
into periods that are assumed to be stable, but separated by moments of change.
These moments of change are marked by a warning signal.
Carina, the prototype of the new warning system generates warnings according to
the methods described above. The warning signal consists of a short melodious
tune, completed with a strong optical signal. The latter does not disturb the surgical
team but alerts the anesthetist effect ively. Al l  measured values can be read from
the system's video display. In addit ion i t  is indicated which of the variables have
recently caused a warning. Trend curves of some of the measured values are shown
on the display as well  as real t ime curves of the ECG and the capnogram.
Furthermore, the system automatical ly writes an anesthesia record.
In order to provide the reader with an impression of how the warning system
works  in  p rac t ice  a  s ing le  anes the t ic  i s  descr ibed in  de ta i l ,  w i th  the  warn ings
generated by f ive dif ferent warning systems. Carina's warnings are given for
changes that an anesthetist probably recognizes as cl inical ly relevant, while a
conventional warning system generates long sustained alarms, once a l imit is
exceeded. For the purpose of comparison unusually narrow l imits were chosen.
For further evaluation f ive studies have been performed. I t  is assumed that
opiates are given when pain is suggested by an increased heart rate or blood
pressure. As an anesthetist gives an opiate when he considers such changes to be
relevant, Carina's warnings can be expected to be associated with opiate gif ts. In
the f irst study i t  is demonstrated that hemodynamic changes are associated with
opiate gif ts. I t  is concluded that opiate gif ts are indeed associated with
therapeutical ly relevant changes. The second study demonstrates that warnings are
given for the events that correlate with opiate gif ts. However, in this study changes
1 1 6 Warn ing  Sys tens  in  Anesthes ia
tha t  ind ica te  the  necess i ty  o f  op ia te  admin is t ra t ion  cannot  be  d is t ingu ished f rom
changes resu l t ing  f rom op ia te  therapy .  A  th i rd  s tudy  was ins t iga ted  to  inves t iga te  i f
warnings were given for increases in blood pressure, as an example of changes that
indicate exclusively the necessity of opiate administrat ion.
Thequest ion  ar ises  i f  a  comparab le  cor re la t ion  be tween changes and warn ings  can
also be achieved with a conventional alarm system, by choosing unusually narrow
alarm l imits, Study four demonstrates that such a correlat ion can indeed be
achieved. However, there are theoretical objections against such a system: many
false alarms are to be expected, alarm sounds wil l  have a sustained character and
not al l  relevant change-events wil l  be detected. Study f ive demonstrates that long
sustained alarm periods are generated i f  a conventional alarm system is so adapted.
Th is  w i l l  obv ious ly  cause much i r r i ta t ion  and a  compara t ive ly  smal l  number  o f
useful warnings wil l  be generated.
The occur rence o f  fa lse  a la rms is  a  p rob lem wi th  many warn ing  sys tems.
Therefore, methods were developed for suppression of two frequent causes of false
a la rms:  no ise  or  rap id  i r regu la r  f luc tua t ions ,  and sp ikes  or  la rge  t rans ien t
f luctuations. False alarms caused by noise can be suppressed by using a running
average over a short period instead of the measured value as such as the input for
the  de tec t ion  sys tem.  Th is  works  as  a  low-pass  f i l te r  and is  a  common method fo r
suppression of noise. I t  is also used in the prototype.
For  the  suppress ion  o f  sp ikes ,  a  low-pass  f i l te r  i s  insu f f i c ien t  because the  h igh
ampl i tude o f  the  sp ikes  cannot  be  a t tenuated  su f f i c ien t ly  by  the  f i l te r .  A  new f i l te r
techn ique was,  there fore ,  in t roduced.  Th is  l im i ts  the  speed o f  inc rease o f  a
var iab le ,  be fore  i t  i s  fed  in to  the  de tec t ion  sys tem.  I t  la rge ly  reduces  the  ampl i tude
of  sp ikes ,  bu t  phys io log ica l  changes w i l l  pass  such a  f i l te r  a lmost  unaf fec ted  s ince
they  occur  more  s lowly  than sp ikes  do .  The resu l t  o f  such a  f i l te r ing  is  ca l led  a
l im i ted  runn ing  average (LRA) .  The normal ized  l im i ted  runn ing  average
d i f fe rence (NLRAD)  is  the  normal ized  d i f fe rence be tween two LRA's ,  and can be
used fo r  de tec t ion  o f  changes ins tead o f  the  NRAD.
An example  o f  sp ike  in te r fe rence is  a f fo rded by  the  d is tu rbances  in  pu lse
oximeter readings caused by interruption of the blood f low to the arm when a blood
cuf f  i s  in f la ted  on  the  same arm.  In  a  vo lun teer  such d is tu rbances  cou ld  be
suppressed effect ively. A decrease in the saturation reading caused by breath
ho ld ing  was used as  an  example  o f  a  c l in ica l l y  re levant  change.  Th is  re levant
change was in f luenced by  th is  f i l te r  techn ique to  a  minor  degree on ly .
In  the  f ina l  chapter  there  is  a  resume o f  the  main  concepts  d iscussed in  th is  thes is .
The NLRAD descr ibed prov ides  op t ima l  de tec t ion  o f  in i t ia l  changes
(change-events )  in  a  de f inab le  t ime w indow,  be ing  insens i t i ve  fo r  no ise  and sp ikes .
Whenever  the  NLRAD exceeds a  cer ta in  l im i t ,  a  shor t  bu t  c lear  s igna l  i s  g iven .  A
reset  machan ism is  added to  p repare  the  sys tem immedia te ly  fo r  de tec t ion  o f
subsequent changes compared to the new reference.
The inf luence that such a warning system has on the dai ly work of the anesthetist
i s  a lso  descr ibed.  Prev ious ly  the  anes the t is t  had the  a lmost  imposs ib le  task  o f  be ing
a le r t  fo r  beg inn ing  changes in  the  pa t ien t ' s  phys io log ica l  var iab les  and,  a t  the  same
t ime,  admin is te r ing  med icaments ,  t rans fus ions  and o ther  therapeut ic  ac t i v i t ies .
Car ina  a l lows h im to  d iv ide  h is  a t ten t ion  more  e f f i c ien t ly ,  g iv ing  a  c lear  s igna l  tha t
i n d i c a t e s  a n y  e v e n t  r e q u i r i n g  a t t e n r i o n .
F ina l l y ,  some suggest ions  are  made fo r  fu tu re  improvements .


University of Groningen

   University of GroningenWarning systems in anesthesia. Human vigilance supported by clinically relevant warningsBallast, AlbertusIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1992Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ballast, A. (1992). Warning systems in anesthesia. Human vigilance supported by clinically relevantwarnings. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-20191 1 4SIJl\ /{ IVIAR)'Warn ing  or  a la rm func t ions  in  commerc ia l  mon i to r ing  equ ipment  genera te  manyf alse alarms, which restr ict their cl inical usef ulness. Nevertheless, warningfunctions are indispensable whenever the possibi l i ty exists that an unexpectedproblem escapes our attention.Anesthesia can be considered as a process of continuous optimization, correctingany disturbance in the desired physiological state. This process requires theanesthetist 's continuous alertness. When an init ial  disturbance escapes oura t ten t ion ,  an  op t ima l  s ta te  i s  no  longer  ma in ta ined.  When the  d is tu rbances  deve lopfur ther ,  management  becomes increas ing ly  d i f f i cu l t  and eventua l l y  permanentdamage may result.I t  i s  a  d i f f i cu l t  task  fo r  a  human be ing  to  keep a le r t  o r  v ig i lan t  con t inuous ly ,espec ia l l y  when the  inc idence o f  unexpected  prob lems is  as  smal l  as  i t  i s  inanesthesia.Mon i to r ing  equ ipment  i s  usua l ly  p rov ided w i th  a la rm func t ions  to  suppor tvigi lance. These conventional alarm systems generate an alarm whenever the valueof a measured variable exceeds a l imit that has been set previously. In this thesisd is tu rbances  tha t  a re  de tec ted  w i th  such an  a la rm sys tem are  ca l led 'va lue  events ' .The l im i ts  a re  usua l ly  se t  a t  the  phys io log ica l  l im i ts .The cont inuous  op t im iza t ion  process  o f  anes thes ia  occurs  a lmost  en t i re ly  w i th inthe  phys io log ica l  l im i ts .  D is tu rbances  wh ich  requ i re  op t im iza t ion  cannot  bedetec ted  by  a  convent iona l  a la rm sys tem unt i l  abnormal  va lues  have occur red .  Thein i t ia t ion  o f  d is tu rbances  -wh ich  requ i res  cor rec t ion-  can on ly  be  de tec ted  as  achange, or more exactly as a dif ference between present and previousmeasurements ,  In  th is  thes is  such in i t ia l  d is tu rbances  are  ca l led  ' change-events ' .A  convent iona l  a la rm sys tem,  be ing  su i tab le  fo r  de tec t ion  o f  va lue-events  on ly ,cannot  suppor t  the  cont inuous  op t im iza t ion  process  o f  anes thes ia  e f fec t i ve ly .There fore ,  there  is  a  need fo r  a  warn ing  sys tem tha t  de tec ts  change-events  longbefore values outside the normal range are measured.Several systems have been described for the detection of changes within thenormal  range.  They  compare  the  var iab le 's  p resent  va lue  w i th  a  runn ing  average,an  average o f  a  p reced ing  per iod .  Other  methods  compare  two runn ing  averagesca lcu la ted  over  a  recent  per iod  and an  ear l ie r  per iod .  The d isadvantages  o f  thesesys tems are  the i r  comple te  insens i t i v i t y  fo r  s low changes and the  pers is ten tcharacter of their alarm states.I t  has  been dec ided to  deve lop  a  warn ing  sys tem fo r  the  de tec t ion  o fchange-events ,  g iv ing  a  shor t  bu t  c lear  warn ing  whenever  necessary .  An impor tan ts tep  in  th is  deve lopment  i s  to  de termine wh ich  change-events  a re  c l in ica l l yre levant  and wor th  a  warn ing .  Both  rap id  and s low changes have to  be  cons idered,a l though they  requ i re  d i f fe ren t  de tec t ion  methods .  For  changes near  thephys io log ica l  l im i ts  an  enhanced sens i t i v i t y  i s  des i red  s ince  they  are  cons ideredmore relevant than those in the centre of the normal range. Furthermore, there levance o f  changes depends on  the i r  ampl i tude and on  pa t ien t -  o ranes the t is t - re la ted  ind iv idua l  fac to rs .A warning signal is required whenever the degree of change exceeds a certainth resho ld .  The cho ice  o f  the  th resho ld  de termines  how many warn ings  aregenera ted .  A  low th resho ld  p roduces  a  h igh  sens i t i v i t y ,  genera t ing  many warn ings ,Sunnary 1 1 5of which a large proport ion is relat ively irrelevant. A high threshold results in a lowsensit ivi ty, with the chance that some cl inical ly relevant events pass withoutgenerating a warning. Numerical ly comparable changes are cl inical ly sometimesrelevant and sometimes irrelevant depending on secondary factors. The choice ofthe threshold for the prototype was based on init ial  experience with the system andis supported by a study of the variabi l i ty of measured values and their relat ion totherapeutical intervention.The numerieal methods for the detection of change events are described in aseparate chapter. The normalized running average dif ference (NRAD) isintroduced as a qual i tat ive and quanti tat ive representation of the average changeoccur r ing  in  a  t ime w indow def ined by  two t ime-cons tan ts .  The NRAD iscalculatedas the product of a correction factor and the dif ference of two runningaverages characterized by the two time constants.Very slow changes can be detected by the Minimax method. I t  was introduced todetec t  changes tha t  occur  ou ts ide  the  t ime w indow def ined fo r  the  NRAD.  TheMinimax method compares the present value of a variable with the very largest andthe very smallest value that has occurred during the whole observation period.Fur thermore  a  change- ind ica tor  has  been in t roduced.  Th is  i s  the  ra t io  o f  theNRAD and the appropriate warning threshold. I t  may be used in the future formathematical processing of coincidental changes occurring in dif ferent variables.A rese t -mechan ism has  a lso  been in t roduced.  Th is  p revents  sus ta ined warn ingstates. As soon as a warning is given -because of a change- a new observationperiod is started, using the value measured after the change as a new referencevalue. There is no need for a sustained warning state provided the change occurredins ide  the  normal  range.  The resu l t  o f  th is  rese t -mechan ism is  tha t  t ime is  d iv idedinto periods that are assumed to be stable, but separated by moments of change.These moments of change are marked by a warning signal.Carina, the prototype of the new warning system generates warnings according tothe methods described above. The warning signal consists of a short melodioustune, completed with a strong optical signal. The latter does not disturb the surgicalteam but alerts the anesthetist effect ively. Al l  measured values can be read fromthe system's video display. In addit ion i t  is indicated which of the variables haverecently caused a warning. Trend curves of some of the measured values are shownon the display as well  as real t ime curves of the ECG and the capnogram.Furthermore, the system automatical ly writes an anesthesia record.In order to provide the reader with an impression of how the warning systemworks  in  p rac t ice  a  s ing le  anes the t ic  i s  descr ibed in  de ta i l ,  w i th  the  warn ingsgenerated by f ive dif ferent warning systems. Carina's warnings are given forchanges that an anesthetist probably recognizes as cl inical ly relevant, while aconventional warning system generates long sustained alarms, once a l imit isexceeded. For the purpose of comparison unusually narrow l imits were chosen.For further evaluation f ive studies have been performed. I t  is assumed thatopiates are given when pain is suggested by an increased heart rate or bloodpressure. As an anesthetist gives an opiate when he considers such changes to berelevant, Carina's warnings can be expected to be associated with opiate gif ts. Inthe f irst study i t  is demonstrated that hemodynamic changes are associated withopiate gif ts. I t  is concluded that opiate gif ts are indeed associated withtherapeutical ly relevant changes. The second study demonstrates that warnings aregiven for the events that correlate with opiate gif ts. However, in this study changes1 1 6 Warn ing  Sys tens  in  Anesthes iatha t  ind ica te  the  necess i ty  o f  op ia te  admin is t ra t ion  cannot  be  d is t ingu ished f romchanges resu l t ing  f rom op ia te  therapy .  A  th i rd  s tudy  was ins t iga ted  to  inves t iga te  i fwarnings were given for increases in blood pressure, as an example of changes thatindicate exclusively the necessity of opiate administrat ion.Thequest ion  ar ises  i f  a  comparab le  cor re la t ion  be tween changes and warn ings  canalso be achieved with a conventional alarm system, by choosing unusually narrowalarm l imits, Study four demonstrates that such a correlat ion can indeed beachieved. However, there are theoretical objections against such a system: manyfalse alarms are to be expected, alarm sounds wil l  have a sustained character andnot al l  relevant change-events wil l  be detected. Study f ive demonstrates that longsustained alarm periods are generated i f  a conventional alarm system is so adapted.Th is  w i l l  obv ious ly  cause much i r r i ta t ion  and a  compara t ive ly  smal l  number  o fuseful warnings wil l  be generated.The occur rence o f  fa lse  a la rms is  a  p rob lem wi th  many warn ing  sys tems.Therefore, methods were developed for suppression of two frequent causes of falsea la rms:  no ise  or  rap id  i r regu la r  f luc tua t ions ,  and sp ikes  or  la rge  t rans ien tf luctuations. False alarms caused by noise can be suppressed by using a runningaverage over a short period instead of the measured value as such as the input forthe  de tec t ion  sys tem.  Th is  works  as  a  low-pass  f i l te r  and is  a  common method fo rsuppression of noise. I t  is also used in the prototype.For  the  suppress ion  o f  sp ikes ,  a  low-pass  f i l te r  i s  insu f f i c ien t  because the  h ighampl i tude o f  the  sp ikes  cannot  be  a t tenuated  su f f i c ien t ly  by  the  f i l te r .  A  new f i l te rtechn ique was,  there fore ,  in t roduced.  Th is  l im i ts  the  speed o f  inc rease o f  avar iab le ,  be fore  i t  i s  fed  in to  the  de tec t ion  sys tem.  I t  la rge ly  reduces  the  ampl i tudeof  sp ikes ,  bu t  phys io log ica l  changes w i l l  pass  such a  f i l te r  a lmost  unaf fec ted  s incethey  occur  more  s lowly  than sp ikes  do .  The resu l t  o f  such a  f i l te r ing  is  ca l led  al im i ted  runn ing  average (LRA) .  The normal ized  l im i ted  runn ing  averaged i f fe rence (NLRAD)  is  the  normal ized  d i f fe rence be tween two LRA's ,  and can beused fo r  de tec t ion  o f  changes ins tead o f  the  NRAD.An example  o f  sp ike  in te r fe rence is  a f fo rded by  the  d is tu rbances  in  pu lseoximeter readings caused by interruption of the blood f low to the arm when a bloodcuf f  i s  in f la ted  on  the  same arm.  In  a  vo lun teer  such d is tu rbances  cou ld  besuppressed effect ively. A decrease in the saturation reading caused by breathho ld ing  was used as  an  example  o f  a  c l in ica l l y  re levant  change.  Th is  re levantchange was in f luenced by  th is  f i l te r  techn ique to  a  minor  degree on ly .In  the  f ina l  chapter  there  is  a  resume o f  the  main  concepts  d iscussed in  th is  thes is .The NLRAD descr ibed prov ides  op t ima l  de tec t ion  o f  in i t ia l  changes(change-events )  in  a  de f inab le  t ime w indow,  be ing  insens i t i ve  fo r  no ise  and sp ikes .Whenever  the  NLRAD exceeds a  cer ta in  l im i t ,  a  shor t  bu t  c lear  s igna l  i s  g iven .  Areset  machan ism is  added to  p repare  the  sys tem immedia te ly  fo r  de tec t ion  o fsubsequent changes compared to the new reference.The inf luence that such a warning system has on the dai ly work of the anesthetisti s  a lso  descr ibed.  Prev ious ly  the  anes the t is t  had the  a lmost  imposs ib le  task  o f  be inga le r t  fo r  beg inn ing  changes in  the  pa t ien t ' s  phys io log ica l  var iab les  and,  a t  the  samet ime,  admin is te r ing  med icaments ,  t rans fus ions  and o ther  therapeut ic  ac t i v i t ies .Car ina  a l lows h im to  d iv ide  h is  a t ten t ion  more  e f f i c ien t ly ,  g iv ing  a  c lear  s igna l  tha ti n d i c a t e s  a n y  e v e n t  r e q u i r i n g  a t t e n r i o n .F ina l l y ,  some suggest ions  are  made fo r  fu tu re  improvements .

ARTS repository - University of Groningen

   University of GroningenWarning systems in anesthesia. Human vigilance supported by clinically relevant warningsBallast, AlbertusIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1992Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ballast, A. (1992). Warning systems in anesthesia. Human vigilance supported by clinically relevantwarnings. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-06-20221 1 4SIJl\ /{ IVIAR)'Warn ing  or  a la rm func t ions  in  commerc ia l  mon i to r ing  equ ipment  genera te  manyf alse alarms, which restr ict their cl inical usef ulness. Nevertheless, warningfunctions are indispensable whenever the possibi l i ty exists that an unexpectedproblem escapes our attention.Anesthesia can be considered as a process of continuous optimization, correctingany disturbance in the desired physiological state. This process requires theanesthetist 's continuous alertness. When an init ial  disturbance escapes oura t ten t ion ,  an  op t ima l  s ta te  i s  no  longer  ma in ta ined.  When the  d is tu rbances  deve lopfur ther ,  management  becomes increas ing ly  d i f f i cu l t  and eventua l l y  permanentdamage may result.I t  i s  a  d i f f i cu l t  task  fo r  a  human be ing  to  keep a le r t  o r  v ig i lan t  con t inuous ly ,espec ia l l y  when the  inc idence o f  unexpected  prob lems is  as  smal l  as  i t  i s  inanesthesia.Mon i to r ing  equ ipment  i s  usua l ly  p rov ided w i th  a la rm func t ions  to  suppor tvigi lance. These conventional alarm systems generate an alarm whenever the valueof a measured variable exceeds a l imit that has been set previously. In this thesisd is tu rbances  tha t  a re  de tec ted  w i th  such an  a la rm sys tem are  ca l led 'va lue  events ' .The l im i ts  a re  usua l ly  se t  a t  the  phys io log ica l  l im i ts .The cont inuous  op t im iza t ion  process  o f  anes thes ia  occurs  a lmost  en t i re ly  w i th inthe  phys io log ica l  l im i ts .  D is tu rbances  wh ich  requ i re  op t im iza t ion  cannot  bedetec ted  by  a  convent iona l  a la rm sys tem unt i l  abnormal  va lues  have occur red .  Thein i t ia t ion  o f  d is tu rbances  -wh ich  requ i res  cor rec t ion-  can on ly  be  de tec ted  as  achange, or more exactly as a dif ference between present and previousmeasurements ,  In  th is  thes is  such in i t ia l  d is tu rbances  are  ca l led  ' change-events ' .A  convent iona l  a la rm sys tem,  be ing  su i tab le  fo r  de tec t ion  o f  va lue-events  on ly ,cannot  suppor t  the  cont inuous  op t im iza t ion  process  o f  anes thes ia  e f fec t i ve ly .There fore ,  there  is  a  need fo r  a  warn ing  sys tem tha t  de tec ts  change-events  longbefore values outside the normal range are measured.Several systems have been described for the detection of changes within thenormal  range.  They  compare  the  var iab le 's  p resent  va lue  w i th  a  runn ing  average,an  average o f  a  p reced ing  per iod .  Other  methods  compare  two runn ing  averagesca lcu la ted  over  a  recent  per iod  and an  ear l ie r  per iod .  The d isadvantages  o f  thesesys tems are  the i r  comple te  insens i t i v i t y  fo r  s low changes and the  pers is ten tcharacter of their alarm states.I t  has  been dec ided to  deve lop  a  warn ing  sys tem fo r  the  de tec t ion  o fchange-events ,  g iv ing  a  shor t  bu t  c lear  warn ing  whenever  necessary .  An impor tan ts tep  in  th is  deve lopment  i s  to  de termine wh ich  change-events  a re  c l in ica l l yre levant  and wor th  a  warn ing .  Both  rap id  and s low changes have to  be  cons idered,a l though they  requ i re  d i f fe ren t  de tec t ion  methods .  For  changes near  thephys io log ica l  l im i ts  an  enhanced sens i t i v i t y  i s  des i red  s ince  they  are  cons ideredmore relevant than those in the centre of the normal range. Furthermore, there levance o f  changes depends on  the i r  ampl i tude and on  pa t ien t -  o ranes the t is t - re la ted  ind iv idua l  fac to rs .A warning signal is required whenever the degree of change exceeds a certainth resho ld .  The cho ice  o f  the  th resho ld  de termines  how many warn ings  aregenera ted .  A  low th resho ld  p roduces  a  h igh  sens i t i v i t y ,  genera t ing  many warn ings ,Sunnary 1 1 5of which a large proport ion is relat ively irrelevant. A high threshold results in a lowsensit ivi ty, with the chance that some cl inical ly relevant events pass withoutgenerating a warning. Numerical ly comparable changes are cl inical ly sometimesrelevant and sometimes irrelevant depending on secondary factors. The choice ofthe threshold for the prototype was based on init ial  experience with the system andis supported by a study of the variabi l i ty of measured values and their relat ion totherapeutical intervention.The numerieal methods for the detection of change events are described in aseparate chapter. The normalized running average dif ference (NRAD) isintroduced as a qual i tat ive and quanti tat ive representation of the average changeoccur r ing  in  a  t ime w indow def ined by  two t ime-cons tan ts .  The NRAD iscalculatedas the product of a correction factor and the dif ference of two runningaverages characterized by the two time constants.Very slow changes can be detected by the Minimax method. I t  was introduced todetec t  changes tha t  occur  ou ts ide  the  t ime w indow def ined fo r  the  NRAD.  TheMinimax method compares the present value of a variable with the very largest andthe very smallest value that has occurred during the whole observation period.Fur thermore  a  change- ind ica tor  has  been in t roduced.  Th is  i s  the  ra t io  o f  theNRAD and the appropriate warning threshold. I t  may be used in the future formathematical processing of coincidental changes occurring in dif ferent variables.A rese t -mechan ism has  a lso  been in t roduced.  Th is  p revents  sus ta ined warn ingstates. As soon as a warning is given -because of a change- a new observationperiod is started, using the value measured after the change as a new referencevalue. There is no need for a sustained warning state provided the change occurredins ide  the  normal  range.  The resu l t  o f  th is  rese t -mechan ism is  tha t  t ime is  d iv idedinto periods that are assumed to be stable, but separated by moments of change.These moments of change are marked by a warning signal.Carina, the prototype of the new warning system generates warnings according tothe methods described above. The warning signal consists of a short melodioustune, completed with a strong optical signal. The latter does not disturb the surgicalteam but alerts the anesthetist effect ively. Al l  measured values can be read fromthe system's video display. In addit ion i t  is indicated which of the variables haverecently caused a warning. Trend curves of some of the measured values are shownon the display as well  as real t ime curves of the ECG and the capnogram.Furthermore, the system automatical ly writes an anesthesia record.In order to provide the reader with an impression of how the warning systemworks  in  p rac t ice  a  s ing le  anes the t ic  i s  descr ibed in  de ta i l ,  w i th  the  warn ingsgenerated by f ive dif ferent warning systems. Carina's warnings are given forchanges that an anesthetist probably recognizes as cl inical ly relevant, while aconventional warning system generates long sustained alarms, once a l imit isexceeded. For the purpose of comparison unusually narrow l imits were chosen.For further evaluation f ive studies have been performed. I t  is assumed thatopiates are given when pain is suggested by an increased heart rate or bloodpressure. As an anesthetist gives an opiate when he considers such changes to berelevant, Carina's warnings can be expected to be associated with opiate gif ts. Inthe f irst study i t  is demonstrated that hemodynamic changes are associated withopiate gif ts. I t  is concluded that opiate gif ts are indeed associated withtherapeutical ly relevant changes. The second study demonstrates that warnings aregiven for the events that correlate with opiate gif ts. However, in this study changes1 1 6 Warn ing  Sys tens  in  Anesthes iatha t  ind ica te  the  necess i ty  o f  op ia te  admin is t ra t ion  cannot  be  d is t ingu ished f romchanges resu l t ing  f rom op ia te  therapy .  A  th i rd  s tudy  was ins t iga ted  to  inves t iga te  i fwarnings were given for increases in blood pressure, as an example of changes thatindicate exclusively the necessity of opiate administrat ion.Thequest ion  ar ises  i f  a  comparab le  cor re la t ion  be tween changes and warn ings  canalso be achieved with a conventional alarm system, by choosing unusually narrowalarm l imits, Study four demonstrates that such a correlat ion can indeed beachieved. However, there are theoretical objections against such a system: manyfalse alarms are to be expected, alarm sounds wil l  have a sustained character andnot al l  relevant change-events wil l  be detected. Study f ive demonstrates that longsustained alarm periods are generated i f  a conventional alarm system is so adapted.Th is  w i l l  obv ious ly  cause much i r r i ta t ion  and a  compara t ive ly  smal l  number  o fuseful warnings wil l  be generated.The occur rence o f  fa lse  a la rms is  a  p rob lem wi th  many warn ing  sys tems.Therefore, methods were developed for suppression of two frequent causes of falsea la rms:  no ise  or  rap id  i r regu la r  f luc tua t ions ,  and sp ikes  or  la rge  t rans ien tf luctuations. False alarms caused by noise can be suppressed by using a runningaverage over a short period instead of the measured value as such as the input forthe  de tec t ion  sys tem.  Th is  works  as  a  low-pass  f i l te r  and is  a  common method fo rsuppression of noise. I t  is also used in the prototype.For  the  suppress ion  o f  sp ikes ,  a  low-pass  f i l te r  i s  insu f f i c ien t  because the  h ighampl i tude o f  the  sp ikes  cannot  be  a t tenuated  su f f i c ien t ly  by  the  f i l te r .  A  new f i l te rtechn ique was,  there fore ,  in t roduced.  Th is  l im i ts  the  speed o f  inc rease o f  avar iab le ,  be fore  i t  i s  fed  in to  the  de tec t ion  sys tem.  I t  la rge ly  reduces  the  ampl i tudeof  sp ikes ,  bu t  phys io log ica l  changes w i l l  pass  such a  f i l te r  a lmost  unaf fec ted  s incethey  occur  more  s lowly  than sp ikes  do .  The resu l t  o f  such a  f i l te r ing  is  ca l led  al im i ted  runn ing  average (LRA) .  The normal ized  l im i ted  runn ing  averaged i f fe rence (NLRAD)  is  the  normal ized  d i f fe rence be tween two LRA's ,  and can beused fo r  de tec t ion  o f  changes ins tead o f  the  NRAD.An example  o f  sp ike  in te r fe rence is  a f fo rded by  the  d is tu rbances  in  pu lseoximeter readings caused by interruption of the blood f low to the arm when a bloodcuf f  i s  in f la ted  on  the  same arm.  In  a  vo lun teer  such d is tu rbances  cou ld  besuppressed effect ively. A decrease in the saturation reading caused by breathho ld ing  was used as  an  example  o f  a  c l in ica l l y  re levant  change.  Th is  re levantchange was in f luenced by  th is  f i l te r  techn ique to  a  minor  degree on ly .In  the  f ina l  chapter  there  is  a  resume o f  the  main  concepts  d iscussed in  th is  thes is .The NLRAD descr ibed prov ides  op t ima l  de tec t ion  o f  in i t ia l  changes(change-events )  in  a  de f inab le  t ime w indow,  be ing  insens i t i ve  fo r  no ise  and sp ikes .Whenever  the  NLRAD exceeds a  cer ta in  l im i t ,  a  shor t  bu t  c lear  s igna l  i s  g iven .  Areset  machan ism is  added to  p repare  the  sys tem immedia te ly  fo r  de tec t ion  o fsubsequent changes compared to the new reference.The inf luence that such a warning system has on the dai ly work of the anesthetisti s  a lso  descr ibed.  Prev ious ly  the  anes the t is t  had the  a lmost  imposs ib le  task  o f  be inga le r t  fo r  beg inn ing  changes in  the  pa t ien t ' s  phys io log ica l  var iab les  and,  a t  the  samet ime,  admin is te r ing  med icaments ,  t rans fus ions  and o ther  therapeut ic  ac t i v i t ies .Car ina  a l lows h im to  d iv ide  h is  a t ten t ion  more  e f f i c ien t ly ,  g iv ing  a  c lear  s igna l  tha ti n d i c a t e s  a n y  e v e n t  r e q u i r i n g  a t t e n r i o n .F ina l l y ,  some suggest ions  are  made fo r  fu tu re  improvements .

English

NARCIS 

   University of GroningenWarning systems in anesthesia. Human vigilance supported by clinically relevant warningsBallast, AlbertusIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1992Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ballast, A. (1992). Warning systems in anesthesia. Human vigilance supported by clinically relevantwarnings. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 09-09-20231 1 4SIJl\ /{ IVIAR)'Warn ing  or  a la rm func t ions  in  commerc ia l  mon i to r ing  equ ipment  genera te  manyf alse alarms, which restr ict their cl inical usef ulness. Nevertheless, warningfunctions are indispensable whenever the possibi l i ty exists that an unexpectedproblem escapes our attention.Anesthesia can be considered as a process of continuous optimization, correctingany disturbance in the desired physiological state. This process requires theanesthetist 's continuous alertness. When an init ial  disturbance escapes oura t ten t ion ,  an  op t ima l  s ta te  i s  no  longer  ma in ta ined.  When the  d is tu rbances  deve lopfur ther ,  management  becomes increas ing ly  d i f f i cu l t  and eventua l l y  permanentdamage may result.I t  i s  a  d i f f i cu l t  task  fo r  a  human be ing  to  keep a le r t  o r  v ig i lan t  con t inuous ly ,espec ia l l y  when the  inc idence o f  unexpected  prob lems is  as  smal l  as  i t  i s  inanesthesia.Mon i to r ing  equ ipment  i s  usua l ly  p rov ided w i th  a la rm func t ions  to  suppor tvigi lance. These conventional alarm systems generate an alarm whenever the valueof a measured variable exceeds a l imit that has been set previously. In this thesisd is tu rbances  tha t  a re  de tec ted  w i th  such an  a la rm sys tem are  ca l led 'va lue  events ' .The l im i ts  a re  usua l ly  se t  a t  the  phys io log ica l  l im i ts .The cont inuous  op t im iza t ion  process  o f  anes thes ia  occurs  a lmost  en t i re ly  w i th inthe  phys io log ica l  l im i ts .  D is tu rbances  wh ich  requ i re  op t im iza t ion  cannot  bedetec ted  by  a  convent iona l  a la rm sys tem unt i l  abnormal  va lues  have occur red .  Thein i t ia t ion  o f  d is tu rbances  -wh ich  requ i res  cor rec t ion-  can on ly  be  de tec ted  as  achange, or more exactly as a dif ference between present and previousmeasurements ,  In  th is  thes is  such in i t ia l  d is tu rbances  are  ca l led  ' change-events ' .A  convent iona l  a la rm sys tem,  be ing  su i tab le  fo r  de tec t ion  o f  va lue-events  on ly ,cannot  suppor t  the  cont inuous  op t im iza t ion  process  o f  anes thes ia  e f fec t i ve ly .There fore ,  there  is  a  need fo r  a  warn ing  sys tem tha t  de tec ts  change-events  longbefore values outside the normal range are measured.Several systems have been described for the detection of changes within thenormal  range.  They  compare  the  var iab le 's  p resent  va lue  w i th  a  runn ing  average,an  average o f  a  p reced ing  per iod .  Other  methods  compare  two runn ing  averagesca lcu la ted  over  a  recent  per iod  and an  ear l ie r  per iod .  The d isadvantages  o f  thesesys tems are  the i r  comple te  insens i t i v i t y  fo r  s low changes and the  pers is ten tcharacter of their alarm states.I t  has  been dec ided to  deve lop  a  warn ing  sys tem fo r  the  de tec t ion  o fchange-events ,  g iv ing  a  shor t  bu t  c lear  warn ing  whenever  necessary .  An impor tan ts tep  in  th is  deve lopment  i s  to  de termine wh ich  change-events  a re  c l in ica l l yre levant  and wor th  a  warn ing .  Both  rap id  and s low changes have to  be  cons idered,a l though they  requ i re  d i f fe ren t  de tec t ion  methods .  For  changes near  thephys io log ica l  l im i ts  an  enhanced sens i t i v i t y  i s  des i red  s ince  they  are  cons ideredmore relevant than those in the centre of the normal range. Furthermore, there levance o f  changes depends on  the i r  ampl i tude and on  pa t ien t -  o ranes the t is t - re la ted  ind iv idua l  fac to rs .A warning signal is required whenever the degree of change exceeds a certainth resho ld .  The cho ice  o f  the  th resho ld  de termines  how many warn ings  aregenera ted .  A  low th resho ld  p roduces  a  h igh  sens i t i v i t y ,  genera t ing  many warn ings ,Sunnary 1 1 5of which a large proport ion is relat ively irrelevant. A high threshold results in a lowsensit ivi ty, with the chance that some cl inical ly relevant events pass withoutgenerating a warning. Numerical ly comparable changes are cl inical ly sometimesrelevant and sometimes irrelevant depending on secondary factors. The choice ofthe threshold for the prototype was based on init ial  experience with the system andis supported by a study of the variabi l i ty of measured values and their relat ion totherapeutical intervention.The numerieal methods for the detection of change events are described in aseparate chapter. The normalized running average dif ference (NRAD) isintroduced as a qual i tat ive and quanti tat ive representation of the average changeoccur r ing  in  a  t ime w indow def ined by  two t ime-cons tan ts .  The NRAD iscalculatedas the product of a correction factor and the dif ference of two runningaverages characterized by the two time constants.Very slow changes can be detected by the Minimax method. I t  was introduced todetec t  changes tha t  occur  ou ts ide  the  t ime w indow def ined fo r  the  NRAD.  TheMinimax method compares the present value of a variable with the very largest andthe very smallest value that has occurred during the whole observation period.Fur thermore  a  change- ind ica tor  has  been in t roduced.  Th is  i s  the  ra t io  o f  theNRAD and the appropriate warning threshold. I t  may be used in the future formathematical processing of coincidental changes occurring in dif ferent variables.A rese t -mechan ism has  a lso  been in t roduced.  Th is  p revents  sus ta ined warn ingstates. As soon as a warning is given -because of a change- a new observationperiod is started, using the value measured after the change as a new referencevalue. There is no need for a sustained warning state provided the change occurredins ide  the  normal  range.  The resu l t  o f  th is  rese t -mechan ism is  tha t  t ime is  d iv idedinto periods that are assumed to be stable, but separated by moments of change.These moments of change are marked by a warning signal.Carina, the prototype of the new warning system generates warnings according tothe methods described above. The warning signal consists of a short melodioustune, completed with a strong optical signal. The latter does not disturb the surgicalteam but alerts the anesthetist effect ively. Al l  measured values can be read fromthe system's video display. In addit ion i t  is indicated which of the variables haverecently caused a warning. Trend curves of some of the measured values are shownon the display as well  as real t ime curves of the ECG and the capnogram.Furthermore, the system automatical ly writes an anesthesia record.In order to provide the reader with an impression of how the warning systemworks  in  p rac t ice  a  s ing le  anes the t ic  i s  descr ibed in  de ta i l ,  w i th  the  warn ingsgenerated by f ive dif ferent warning systems. Carina's warnings are given forchanges that an anesthetist probably recognizes as cl inical ly relevant, while aconventional warning system generates long sustained alarms, once a l imit isexceeded. For the purpose of comparison unusually narrow l imits were chosen.For further evaluation f ive studies have been performed. I t  is assumed thatopiates are given when pain is suggested by an increased heart rate or bloodpressure. As an anesthetist gives an opiate when he considers such changes to berelevant, Carina's warnings can be expected to be associated with opiate gif ts. Inthe f irst study i t  is demonstrated that hemodynamic changes are associated withopiate gif ts. I t  is concluded that opiate gif ts are indeed associated withtherapeutical ly relevant changes. The second study demonstrates that warnings aregiven for the events that correlate with opiate gif ts. However, in this study changes1 1 6 Warn ing  Sys tens  in  Anesthes iatha t  ind ica te  the  necess i ty  o f  op ia te  admin is t ra t ion  cannot  be  d is t ingu ished f romchanges resu l t ing  f rom op ia te  therapy .  A  th i rd  s tudy  was ins t iga ted  to  inves t iga te  i fwarnings were given for increases in blood pressure, as an example of changes thatindicate exclusively the necessity of opiate administrat ion.Thequest ion  ar ises  i f  a  comparab le  cor re la t ion  be tween changes and warn ings  canalso be achieved with a conventional alarm system, by choosing unusually narrowalarm l imits, Study four demonstrates that such a correlat ion can indeed beachieved. However, there are theoretical objections against such a system: manyfalse alarms are to be expected, alarm sounds wil l  have a sustained character andnot al l  relevant change-events wil l  be detected. Study f ive demonstrates that longsustained alarm periods are generated i f  a conventional alarm system is so adapted.Th is  w i l l  obv ious ly  cause much i r r i ta t ion  and a  compara t ive ly  smal l  number  o fuseful warnings wil l  be generated.The occur rence o f  fa lse  a la rms is  a  p rob lem wi th  many warn ing  sys tems.Therefore, methods were developed for suppression of two frequent causes of falsea la rms:  no ise  or  rap id  i r regu la r  f luc tua t ions ,  and sp ikes  or  la rge  t rans ien tf luctuations. False alarms caused by noise can be suppressed by using a runningaverage over a short period instead of the measured value as such as the input forthe  de tec t ion  sys tem.  Th is  works  as  a  low-pass  f i l te r  and is  a  common method fo rsuppression of noise. I t  is also used in the prototype.For  the  suppress ion  o f  sp ikes ,  a  low-pass  f i l te r  i s  insu f f i c ien t  because the  h ighampl i tude o f  the  sp ikes  cannot  be  a t tenuated  su f f i c ien t ly  by  the  f i l te r .  A  new f i l te rtechn ique was,  there fore ,  in t roduced.  Th is  l im i ts  the  speed o f  inc rease o f  avar iab le ,  be fore  i t  i s  fed  in to  the  de tec t ion  sys tem.  I t  la rge ly  reduces  the  ampl i tudeof  sp ikes ,  bu t  phys io log ica l  changes w i l l  pass  such a  f i l te r  a lmost  unaf fec ted  s incethey  occur  more  s lowly  than sp ikes  do .  The resu l t  o f  such a  f i l te r ing  is  ca l led  al im i ted  runn ing  average (LRA) .  The normal ized  l im i ted  runn ing  averaged i f fe rence (NLRAD)  is  the  normal ized  d i f fe rence be tween two LRA's ,  and can beused fo r  de tec t ion  o f  changes ins tead o f  the  NRAD.An example  o f  sp ike  in te r fe rence is  a f fo rded by  the  d is tu rbances  in  pu lseoximeter readings caused by interruption of the blood f low to the arm when a bloodcuf f  i s  in f la ted  on  the  same arm.  In  a  vo lun teer  such d is tu rbances  cou ld  besuppressed effect ively. A decrease in the saturation reading caused by breathho ld ing  was used as  an  example  o f  a  c l in ica l l y  re levant  change.  Th is  re levantchange was in f luenced by  th is  f i l te r  techn ique to  a  minor  degree on ly .In  the  f ina l  chapter  there  is  a  resume o f  the  main  concepts  d iscussed in  th is  thes is .The NLRAD descr ibed prov ides  op t ima l  de tec t ion  o f  in i t ia l  changes(change-events )  in  a  de f inab le  t ime w indow,  be ing  insens i t i ve  fo r  no ise  and sp ikes .Whenever  the  NLRAD exceeds a  cer ta in  l im i t ,  a  shor t  bu t  c lear  s igna l  i s  g iven .  Areset  machan ism is  added to  p repare  the  sys tem immedia te ly  fo r  de tec t ion  o fsubsequent changes compared to the new reference.The inf luence that such a warning system has on the dai ly work of the anesthetisti s  a lso  descr ibed.  Prev ious ly  the  anes the t is t  had the  a lmost  imposs ib le  task  o f  be inga le r t  fo r  beg inn ing  changes in  the  pa t ien t ' s  phys io log ica l  var iab les  and,  a t  the  samet ime,  admin is te r ing  med icaments ,  t rans fus ions  and o ther  therapeut ic  ac t i v i t ies .Car ina  a l lows h im to  d iv ide  h is  a t ten t ion  more  e f f i c ien t ly ,  g iv ing  a  c lear  s igna l  tha ti n d i c a t e s  a n y  e v e n t  r e q u i r i n g  a t t e n r i o n .F ina l l y ,  some suggest ions  are  made fo r  fu tu re  improvements .

Warning systems in anesthesia. Human vigilance supported by clinically relevant warnings

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