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 MeasurementsNormal Dog:

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