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Electrocardiogram. Wendy Blount, DVM Nacogdoches TX. ECG – What it Detects. Heart chamber enlargement Eccentric hypertrophy Dilation and growth Due to volume overload Concentric hypertrophy Wall thickening Due to pressure overload Conduction Disturbances. ECG – When to Do.

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Wendy Blount, DVM

Nacogdoches TX

ecg what it detects
ECG – What it Detects

Heart chamber enlargement

  • Eccentric hypertrophy
    • Dilation and growth
    • Due to volume overload
  • Concentric hypertrophy
    • Wall thickening
    • Due to pressure overload

Conduction Disturbances

ecg when to do
ECG – When to Do
  • Pulse deficits detected on exam
  • Chaotic heart sounds (arrhythmia) detected on exam
  • Tachycardia
  • Bradycardia
  • Episodes of weakness or collapse

Intermittent arrhythmias may not be detected in a single ECG

ecg when to do4
ECG – 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


ecg helpful hints
ECG – 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 hints6
ECG – 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 cycle
ECG – The Cardiac Cycle

P wave

  • SA node fires
    • Atrial depolarization (contraction)
      • HS4
    • Iternodal tracts (shortcut to AV node)

PR interval (no deflection - baseline)

  • Beginning of P wave to beginning of QRS
  • AV node (*most of the PR interval is here*)
  • Bundle of HIS
  • bundle branches (R and L)
  • Purkinje fiber network
ecg the cardiac cycle8
ECG – The Cardiac Cycle

QRS complex

  • ventricular depolarization (systole)
  • Q wave 1st negative deflection
  • R wave 1st positive deflection
  • S wave 2nd negative deflection
  • HS1 at beginning of QRS
  • HS2 at end of QRS
  • Pulse is generated
ecg the cardiac cycle9
ECG – The Cardiac Cycle

T wave

  • Ventricular repolarization (diastole)
  • HS3 if myocardium is stiff

QT interval (no deflection - baseline)

  • beginning of QRS to end of T wave
  • ventricular depolarization & repolarization
  • QRS Contains HS1 and HS2
  • HS3 during T wave
  • Pulse generated
ecg 6 leads
ECG – 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
  • Heart Rate and Rhythm
  • Measurements of Deflections
    • 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
  • Mean Electrical Axis


ecg measurements
ECG – Measurements

Take 3-5 measurements and average

All measurements done in lead II

Use calipers

Measure from the center of the line

ecg heart rate
ECG – 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

  • 2 Bic Pens times Ten
  • Accurate within 20 beats per minute


  • Giant dogs 60-140 Med-Lg dogs 70-160
  • Toy dogs 80-180 Puppies 70-220
  • Cats 100-240
ecg rhythm
ECG – Rhythm

Normal Sinus rhythm

  • Regular heart rate
  • P, QRS and T waves in each complex

Respiratory Sinus Arrhythmia

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


ecg p wave measurements
ECG – P Wave Measurements

Atrial depolarization (contraction)

Normal Dog: <0.4 mV x <0.04 sec

Normal Cat: <0.2 mV x <0.04 sec

  • Wide P wave, Notched P wave
    • LA enlargement
  • Tall P wave (spiked)
    • RA enlargement
  • Variable P wave
    • “wandering pacemaker” – increased vagal tone
  • Lack of P wave
    • Atrial standstill
ecg pr interval
ECG – 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
    • Can try calcium channel blockers
ecg pr interval17
ECG – PR Interval
  • 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
ecg pr interval18
ECG – PR Interval

2nd degree AV Block

Some P waves not followed by a QRS

  • Mobitz type I – PR progressively longer until QRS dropped
  • Mobitz type 2 – no pattern
  • Not necessarily pathogenic

3rd degree AV Block (complete AV block)

No relationship between P waves and QRS

  • P waves have their own rate (faster)
  • QRS has its own rate (slower)
  • Treated by pacemaker
ecg qrs complex measurements
ECG – QRS Complex Measurements

Normal Dog: <40 lbs - <0.05sec x <3.0 mV

>40 lbs - <0.06sec x <3.0 mV

Normal Cat: <0.04sec x <0.9 mV

R wave measured from baseline to top

  • Tall R wave, wide QRS
    • LV enlargement
    • Left Bundle branch block
ecg st segment
ECG – ST Segment

Late ventricular depolarization

“J point” should be within 0.2 mV of baseline in dogs

Should be on baseline in cats

  • Between QRS and T waves
  • Elevation or depression means regional myocardial hypoxia
  • Myocardial infarction not as common in dogs and cats as in people
    • Can see in cats with HCM
ecg st segment21
ECG – ST Segment

Other Causes of ST segment depression or elevation

  • hypothermia
  • hypokalemia
  • Digitalis toxicity
  • Bundle branch block
ecg mean electrical axis mea
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
ecg mean electrical axis mea23
ECG – Mean Electrical Axis (MEA)
  • The normal MEA is 40o to 100o (dog)
  • Lead II is most perpendicular to the normal MEA
  • Lead II shows the largest deflections in the dog with normal MEA
  • This is why lead II is the most often used lead in veterinary medicine
  • aVL is most often the isoelectric lead
ecg mean electrical axis mea24
ECG – Mean Electrical Axis (MEA)

+1.5 - 5 = -3.5

Estimating MEA

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

Calculating MEA by graph

  • Graph net deflection in lead I on “x axis”
  • Graph net deflection in lead aVF on “y axis”
  • Draw the vector between the two (MEA)

+10 -1.5 = +8.5

ecg mean electrical axis mea25
ECG – Mean Electrical Axis (MEA)

Right Axis Shift

  • Right ventricular enlargement
    • RV hypertrophy or dilation
  • Right bundle branch block

Left Axis Shift

  • HCM in cats
  • hyperkalemia
ecg mean electrical axis mea26
ECG – Mean Electrical Axis (MEA)





Right Bundle Branch Block (RBBB)

Heart Rate – 160 per minute (normal 80-180)

Rhythm – normal sinus with 2 complex types








J point

Complex 1

P wave0.05 sec x 0.35 mV

PR interval 0.104 sec

QRS wave0.056 sec x 1.75 mV(R)

ST segment -0.1 mV depressed

MEA – 85o

Complex 2

P wave 0.05 sec x 0.35 mV

PR interval 0.102 sec

QRS wave 0.06 sec x -1.55 mV(S)

ST segment 0 deviation from PR

MEA -90o to -120o

ecg bundle branch blocks
ECG – Bundle Branch Blocks

Bundle Branch blocks

  • Depolarization wave through myocardium rather than through Purkinje network on affected side
  • depolarization takes longer
  • depolarization wave “appears bigger” on ECG
  • Can be persistent or intermittent
  • Intermittent often precipitated by increased heart rate (delayed refractory period)
  • Bilateral BBB looks like 3rd degree AV block
ecg bundle branch blocks28
ECG – Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

  • Causes:
    • primary conduction system disease
    • Disruption of moderator band
    • RV enlargement
    • Congenital (beagles)
  • ECG
    • Deep S save leads I, II, III, aVF
    • Wide QRS
  • May cause a split S2
ecg bundle branch blocks29
ECG – Bundle Branch Blocks

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 a P wave with normal PR interval
physiology cardiac pacemakers
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)
  • Escape rhythm – pacemaker other than SA node, because it fails to fire