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Faisal Mohammed

Ammar Ramadan

Ghufran Touma

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Recall:

TheQRSistherecordingofthedepolarizationoftheventricles.Duringventriculardepolarization,everyinstanttheisavector,whichistheresultantofallvectors(rememberthatthevectorgoesfromthedepolarizedareatothestillpolarizedarea).

InfigureA,thisisthedepolarizationoftheseptum.Asyoucanseethereisavectorwhichistheresultantofallvectorsatthatinstant.Howcanweknowitsvalueonanylead?

Simplywedrawaperpendicularlinefromthetopofthisvectortointersectwiththeplaneofanylead,herewecanseeleadI,leadII,andleadIII.

YoucanmeasureQRSatanyinstant,butherewetook4instances;ineachoneofthemwehavetheresultantvectoranditsvalueonleadI,leadIIandleadIII.

Figureb,afteranalysingthevector,youcanseethatthevaluehasincreasedinleadI,leadIIandleadIII.Whenhalfoftheventricleisdepolarized,thevectorwillhavethehighestvalue.

Figurec(whenthedepolarizationspreadstoventricles),afteranalysingthevector,youcanseethatthevaluehasdecreasedinleadI,leadIIandleadIII.

Figured,thelastpartoftheheartthatgetsdepolarizedistheposterioraspectoftheleftventricle.Themeanelectricalaxis(theresultantvector)atthatinstantisgoingtobereflected.Ifyouanalysedthevector,youcanseethatithasanegativevalueonbothleadIIIandleadIIandadecreasedvalueonleadI.

**thevalues(oftheanalysisoftheinstantaneousvectorsonleadI,II,III)arewhatgivestheQRScomplexitsshape.

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**YoucanalsoanalysetheTwavesandseehowitoccurs,buthereyouwillseethemeanelectricalaxisofventricularrepolarization.Thedoctordidn’tsaymuchaboutit,youcanrefertoslide50forfurtherdetails.

**AtrialrepolarizationwavesorAtrialTwavescannotbeseen,becauseitismaskedbyQRScomplex.Itmayappearsinthecaseofcompleteheartblock(whenPRintervalislong).Ifitappeared,itwouldhaveadownwarddeflection.Why?

Becausethepressureintheatriaisn’tashighasthepressureintheventricle.Thehighpressureintheventriclecausedareverseintherepolarization(firstareatodepolarizeisthelastareatorepolarize).

Note:aftertheventricleiscompletelydepolarized,itwillgobacktotheisoelectricline.Aftertheventricleiscompletelyrepolarized,itwillgobacktotheisoelectricline.

Howtocalculatethemeanelectricalaxis:

●Useany2differentleads.Forthesimplicity,useleadIandAVFastheyareperpendiculartoeachother.Wecalculatethemeanelectricalaxisinthefrontalplaneonly(wedon’tusethechestleads).InexamplebelowweusedleadIandleadIII

●MeasurethesumoftheheightandthenegativedepthoftheQRScomplexandmarkitontheplaneofthelead:+13inleadI,and+9intheleadIII.

●Drawaperpendicularline(perpendiculartotheplaneofthelead)onthevaluesthatyoumarked.

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●Thetwoperpendicularlineswilleventuallyintersect.Afterthat,drawalinefromthecentreofthecircletothepointofintersection.

●nowyoucancalculatethevalueofthemeanelectricalaxisandtheangle.Heretheangleequals58°>>normalbecauseitisbetween-30and110°.Pleasecheckslide54andslide55forextraexamples.

*InthisfigureweusedleadIandAVFforthesimplicity.

*youcancalculatetheanglebypressingshift+tan(valueonAVF/valueonleadI).Becausethesetwoleadsareperpendiculartoeachother,itiseasiertocalculatetheangle.

*Theangleofthemeanelectricalaxisθ“moreexplanation“

tanθ=Opposite/Adjacent;theoppositeisaVFandtheadjacentisleadI

θ=tan-1(aVFvalue/leadIvalue)

Afterlookingatthefigureabove:

•IfBothleadIandAVFwerepositive>>thisisnormal

•IfAVFwasnegativeandleadIwaspositive>>leftaxisdeviation

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1-Thisiscausednormallyinshortandobesepeople>>theheartisshiftedtotheleft.

2-Causedalsobyhypertrophyintheleftventriclecausedbyhypertension,aorticstenosisoraorticregurgitation.How?Leftaxisdeviationmeansthatmostofthetimethevectorsaregoingfromtheright(depolarized)totheleft(stillpolarized).Becausetheventricleishypertrophic,itwillneedmoretimetobedepolarized.Thusthevectorswillpointtowardstheleftmostofthetime.

3- Leftbundlebranchblock.How?Iftheleftbundlebranchwasblockedthedepolarizationintheventricledoesn’toccurfast;becausetherateofconductionintheventricularmusclesisslowercomparedtothebranch.However,thedepolarizationintherightventricleisnormal.Thusthevectorswillpointtotheleft.

•IfAVFwaspositiveandleadIwasnegative>>rightaxisdeviation

1- Causednormallyintallandthinpeople>>Becausetheheartisshiftedtotheright.Hihamzeh:p

2- Rightbundlebranchblock3- Hypertrophyintherightventriclecausedbypulmonary

hypertension,pulmonaryvalvestenosis.

•IfbothleadIandAVFwerenegative>>severerightorleftaxisdeviation.Youdecideeitherrightorleftdependingonthequestions(history)thatyouasktoyourpatient.Ifyourpatienthasrightheartproblems,thepatienthassevererightaxisdeviationandviceversa.

HeartRateCalculation

Iftheheartratewasregular,youtakeonecycle(R-Rinterval)andcalculateitstime>>timeforonecycle.Thenyoudivide60secbytimeforonecycletogettheheartrateperminute.

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IntheECGwecareabout3intervals:

PRinterval:fromthestartofPtothestartofQorR.

QRSinterval:fromthebeginningofQtotheendofS.

QTinterval:fromthebeginningofQtotheendofT.

Isoelectriclines:

-PRsegment:fromtheendofPtothebeginningofRorQ

-STsegment:fromtheendofStothebeginningofT

-ThemostimportantisoelectriclineistheTPsegment.

**rarelyaUwaveappearintheTPsegment,duetodepolarizationofthepapillarymuscle.

**Regularityoftheheartratecouldberegularorirregular:

-Theregularheartrateisexplainedabove.

-Theirregularheartratemightbe:

regularirregularity:incaseof2nddegreeheartblock.

irregularirregularity:incaseof3rddegreeheartblock.

☆ThefirstthingyoudowhenyouhaveanECG;lookattherate(regularornot)thencalculateit.Ifitwas<40thenthisis3rddegreeheartblock.

Toknowiftheheartrateisregularornot.Takeapaperanddrawtwopointsonit;oneononeRandtheotheronthenextR.SlidethepaperontheECGtothenextcycle,ifthe2pointsstoppedexactlyorcloseonthetwoRthenitisregular.

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Abnormalities(Arrhythmias)CausesofCardiacArrythmias

▪Abnormalrhythmicityofthepacemaker

▪Shiftofpacemakerfromsinusnode

▪Blocksatdifferentpointsinthetransmissionofthecardiacimpulse

▪Abnormalpathwaysoftransmissionintheheart;bundlebranchblockintherightsidewillleadtorightaxisdeviationbecausetheimpulsewillbeconductedattherateoftheventricularmuscleswhichisslower.

▪Spontaneousgenerationofabnormalimpulsesfromanypartoftheheart;PurkinjifibresincaseofMIstartstogenerateimpulsesatrateof140perminuteandbecometheectopicpacemaker.

AbnormalSinusRhythms

Tachycardiawhenheartrateisabove100beats/minute(thenormalbetween60and100).Itiscausedby:

(1)increasedbodytemperature;especiallyinchildren(fever).Heartratemightreachupto120and150beats/minute.Sotherewillbeproblemsinfillingoftheheartandwillendinheartfailure.

(2)sympatheticstimulation(Whenyouareinastateofstress)

(3)toxicconditionsoftheheart;hypercalcemia,hyperthyroidism,anddrugs.

Sinustachycardia:thereisPwavefollowedbyQRSfollowedbyTwavebutathigherrate.

Etiology:SAnodeisdepolarizingfasterthannormal:

**sinustachycardiaisaresponsetophysicalorpsychologicalstress,notaprimaryarrhythmia.

Tocalculatetheheartrate:ifitwasregularyoucancalculateitsimplyaswediscusseditpreviously.However,ifitwasn’tregular;youcounthowmanycyclesin20secondsforexamplethenyoumultiplyby3togettheheartrateperminute,oryoucountfor15secondsandmultiplyby

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4.Rememberthateachminutehas1500smallsquaresor300bigsquares,andeachsecondhas5bigsquaresor25smallsquares.

AfastermethodfromDrFaisal:youlookforonecyclethathastheRonahardline,ifthenextRislocatedonthenexthardlinethentherateis300beats/minutewhy?Becausethedistancebetweentwoadjacenthardlinesequals5smallsquaresor1bigsquare,whichmeansthateachcycledurationis0.2secondsandtherate=60/0.2=300.

WhatisheartrateifoneRoccursonethefirsthardlineandthesecondRoccursonthe:

-3rdhardline?

-4thhardline?

Bradycardia:whentheheartrateisbelow60beats/min.

Presentsnormallyinathleteswhohavehypertrophyintheirhearts,sotheirheartshavealargestrokevolume,whichmeansthattheypumpmorebloodperbeat.

Cardiacoutput=strokevolume*heartrate

Strokevolume:amountofbloodejectedfromeitherventriclesperbeat

Normalcardiacoutputisaround5litresperminute.(70heartbeatsin1minuteand70mlstrokevolume)

Astheyhavehighstrokevolumelet’ssay100ml,theywillhavelowerheartrateatphysiologicalstates,about50heartbeatsperminute.Toachievethenormalcardiacoutput.sotheyhavebradycardiaatphysiologicalstates.

Sinusbradycardia:

NoticethatPRintervalislessthan0.2>>whichmeansthatthisisanormalPRintervalnotaheartblock.

Etiology:SAnodeisdepolarizingslowerthannormal,impulseisconductednormally(i.e.normalPRandQRSinterval)rateisslowerthan60beatsperminute.

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figureashowsasinusrhythm:alternativeP,QRS,andTwaves.

whatistheheartrateinthiscase?

InfigurebthereisnoSAnodeactivity,thisiscalledatrialfibrillation.

ThereisnoPwaves.

SinoatrialBlock

Inrarecases,impulsesfromS-Anodeareblocked.ThiscausescessationofPwaves(noPwaves).Thenewpacemakerisregionofheartwiththefastestdischargerate,usuallytheA-Vnode(ectopicpacemaker).

Figure(c)showsheartblock.WehaveheartblockbecausethePRintervalismorethan0.2

NoticethattherearesomePwavesnotfollowedbyQRS.

Thisfigureshowsventricularfibrillation.ThereisnoQRS.(SawshapedECG).

Thisisverydangerousbecausethereisnopumpingactionoccurs.Youhavetointerfereimmediatelybydefibrillation.

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AtrioventricularBlock:ImpulsesthroughA-VnodeandA-Vbundlearesloweddown(1stand2nddegree)orblocked(3rddegree).

Causes:

(1)IschemiaofA-VnodalorA-Vbundlefibres(decreaseinbloodfloworinfarction).

(2)CompressiononA-Vbundle(byscartissueorcalcifiedtissue)

(3)A-VnodalorA-Vbundleinflammation

(4)Excessivevagalstimulation;theheartwillstopforawhilethentheventriclewillstarttocontractattherateofPurkinjefibres.

IncompleteHeartBlock:FirstDegreeBlock

•everyPisfollowedbyQRS.

•PRintervalismorethan0.2sec

•Etiology:ProlongedconductiondelayintheAVnodeorBundleofHis.

SecondDegreeIncompleteBlock

•P-Rintervalisalsoabove0.2sec

•SomeimpulsespassthroughtheA-Vnodeandsomedonotthuscausing“droppedbeats”.(SomePwavesarenotfollowedbyQRS)

•atrialbeatisfasterthantheventricles.

•sometimesitiscalled2:12nddegreeheartblockwhy?Becausetheskipped/droppedbeatisusuallythethirdone(A2:1blockimpliesthatthereare2pwavesforeveryQRScomplex)).

**whenthereisnoQRSafterthePwave,thismeansthatthereisnopumpingintheheart.

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ThirdDegreeCompleteBlock

•TotalblockthroughtheA-VnodeorA-Vbundle.

•heartrateisbelow40(around37);ventriclesownintrinsicpacemakerbeatsataround15-40beats/minute(thePurkinjefibresrate).

•ThereisnoassociationbetweenthePwavesandQRScomplexes,eachoccursonitsown.BecausethereiscompleteblockofconductionintheAVjunction,sotheatriaandventriclesformimpulsesindependentlyfromeachother.

•Ventriclesescape,andA-Vnodalrhythmensues.

•ProlongedQRS;duetobundlebranchblock.

Stokes-AdamsSyndrome

Compressionfromthecarotidonthevagusnervecausesstimulationofthevagusnerve.Thustheheartratewilldecreaseanditwillstopfor15-30sec,thenitresumesthecontractionbytherateofPurkinjefibres.RememberthatPurkinjefibresdoesn’tworkbyitsratebecauseofoverdrivesuppression.

Wetreatthisconditionsbyartificialpacemakersconnectedtorightventricle.

Extradetailswillbediscussedinlecture10