Aaron Wey, DVM, DACVIM (Cardiology) This lecture will be useful for both new and experienced LVTs and will review the clinical ECG as used in small animal practice. Lecture topics will begin with suggestions for obtaining a good quality ECG and will finish with recognition of common rhythm abnormalities encountered in companion animal practice. Audience participation will enhance the lecture and allow attendees to test their knowledge acquired during the presentation.

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“IF IT QUACKS LIKE A DUCK, IT’S PROBABLY A DUCK!”
PATTERN RECOGNITION IN CLINICAL
ELECTROCARDIOGRAPHY
Aaron C. Wey, DVM
Diplomate ACVIM (Cardiology)
Upstate Veterinary Specialties, PLLC
October 17, 2020

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“IF IT QUACKS LIKE A DUCK, IT’S PROBABLY A DUCK!”
PATTERN RECOGNITION IN CLINICAL
ELECTROCARDIOGRAPHY
Audience Participation: Poll
Anywhere
Polling results will be displayed live during the
presentation for discussion

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 Introduction
 Obtaining a diagnostic ECG
 Review: Frontal Plane, Hexaxial Lead System, ECG
waveforms
 Normal ECGs
 ECG interpretation
 Abnormal ECG findings
 Arrhythmias
 Conduction disturbances
 Artifacts
 Conclusion – you can obtain and interpret veterinary ECGs!
PATTERN RECOGNITION IN CLINICAL
ELECTROCARDIOGRAPHY
10/17/20202020 CE: PATTERN RECOGNITION IN CLINICAL ELECTROCARDIOGRAPHY

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 Electrocardiography is a useful tool in clinical practice
 Only way to diagnose arrhythmias
 Obtain an accurate heart rate
 May indicate cardiac is present
 Pre-anesthetic evaluation
 Breed screening for cardiac disease
 Anesthetic monitoring
 Intra-operative
 Post-operative
WHY CARE ABOUT THIS?
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 Electrocardiography is useful (continued)
 Adjunctive evaluation/monitoring of non-cardiac disease.
 Hypoadrenocorticism
 Hyper/Hypothyroidism
 Urinary Obstruction
 Renal disease
 Shock/Trauma
 Neoplasia
 Systemic inflammation, infection, pain, etc.
 Monitoring antiarrhythmic therapy
 Monitoring chemotherapy
 …..ECGs are just so COOL!
WHO CARES ABOUT THIS?
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Which types of cancer are commonly associated
with arrhythmias?
10/17/20202020 CE: PATTERN RECOGNITION IN CLINICAL ELECTROCARDIOGRAPHY
A. Pheochromocytoma
B. Functional Thyroid Carcinoma
C.Hemangiosarcoma
D.All of the above

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 Insulate table
 Avoid rooms with lots of electrical interference
 Any motorized equipment
 Clippers, Dryers
 Lights (60-cycle interference)
 Ideally isolated electrical outlet
 Right lateral recumbency preferred
 All measurements obtained from this orientation
 Sometimes standing or other positioning is necessary – unable
to perform standardized measurements
OBTAINING THE ELECTROCARDIOGRAM – SETUP MAKES A
DIFFERENCE!
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 Two people:
 One to restrain
 One to hook up and run the ECG
 Electrodes attached to limbs as close to the trunk as
possible
 Axillary skin fold (caudal to triceps muscle)
 Inguinal skin fold (cranial aspect of quadriceps muscle)
 Wet skin with alcohol, ultrasound gel, or water
 Remember alcohol is flammable!
OBTAINING THE ELECTROCARDIOGRAM – SETUP MAKES A
DIFFERENCE!
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OBTAINING THE ELECTROCARDIOGRAM – SETUP MAKES A
DIFFERENCE!
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 ECG settings
 Graphed paper
 Paper speed: 25 or 50mm/sec are most common
 Calibration: 1cm (10mm) = 1mV considered standard
 High-frequency filter – helpful in eliminating common veterinary
artifacts
 Settings should be specified on the ECG!
OBTAINING THE ELECTROCARDIOGRAM – SETUP MAKES A
DIFFERENCE!
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Avoiding errors in Recording: What’s missing
from this ECG?
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A. Calibration
B. Paper speed
C.Graph lines
D.Lead Designation

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Avoiding errors in Recording: What’s missing
from this ECG?
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A. Calibration
B. Paper speed
C.Frequency Filter
D.All of the above

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Avoiding errors in Recording: What’s missing
from this ECG?
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A. Calibration
B. Paper speed
C.Lead designation
D.Graph lines

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THE FRONTAL PLANE AND HEXAXIAL LEAD SYSTEM
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• Limb leads: Monitor electrical activity along an axis defined by
two limbs
• Lead I: right arm (negative pole) to left arm (positive pole)
• Lead II: right arm (negative pole) to left leg (positive pole)
• Lead III: left arm (negative pole) to left leg (positive pole)

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THE FRONTAL PLANE AND LEADS
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• Augmented limb leads: Monitor electrical activity along an axis defined by
three limbs, with one pole being the sum of two limbs, and the other pole
being the third limb
• aVR: [left arm+left leg] (negative pole) to right arm (positive pole)
• aVL: [right arm+left leg] (negative pole) to left arm (positive pole)
• aVF: [right arm+left arm] (negative pole) to left leg (positive pole)

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THE FRONTAL PLANE AND LEADS
• Augmented limb leads: Monitor electrical activity along an axis
defined by three limbs, with one pole being the sum of two limbs,
and the other pole being the third limb
• aVR: [left arm+left leg] (negative pole) to right arm (positive pole)
• aVL: [right arm+left leg] (negative pole) to left arm (positive pole)
• aVF: [right arm+left arm] (negative pole) to left leg (positive pole)
+II
+I
+III
+aVF
RV
LV
+aVR +aVF
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Definitions
 Waveform: any deflection
from the baseline
 Complex: a group of
waveforms
 Segment: time between 2
waveforms
 Interval: time from
beginning of one
waveform or segment to
end of another waveform
NORMAL ECG WAVEFORMS
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II

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Myocardial electrical
activity
 P-wave: atrial depolarization
 PR interval: atrial
depolarization and A-V nodal
conduction
 QRS complex: ventricular
depolarization
 T-wave: ventricular
repolarization
NORMAL ECG WAVEFORMS
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II

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Myocardial electrical
activity
 P-wave: atrial
depolarization
 PR interval: atrial
depolarization and A-V nodal
conduction
 QRS complex: ventricular
depolarization
 T-wave: ventricular
repolarization
NORMAL ECG WAVEFORMS
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NORMAL ECG WAVEFORM - CAT
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NORMAL ECG WAVEFORM - DOG
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In dogs and cats,
which limb lead
typically has the
largest positive
deflection?
A. Lead I
B. Lead II
C. Lead III
D. Lead aVF
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 Heart rate
 Normal/Tachycardic/Bradycardic
 Predominant Rhythm
 Sinus/Supraventricular/Ventricular origin
 Arrhythmias
 Conduction disturbances
 Mean electrical axis
 Measurements
Standard ECG interpretation:
“Heart rate X, Rhythm (origin, irregularities), mean electrical axis,
measurements”
METHODICAL APPROACH TO INTERPRETATION
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METHODICAL APPROACH TO INTERPRETATION
 Heart Rate
 Average method
 “Bic pen” variation
 Instantaneous method
 Count millimeters (boxes) between two beats
 At 25mm/sec = 1500mm/min, so divide mm measured into 1500 to get heart
rate/min
 At 50mm/sec – 3000mm/min, so divide mm measured into 3000 to get heart
rate/min
 Example: 3000/22 = 136bpm
22mm
140bpm
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HEART RATE – FAST OR SLOW?
 Normal
 60-140 for adult dogs
 140-180 for adult cats
 Tachycardic
 >140 dogs, >180 cats
 Bradycardic
 <60 dogs, <140 cats
 “Is this normal or abnormal?”
 Interpret HR with signalment/situation in mind
 Correlation of heart rate to body weight in apparently normal dogs. AP
Lamb; KM Meurs; RL Hamlin. J Vet Cardiol. August 2010;12(2):107-10.
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IS THE RHYTHM REGULAR OR IRREGULAR?
 Irregular – If yes, an arrhythmia is present
 Pattern to the arrhythmia?
 “No” (a.k.a. Irregularly irregular) – always abnormal
 “Yes” (a.k.a. Regularly irregular) – may be normal or abnormal
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IS THE RHYTHM REGULAR OR IRREGULAR?
 Regular
 Regular
 If rhythm is regular, an arrhythmia may or may not be
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25mm/sec 10mm/mV

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 Rate:
 Does the predominant rhythm originate above or below the
atrioventricular (AV) node (supraventricular vs. ventricular)?
 Evaluate morphology of QRS complexes
 Direction:
 Net positive deflection in the left and caudal limb leads (I, II, III, aVF) – indicates normal
direction of cardiac vector
 Duration:
 Narrow, pointed, or “spiked” QRS complexes – indicate depolarization travelling rapidly (i.e.
through the normal His-Purkinje system)
DETERMINING THE ORIGIN OF THE RHYTHM
VENTRICULAR/SUPRAVENTRICULAR/JUNCTIONAL
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120-130bpm
23mm

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 Does the predominant rhythm originate above or below the
atrioventricular (AV) node (supraventricular vs. ventricular)?
 Is there a P-wave for every QRS complex?
 “Yes” – rhythm originates in the atrium (usually sinus node) – “Trumps” all other criteria!
 “No” – An arrhythmia is present (rhythm originates in AV junction or ventricles)
 Is there a QRS for every P wave?
 “Yes” – When atrial impulse reached AV node, conduction occurs through it to ventricles
 “No” - An arrhythmia is present (conduction block).
 HR 120-130, Normal sinus rhythm with an isolated ventricular
premature contraction
DETERMINING THE ORIGIN OF THE RHYTHM
VENTRICULAR/SUPRAVENTRICULAR/JUNCTIONAL
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DETERMINING THE ORIGIN OF THE RHYTHM
 Rate:
 Rhythm:
 Positive (upright) in lead II
 Narrow/pointed QRS complexes???
 P-wave for each QRS and QRS for every P-wave?
 Heart rate 200, Sinus tachycardia with a possible
conduction disturbance (left bundle branch block)
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200bpm
15mm
50mm/sec

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DETERMINING THE ORIGIN OF THE RHYTHM
 Rate:
 Rhythm:
 Positive (upright) in lead II???
 Narrow/pointed QRS complexes???
 P-wave for each QRS complex
 Sinus bradycardia
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49bpm
61.5mm

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What is the origin of the rhythm below?
A. Supraventricular
B. Ventricular
C. Junctional (AV node)
D. None of the above

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DETERMINING THE ORIGIN OF THE RHYTHM
 Rate:
 Rhythm:
 Regular or irregular?
 If irregular, is there a pattern to it?
 Supraventricular or ventricular in origin?
 Positive (upright) in lead II - yes
 Narrow/pointed QRS complexes - yes
 P-wave for each QRS complex???
 Fast, irregular, supraventricular:
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240bpm
Atrial Fibrillation until proven otherwise!

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DETERMINING THE ORIGIN OF THE RHYTHM
 Rate:
 Rhythm:
 Regular or irregular?
 Supraventricular or ventricular in origin?
 Positive (upright) in lead II - no
 Narrow/pointed QRS complexes - no
 P-wave for each QRS complex – no
 Ventricular tachycardia
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300bpm
10mm

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ECG MEASUREMENTS
 Very useful for intervals/segments
 Less useful for chamber enlargement
 Researched in era before echocardiography to try and glean as much
information about anatomy as possible
 Relatively poor sensitivity, relatively good specificity
 Left ventricular hypertrophy: Tall/wide R-waves
 Left atrial enlargement: Wide or notched P-waves
 Numerous references
 Essentials of Canine and Feline Electrocardiography: Interpretation
and Treatment. 3rd Ed. LP Tilley. Lea&Febiger; Philadelphia, 1992.
 Obtaining and Interpreting Electrocardiograms. M St. John. Veterinary
Technician, May 2010.
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What are some potential causes of low-amplitude
QRS complexes in a canine patient?
A. Hypothyroidism
B. Obesity
C.Pericardial
effusion
D.All of the above

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 Heart rate
 Normal/Tachycardic/Bradycardic
 Predominant Rhythm
 Sinus/Supraventricular/Ventricular origin
 Arrhythmias
 Conduction disturbances
 Mean electrical axis and measurements
 Conduction disturbances
 Chamber enlargement
METHODICAL APPROACH TO INTERPRETATION
REVIEW
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CONDUCTION DISTURBANCES
 Anything that delays or blocks electrical depolarization
through the normal conduction system
 Bundle branch block
 AV block
 Intra-ventricular conduction disturbance
10/17/20202020 CE: PATTERN RECOGNITION IN CLINICAL ELECTROCARDIOGRAPHY
SA node
Atrial
myocardium AV node
His/Purkinje
system (left/right
bundle branches)
Ventricles

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 Seen with damage to the right branch of the Bundle of His
 Can occur with severe right ventricular pathology
 May be mild or undetectable RV pathology
 Hallmark characteristics
 Wide, deep S-waves in the left and caudal limb leads
 Prolonged QRS duration (Dog:>60msec, Cat>40msec)
 Right shift of MEA
RIGHT BUNDLE BRANCH BLOCK
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 Initial depolarization is rapid
and toward LV
 Utilizing left Bundle of His
 Later depolarization is slow
toward RV
 Conduction travelling from
myocyte to myocyte
 Result looks like VPC
from left ventricle
RIGHT BUNDLE BRANCH BLOCK
10/17/20202020 CE: PATTERN RECOGNITION IN CLINICAL ELECTROCARDIOGRAPHY
+II
+I
+III
+aVF
+aVR +aVL
RV
LV

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LEFT BUNDLE BRANCH BLOCK
 Seen with damage to the left branch of the Bundle of His
 Usually in conjunction with severe left ventricular pathology
(hypertrophy, dilation, infarction, etc.)
 Hallmark characteristics
 Wide QRS complex (dog: >60msec, cat: >40msec)
 Normal mean electrical axis (doesn’t change direction of cardiac
electrical vector, just magnitude/duration).
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 Initial depolarization is
rapid and toward RV
 Utilizing right branch or
Bundle of His
 Later depolarization is
slow toward LV
 Conduction travelling from
myocyte to myocyte
 Looks like VPC from right
ventricle
LEFT BUNDLE BRANCH BLOCK
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+II
+I
+III
+aVF
+aVR +aVL
RV
LV

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 Varying degrees of conduction delay through the A-V node
 Slowed conduction = 1st degree A-V block (prolonged PR
interval)
 Intermittent block = 2nd degree A-V block
 Physiologic: Mobitz I (progressively longer PR intervals before
block)
 Pathologic: Mobitz II (static PR intervals)
 “High grade”: No consecutively conducted P-waves
ATRIOVENTRICULAR (A-V) BLOCK
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 Complete block = 3rd degree A-V block
 No relationship between sinus activity (p-waves) and ventricular
activity
 Escape rhythm drives the heart rate and keeps animal alive
 Ventricular escape (RV or LV): 20-40bpm
 Junctional escape (A-V node, below level of block): 40-70bpm
ATRIOVENTRICULAR (A-V) BLOCK
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What type of conduction disturbance is present?
A. First degree AV block
B. Mobitz II second
degree AV block
C. High grade second
degree AV block
D. Third degree AV
block

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 Common problem for veterinary medicine
 Patient movement
 Shivering
 Purring
 Growling/Hissing/Biting…
 Won’t sit still for 10 seconds!
 Electrocardiographs often not optimal quality
 Poor technique
 Can make interpretation difficult/impossible
ECG ARTIFACTS
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PURRING/SHIVERING ARTIFACT
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A. Too much artifact, can’t tell
B. Atrial fibrillation
C. Sinus tachycardia
D. Ventricular tachycardia
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What is the rhythm diagnosis for this cat?

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 ECG Manual for the
Veterinary Technician, 1st
Ed. NJ Edwards, 1993
 Obtaining and Interpreting
Electrocardiograms. M St.
John, Veterinary
Technician, May 2010
 Essentials of Canine and
Feline Electrocardiography:
Interpretation and
Treatment. 3rd Ed. LP
Tilley, 1992
REFERENCES
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