ElectrocardiogramWendy Blount, DVMNacogdoches TX
ECG What it DetectsHeart chamber enlargementEccentric hypertrophyDilation and growth of heart chambersDue to volume overloadConcentric hypertrophyWall thickening of heart chambersDue to pressure overload
Conduction Disturbances
ECG What Doesnt DetectType of Heart chamber enlargementEccentric vs. Concentric hypertrophyCongestive Heart Failure
A Short ECG wont detect many arrhythmiasArrhythmias can be intermittent10 minutes is
ECG When to DoPulse deficits detected on examChaotic heart sounds (arrhythmia) detected on examTachycardiaBradycardiaEpisodes of weakness or collapsePre-anesthetic in sick or geriatric animalAbdominal mass (especially spleen)Heart murmur
ECG When to DoEvent RecordersOwner/witness starts recording during an event
Holter MonitorsContinuously record ECG for 24 hoursCan rent for Dr. Kate Meurs at Washington State Vet School
http://www.vetmed.wsu.edu/deptsVCGL/holter/requestform.aspx
ECG Helpful HintsAlways in right lateral recumbency Patient on a towel or rubber matMetal tables are more problematicLimbs perpendicular to bodyPlace leads at the elbow and kneeNo one moves while the ECG is being recordedEnhance lead contact with gel or alcohol
Alcohol is FLAMMABLE!!
ECG Helpful HintsWhich lead goes whereSnow and Grass are on the groundWhite and green leads are on the bottom (R)Christmas comes at the end of the yearRed and green are on the back legsRead the newspaper with your handsWhite and black are on front legs
White RF Green RR (ground) Black LF Red LR
ECG The Cardiac CycleP waveSA node fires
Atrial depolarization (contraction)HS4
2. Iternodal tracts (shortcut to AV node)
ECG The Cardiac CyclePR interval Beginning of P wave to
beginning of QRSAV node *most of the PR interval
is here*Bundle of HISbundle branches (R&L)Purkinje fiber network
ECG The Cardiac CycleQRS complexventricular
depolarization (systole)Q wave 1st negative
deflectionR wave 1st positive
deflectionS wave 2nd negative
deflection
ECG The Cardiac CycleQRS complexHS1 AV valves closingbeginning of QRSHS2 Semilunar valves
closing (AoV, PV)end of QRSPulse is generated
ECG The Cardiac CycleT waveVentricular
repolarization (diastole)HS3Ventricular fillingif myocardium is
stiff
ECG The Cardiac CycleQT intervalbeginning of QRS to
end of T waveventricular depolari-
zation & repolarizationHS1, HS2, HS3Pulse generated
ECG The Cardiac CycleST segmentBetween S & T wavesBetween ventricular
contraction (depolarization systole) and ventricular relaxation (repolarization diastole)Isnt measured per seBut its relationship with
baseline is noted
ECG 6 LeadsBipolar leadsI LF+ RF-II LR+ RF-III RR+ LF-
Unipolar leadsaVR RF+ (summation lead III)-aVL LF+ (summation lead II)-aVF - LR+ (summation lead I)-
ECG Systematic InterpretationHeart Rate and RhythmMeasurements of the partsP wave - width and heightPR interval - lengthQRS - width and heightQT interval lengthST segment relative to PR interval
T wave - width and heightMean Electrical Axis
Form
ECG MeasurementsTake 3-5 measurements and averageAll measurements done in lead IIUse calipersMeasure from the
center of the line
ECG Heart Rate
At 25 mm/sec, 150mm = 6 secBic Pen Times TenAccurate within 10 beats per minute
At 50 mm/sec, 300mm = 6 secA Bic Pen times TwentyAccurate within 20 beats per minute
ECG Heart RateNormalsGiant dogs 60-140Med-Lg dogs 70-160Toy dogs 80-180Puppies 70-220Cats 100-240
Get Baseline heart rates for individuals on every visit
ECG RhythmNormal Sinus rhythmRegular heart rateMeasure from one P wave to the next with calipersP, QRS and T waves in each complex
Respiratory Sinus Arrhythmiaheart rate regularly irregularSpeeds up with inhale, slows with exhale (vagal tone variance, in a regular cycle)P, QRS and T waves in each complexVariable P wave wandering pacemakerHeart rate less than 200
Arrhythmia
ECG Rhythm
Respiratory Sinus Arrhythmia
ECG P Wave MeasurementsAtrial depolarization
(contraction)Normal Dog:
ECG P Wave MeasurementsWide P wave (Sometimes Notched)25 mm/sec > 1 box wide50 mm/sec > 2 boxes wideLA enlargementTall P wave (often spiked)Dog > 4 boxes tall, cat > 2 boxes tallRA enlargementVariable P wave normal variationwandering pacemaker increased vagal toneLack of P waveAtrial standstill
ECG P Wave Measurements
Wandering pacemaker
ECG PR Interval
ECG PR IntervalConduction from atria to ventricles (AV node)Establishes the ECG baselineNormal Dog: 0.06-0.13 sec Normal Cat: 0.05-0.09 sec
ECG PR IntervalConduction from atria to ventricles (AV node)Establishes the ECG baselineNormal Dog: 0.06-0.13 sec Normal Cat: 0.05-0.09 sec
ECG PR IntervalShort PR Interval (tachycardia)AV node is bypassedAccessory pathway (Wolff-Parkinson-White)Congenital or acquiredTreated in people by radioablation of the pathwaySudden onset of tachycardia in a dogCan try calcium channel blockersDiltiazem SR (Plumb dose)If you dont treat right away, the myocardium will poop out & rapidly progressive CHF will ensue
ECG PR IntervalNormal Dog: 0.06-0.13 sec Normal Cat: 0.05-0.09 sec
Long PR IntervalSlow conduction through abnormal AV nodeAV Blocks
ECG PR IntervalNormal Dog: 0.06-0.13 sec Normal Cat: 0.05-0.09 sec
1st degree AV BlockEvery P wave is followed
by a QRSDue to increased vagal toneNon-pathogenic
50 mm/sec
ECG PR Interval
ECG PR Interval2nd degree AV BlockSome P waves not followed by a QRSMobitz type I PR progressively longer until QRS dropped (Wenkebach Phenomenon)
ECG PR Interval2nd degree AV BlockSome P waves not followed by a QRSMobitz type 2 no patternPR interval does not changeP-P interval is consistent, so SA node is working finePR interval may be prolonged and may be normalOccasionally, a P wave is not followed by a QRSNot necessarily pathogenic
Physiology - Cardiac PacemakersAutomatic cells in the heartDepolarize on their own during phase 4 of the cardiac cycleRate of depolarization affected by autonomic nervous systemSA node (60-180 beats/min dog) (100-240 cat)AV node (40-60 beats/min dog) (80-130 cat)Purkinje fibers (20-40 beats/min)Bundle of HIS (20-40 beats/min)Ventricular myocytes (20-40 beats/min)
Physiology - Cardiac PacemakersAutomatic cells in the heartThe fastest functioning pacemaker in the heart takes over, by defaultThe closer to the AV node, the more the escape beat will resemble normal QRSThe closer to the ventricle, the more wide and bizarre the QRS will appearEscape rhythm pacemaker other than SA node takes over, because SA node fails to fire
ECG PR Interval3rd degree AV Block (complete AV block)No relationship between P waves and QRSP waves have their own rate (faster), determined by the normal SA nodeQRS has its own rate (slower), determined by the automaticity of the fastest remaining functioning pacemakerTreatmentpacemakerPrognosisCats without anesthesia, potentially very goodDogs eventual asystole is likely, if no pacemaker implanted
ECG PR Interval3rd degree AV Block (complete AV block)Pacemaker above bifurcation of bundle of His
Pacemaker left ventricle
ECG QRS Complex MeasurementsR wave measured from baseline to topTall R wave, wide QRSLV enlargementLeft Bundle branch blockDeep S wave in leads
I, II & IIIRV enlargement
ECG Bundle Branch BlocksDepolarization wave through myocardium rather than through Purkinje network on affected sidetakes longerappears bigger on ECGCan be persistent or intermittentIntermittent often precipitated by increased heart rate (delayed refractory period)Left side, right side or bothBilateral BBB looks like 3rd degree AV block
ECG Bundle Branch BlocksRight Bundle Branch Block (RBBB)Causes:primary conduction system diseaseDisruption of moderator bandRV enlargementCongenital (especially beagles)ECGDeep S wave leads I, II, III, aVFWide QRSMay cause a split S2
ECG Bundle Branch BlocksLeft Bundle Branch Block (RBBB)Causes:primary conduction system diseaseWidespread LV myocardial diseaseUnlike RBBB, not usually benignECGTall R waveWide QRSLooks like a VPC, but follows
normal PR interval
Ventricular Premature Complexes
Depolarization wave through myocardium rather than through Purkinje network on affected sidetakes longerappears bigger on ECG
Ventricular Premature ComplexesVPCs are like escape beats in that they both originate from the ventricular myocardiumVPCs are abnormal due to primary LV pathology or secondary to metabolic diseaseEscape beats are the normal life saving response to a failure of upline pacemakerVPCs can be persistent or intermittentIntermittent often precipitated by increased heart rate (delayed refractory period)Multiform VPCs are more seriousMultifocal areas of LV pathology
ECG ST SegmentST segment depression orelevation >0.2mV between baseline
and SThypothermiahypokalemiaDigitalis toxicityBundle branch blockMyocardial infarctionRare in dogsCan be seen in feline HCM
ECG Mean Electrical Axis (MEA)when a wavefront spreads toward an electrode, the largest possible deflection will occurWhen a wavefront spreads perpendicular to a lead, the smallest or no deflection occursECG shows the sum of all wavefronts relative to the lead being used to measure (MEA)Isoelectric leadlead with the smallest deflectionPerpendicular to the MEA
ECG Mean Electrical Axis (MEA)The normal MEA is 40o to 100o
in the dogLead II is most perpendicular
to the normal MEAlargest deflectionsbest for measurementsaVL is most often the
isoelectric leadApproximates MEA in normal
dogs
ECG Mean Electrical Axis (MEA)Calculating MEA by graphCalculate the net deflection in lead IGraph on x axisCalculate net deflection in head aVFGraph on y axisDraw the vector between the two (MEA)
ECG Mean Electrical Axis (MEA)
+3 - 5 = -2+10 -1.5 = +8.5MEA = 105o+-
ECG Mean Electrical Axis (MEA)Estimating MEAFind the isoelectric leadNOT the lead with smallest deflectionsLead with smallest NET DEFLECTIONMEA is perpendicular to that, in the direction of net deflection
ECG Mean Electrical Axis (MEA)Estimating MEA+3-5-2+8-0+8+13-2+11+2-20+1-8-7+9.5-1+8.5Isoelectric lead = aVRMEA = +120oRight Axis Shift
ECG Mean Electrical Axis (MEA)
Normal Canine MEA40-110oNormal Feline MEA0-160o
ECG Mean Electrical Axis (MEA)Right Axis ShiftRight ventricular enlargementRV hypertrophy or dilationRight bundle branch block
Left Axis ShiftHCM in catshyperkalemia
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