Electrocardiogram

ElectrocardiogramElectrocardiogram

Wendy Blount, DVMNacogdoches TXWendy Blount, DVMNacogdoches TX

ECG – What it DetectsECG – What it Detects

Heart chamber enlargement• Eccentric hypertrophy

– Dilation and growth of heart chambers– Due to volume overload

• Concentric hypertrophy– Wall thickening of heart chambers– Due to pressure overload

Conduction Disturbances

ECG – What Doesn’t DetectECG – What Doesn’t Detect

Type of Heart chamber enlargement• Eccentric vs. Concentric hypertrophy• Congestive Heart Failure

A Short ECG won’t detect many arrhythmias

• Arrhythmias can be intermittent• 10 minutes is <1% of the day

ECG – When to DoECG – When to Do

• Pulse deficits detected on exam• Chaotic heart sounds (arrhythmia)

detected on exam• Tachycardia• Bradycardia• Episodes of weakness or collapse• Pre-anesthetic in sick or geriatric animal

– Abdominal mass (especially spleen)– Heart murmur

ECG – When to DoECG – When to Do

Event Recorders• Owner/witness starts recording during an event

Holter Monitors• Continuously record ECG for 24 hours• Can rent for Dr. Kate Meurs at Washington State

Vet School

http://www.vetmed.wsu.edu/deptsVCGL/holter/requestform.aspx

ECG – Helpful HintsECG – Helpful Hints

• Always in right lateral recumbency • Patient on a towel or rubber mat• Metal tables are more problematic• Limbs perpendicular to body• Place leads at the elbow and knee• No one moves while the ECG is being

recorded• Enhance lead contact with gel or alcohol

Alcohol is FLAMMABLE!!

ECG – Helpful HintsECG – Helpful Hints

Which lead goes where• “Snow and Grass are on the ground”

– White and green leads are on the bottom (R)

• “Christmas comes at the end of the year”– Red and green are on the back legs

• “Read the newspaper with your hands”– White and black are on front legs

White – RF Green – RR (ground) Black – LF Red – LR

ECG – The Cardiac CycleECG – The Cardiac Cycle

P wave• SA node fires

1. Atrial depolarization (contraction)

• HS42. Iternodal tracts (shortcut to AV node)


PR interval • Beginning of P wave to beginning of QRS• AV node

– *most of the PR interval is here*

• Bundle of HIS• bundle branches (R&L)• Purkinje fiber network


QRS complex• ventricular depolarization (systole)• Q wave 1st negative deflection• R wave 1st positive deflection• S wave 2nd negative deflection


QRS complex• HS1

– AV valves closing– beginning of QRS

• HS2 – Semilunar valves closing (AoV, PV)– end of QRS

• Pulse is generated


T wave• Ventricular repolarization (diastole)• HS3

– Ventricular filling– if myocardium is stiff


QT interval• beginning of QRS to end of T wave• ventricular depolarization & repolarization• HS1, HS2, HS3• Pulse generated


ST segment• Between S & T waves• Between ventricular contraction (depolarization – systole) and ventricular relaxation (repolarization – diastole)• Isn’t measured per se• But it’s relationship with baseline is noted

ECG – 6 LeadsECG – 6 Leads

Bipolar leads• I – LF+ RF-• II – LR+ RF-• III – RR+ LF-Unipolar leads• aVR – RF+ (summation lead III)-• aVL – LF+ (summation lead II)-• aVF - LR+ (summation lead I)-

ECG – Systematic Interpretation

ECG – Systematic Interpretation

1. Heart Rate and Rhythm2. Measurements of the parts

• P wave - width and height• PR interval - length• QRS - width and height• QT interval – length

• ST segment – relative to PR interval• T wave - width and height

3. Mean Electrical Axis

Form

ECG – MeasurementsECG – Measurements

• Take 3-5 measurements and average• All measurements done in lead II• Use calipers• Measure from the center of the line

ECG – Heart RateECG – Heart Rate

At 25 mm/sec, 150mm = 6 sec• “Bic Pen Times Ten”• Accurate within 10 beats per minute

At 50 mm/sec, 300mm = 6 sec• A Bic Pen times Twenty• Accurate within 20 beats per minute

ECG – Heart RateECG – Heart Rate

Normals• Giant dogs 60-140 Med-Lg dogs 70-

160• Toy dogs 80-180 Puppies 70-220• Cats 100-240

Get Baseline heart rates for individuals on every visit

ECG – RhythmECG – Rhythm

Normal Sinus rhythm• Regular heart rate

– Measure from one P wave to the next with calipers

• P, QRS and T waves in each complex

Respiratory Sinus Arrhythmia• heart rate regularly irregular

– Speeds up with inhale, slows with exhale (vagal tone variance, in a regular cycle)

• P, QRS and T waves in each complex• Variable P wave – wandering pacemaker• Heart rate less than 200

Arrhythmia

ECG – RhythmECG – Rhythm

Respiratory Sinus Arrhythmia

• Atrial depolarization (contraction)• Normal Dog: <0.4 mV x <0.04 sec <0.5 sec in giant breeds

– 4 boxes tall– 25 mm/sec 1-1.25 boxes wide– 50 mm/sec 2-2.5 boxes wide

• Normal Cat: <0.2 mV x <0.04 sec

– 2 boxes tall

ECG – P Wave MeasurementsECG – P Wave

Measurements


Measurements

• Wide P wave (Sometimes Notched)– 25 mm/sec > 1 box wide– 50 mm/sec > 2 boxes wide– LA enlargement

• Tall P wave (often spiked)– Dog > 4 boxes tall, cat > 2 boxes tall– RA enlargement

• Variable P wave – normal variation– “wandering pacemaker” – increased vagal tone

• Lack of P wave– Atrial standstill


Measurements

Wandering pacemaker

ECG – PR IntervalECG – PR Interval


Conduction from atria to ventricles (AV node)

Establishes the ECG baseline

Normal Dog: 0.06-0.13 sec

Normal Cat: 0.05-0.09 sec


• Short PR Interval (tachycardia)– AV node is bypassed– “Accessory pathway” (Wolff-Parkinson-White)– Congenital or acquired– Treated in people by radioablation of the pathway– Sudden onset of tachycardia in a dog– Can try calcium channel blockers

• Diltiazem SR (Plumb dose)– If you don’t treat right away, the myocardium will

poop out & rapidly progressive CHF will ensue




• Long PR Interval– Slow conduction through abnormal AV node– AV Blocks




1st degree AV Block• Every P wave is followed

by a QRS• Due to increased vagal tone• Non-pathogenic

50 mm/sec50 mm/sec


2nd degree AV Block

Some P waves not followed by a QRS• Mobitz type I – PR progressively longer until QRS

dropped (Wenkebach Phenomenon)


2nd degree AV Block

Some P waves not followed by a QRS• Mobitz type 2 – no pattern• PR interval does not change• P-P interval is consistent, so SA node is working fine• PR interval may be prolonged and may be normal• Occasionally, a P wave is not followed by a QRS• Not necessarily pathogenic

Physiology - Cardiac Pacemakers


Automatic cells in the heart• Depolarize on their own during phase 4 of

the cardiac cycle• Rate of depolarization affected by

autonomic nervous system– SA 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)



Automatic cells in the heart• The fastest functioning pacemaker in the

heart takes over, by default• The closer to the AV node, the more the

escape beat will resemble normal QRS• The closer to the ventricle, the more wide

and bizarre the QRS will appear• Escape rhythm – pacemaker other than

SA node takes over, because SA node fails to fire


3rd degree AV Block (complete AV block)

No relationship between P waves and QRS• P waves have their own rate (faster), determined by the

normal SA node• QRS has its own rate (slower), determined by the

automaticity of the fastest remaining functioning pacemaker

• Treatment– pacemaker

• Prognosis– Cats – without anesthesia, potentially very good– Dogs – eventual asystole is likely, if no pacemaker implanted


3rd degree AV Block (complete AV block)

•Pacemaker above bifurcation of bundle of His

•Pacemaker left ventricle

ECG – QRS Complex Measurements


Normal Dog:

<40 lbs: <0.05sec x <3.0 mV• 30 boxes tall

• 25 mm/sec 1.25 boxes wide

• 50 mm/sec 2.5 boxes wide

>40 lbs: <0.06sec x <3.0 mV• 25 mm/ sec 1.5 boxes wide

• 50 mm/sec 3 boxes wide

Normal Cat:

<0.04sec x <0.9 mV• 9 boxes tall

• 25 mm/sec 1 box wide

• 50 mm/sec 2 boxes wide



R wave measured from

baseline to top• Tall R wave, wide QRS

– LV enlargement– Left Bundle branch block

• Deep S wave in leads

I, II & III– RV enlargement

ECG – Bundle Branch BlocksECG – Bundle Branch Blocks

• Depolarization wave through myocardium rather than through Purkinje network on affected side– takes longer– “appears bigger” on ECG

• Can be persistent or intermittent– Intermittent often precipitated by increased heart

rate (delayed refractory period)

• Left side, right side or both– Bilateral BBB looks like 3rd degree AV block


Right Bundle Branch Block (RBBB)• Causes:

– primary conduction system disease– Disruption of moderator band– RV enlargement– Congenital (especially beagles)

• ECG– Deep S wave leads I, II, III, aVF– Wide QRS

• May cause a split S2


Left Bundle Branch Block (RBBB)• Causes:

– primary conduction system disease– Widespread LV myocardial disease– Unlike RBBB, not usually benign

• ECG– Tall R wave– Wide QRS– Looks like a VPC, but follows

normal PR interval

Ventricular Premature Complexes


• Depolarization wave through myocardium rather than through Purkinje network on affected side– takes longer– “appears bigger” on ECG



• VPCs are like escape beats in that they both originate from the ventricular myocardium

• VPCs are abnormal due to primary LV pathology or secondary to metabolic disease

• Escape beats are the normal life saving response to a failure of upline pacemaker

• VPCs can be persistent or intermittent– Intermittent often precipitated by increased heart

rate (delayed refractory period)

• Multiform VPCs are more serious– Multifocal areas of LV pathology

ECG – ST SegmentECG – ST Segment

ST segment depression orelevation • >0.2mV between baseline and ST• hypothermia• hypokalemia• Digitalis toxicity• Bundle branch block• Myocardial infarction

– Rare in dogs– Can be seen in feline HCM

ECG – Mean Electrical Axis (MEA)


• when a wavefront spreads toward an electrode, the largest possible deflection will occur

• When a wavefront spreads perpendicular to a lead, the smallest or no deflection occurs

• ECG shows the sum of all wavefronts relative to the lead being used to measure (MEA)

• Isoelectric lead– lead with the smallest deflection– Perpendicular to the MEA



• The normal MEA is 40o to 100o

in the dog• Lead II is most perpendicular

to the normal MEA– largest deflections– best for measurements

• aVL is most often the

isoelectric lead– Approximates MEA in normal

dogs



Calculating MEA by graph• Calculate the net deflection in lead I

– Graph on “x axis”

• Calculate net deflection in head aVF– Graph on “y axis”

• Draw the vector between the two (MEA)



+3 - 5 = -2

+10 -1.5 = +8.5

MEA = 105MEA = 105oo

+-



Estimating MEA• Find the isoelectric lead

– NOT the lead with smallest deflections– Lead with smallest NET DEFLECTION

• MEA is perpendicular to that, in the direction of net deflection



Estimating MEA

+3

-5

-2

+8

-0

+8

+13

-2

+11

+2

-2

0

+1

-8

-7

+9.5

-1

+8.5

Isoelectric lead = aVR

MEA = +120o

Right Axis Shift



Normal Canine MEA

40-110o

Normal Feline MEA

0-160o



Right Axis Shift• Right ventricular enlargement

– RV hypertrophy or dilation

• Right bundle branch block

Left Axis Shift• HCM in cats• hyperkalemia

Electrocardiogram

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