This document provides an overview of canine and feline electrocardiography for veterinary technicians. It discusses normal cardiac anatomy and conduction, how to perform an ECG, how to calculate heart rate, normal ECG waves and intervals, common arrhythmias including bradyarrhythmias, tachyarrhythmias, and conduction abnormalities. Specific arrhythmias covered include sinus bradycardia, sick sinus syndrome, atrioventricular block, ventricular escape complexes, supraventricular tachycardia, atrial fibrillation, and ventricular premature complexes.

1. AGNIESZKA KENT DVM, MS, DACVIM (CARDIOLOGY)
ELECTROCARDIOGRAPHY FOR THE VETERINARY
TECHNICIAN

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 Sinoatrial (sinus) node
 Internodal tracts
 Atrioventricular node
 “gate-keeper” to ventricles
 His bundle
 Left bundle branch
 Right bundle branch
 Allow rapid and organized
conduction
CONDUCTION SYSTEM
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http://nyp.org/health/cardiac-electric.html

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NORMAL CARDIAC CONDUCTION
http://cnx.org/contents/ef365738-1b72-4234-8bb3-f994538801c8@1/Cardiovascular_System_Module_4
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ECG WAVES AND INTERVALS
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researchgate.net

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 P-wave: Conduction of electrical current through the atria
 PR (PQ) interval: Time for conduction from the SA node across
the AV node to the start of ventricular depolarization
 QRS: Ventricular depolarization
 ST segment: Ventricles are between depolarization and
repolarization
 T-wave: Repolarization of the ventricles
 Electrical currents reset and get ready for the next impulse
 QT interval: Time for ventricular depolarization and
repolarization
WAVE FORMS AND INTERVALS
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MEAN ELECTRICAL AXIS
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cvphysiology.com

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 Patient should be in right lateral recumbency
 Allows you to see whether conduction is normal
 Important for the “mean electrical axis”
 Place leads
 “Snow and grass are on the ground, Christmas comes at the end of
the year”
 Place small quantity of alcohol unless:
 Concern for ventricular fibrillation and need for defibrillation
 Use conducting gel instead
SET UP
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 First ensure you know the paper speed in order to acquire a heart rate
(generally 25 mm/s)
 You can’t always trust the machine for accuracy
 To determine the rate:
 Average HR
15 big boxes (5mm/box) = 3 sec at 25 mm/sec
Count # of QRS complexes and multiply by 20
 Instantaneous HR
Count number of small (1mm) boxes within R-R interval
 50 mm/sec HR =3000/#boxes
 25 mm/sec HR=1500/# boxes
HEART RATE (HR)
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HEART RATE CALCULATION EXAMPLES
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Average HR
# of complexes within 15 big boxes: 7
HR = 7 x 20
HR = 140 bpm
Instantaneous HR
# of small boxes between R-waves: 11
HR = 1500 ÷ 11
HR = 136 bpm

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Canine
Rate
70-160 bpm (adult)
Can go to ~30-40
bpm when sleeping
Some very stressed
or excited dogs can
have heart rates in
the low 200’s
Feline
Rate
120-240 bpm
If hear rate is 140 or
lower in the
hospital likely
abnormal
Caution if receiving
a ß-blocker
(atenolol)
NORMAL HEART RATE
There is no correlation between heart rate and body size in dogs
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 Look for normal complexes first
 Do you see P-waves?
 Do you see QRS complexes that are narrow and look normal?
 Look at the overall rhythm
 Is it regular or irregular
 If irregular
 Is it regularly irregular?
 Is there a pattern to it?
 Possible respiratory sinus arrhythmia
 Or irregularly irregular?
 No pattern at all
RHYTHM ANALYSIS
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 Is every QRS preceded by a P-wave?
 Is every P wave followed by a QRS?
 Is the R-R interval consistent?
 Is the P-P interval consistent?
 Is the P-R interval consistent?
 All normal beats will have a P-wave, QRS, and T-wave
RHYTHM ANALYSIS
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WATCH OUT FOR ARTIFACTS
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 Bradyarrhythmias
 Abnormally slow
 Sinus bradycardia
 Sick sinus syndrome (sinus node dysfunction)
 Atrioventricular block
 Atrial standstill
ARRHYTHMIA CLASSIFICATION
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 Premature complexes
 Abnormally early beats
 Atrial and ventricular premature complexes
 Tachyarrhythmias
 Abnormally fast
 Ventricular tachycardia
 Supraventricular tachyarrhythmia
 Atrial fibrillation, supraventricular tachycardia
ARRHYTHMIA CLASSIFICATION
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 Transient loss of consciousness
 Generally collapse acutely and lose consciousness
 If does not lose consciousness: partial syncope
 Generally lasts few seconds before start to recover
 May be disoriented for few minutes but recover quickly
 Return to normal within minutes
 Can paddle legs and have some facial twitching
 NO pre or post-ictal behaviors**
SYNCOPE
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 High vagal tone
 Gastrointestinal disease
 Respiratory disease
 Neurologic disease
 Severe ocular disease
 Hypothermia
 Endocrinopathy
 Hypothyroidism
 Systemic hypertension
SINUS BRADYCARDIA
50 mm/sec, HR ~30
Normal in appearance but slow
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
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 Abnormal function of the sinus node and conduction system
 Periods of no atrial activity and asystole (“flat-line”)
 Sinus node dysfunction
 No clinical signs, ECG abnormality only
 Sick sinus syndrome
 ECG abnormality with syncope
SICK SINUS SYNDROME
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SICK SINUS SYNDROME
14yo Mini Schnauzer; Collapse 50 mm/sec; HR ~20
3.8 s pause
ECGs courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
10/16/2018CE FALL 2017/ECH FOR TECHS
Generally need a pacemaker

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 1st degree = prolonged PR interval
 Takes longer for electrical activity to go through the AV node
 High vagal tone
 2nd degree = intermittent conduction
 Some P waves without QRS
 Mobitz I (Wenckebach) = PR progressively lengthens before block
 Mobitz II = PR is constant
 3rd degree = complete block/no conduction
 No association between P-waves and QRS complexes
 Ventricular escape beats/complexes
ATRIOVENTRICULAR BLOCK
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 Wide and bizarre complexes
 Look like ventricular premature complexes
 Have a regular and slow heart rate
 Not early or premature
 Secondary pacemakers in the ventricles
 Fire when there is a pause or lack of normal electrical activity
VENTRICULAR ESCAPE COMPLEXES
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 Drugs that prolong AV conduction
 Calcium channel blocker (diltiazem), digoxin, Beta-blocker (atenolol)
 Opioids can also increase vagal tone
FIRST DEGREE ATRIOVENTRICULAR BLOCK
3yo Golden Retriever
50 mm/sec, HR ~80
PR interval 0.14s (normal: 0.06-0.13s)
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
10/16/2018CE FALL 2017/ECH FOR TECHS

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 Mobitz ISECOND DEGREE ATRIOVENTRICULAR BLOCK (MOBITZ 1)
 PR interval prolongs before the blocked P-wave
 Generally physiologic
 High vagal tone
 Drugs that affect AV conduction
VetGo Cardiology
Dog, 25 mm/s, 10 mm/mV
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SECOND DEGREE ATRIOVENTRICULAR BLOCK (MOBITZ 2)
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 Pathologic: Disease in the AV node or His Bundle
ECGs courtesy of Dr. Brian Scansen, DVM, MS, DACVIM

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 Exercise/stress may resolve Mobitz 1 block
 Atropine Response Test
 0.04 mg/kg IM, re-evaluate ECG in 20-30 minutes
 Block will resolve if Mobitz 1, will not if Mobitz 2
 Low grade Mobitz 2 second degree AVB
 Not many P-waves getting blocked
 Generally no treatment
 High grade Mobitz 2 second degree AVB
 4:1 or worse conduction (4 P-waves and only 1 conducted)
 Patient generally symptomatic: syncope, weak, lethargic
 PACEMAKER
SECOND DEGREE ATRIOVENTRICULAR BLOCK
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 PACEMAKER
THIRD DEGREE ATRIOVENTRICULAR BLOCK
6yo Pug; HR ~55 bpm 50 mm/sec
Cat
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
10/16/2018CE FALL 2017/ECH FOR TECHS
Dog
P-waves and QRS complexes are completely independent

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Some patients with AV block can also have ventricular
arrhythmias (ventricular tachycardia)
Do not give lidocaine!
You can inhibit the escape rhythm
 Pacemaker first
 Then begin antiarrhythmic treatment
Patient may be in congestive heart failure
**PLEASE NOTE**
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 Hyperkalemia
 >5.5 mmol/L: T waves larger and peaked
 >6.5 mmol/L: decreased R wave amplitude, prolonged
QRS and PR interval, ST segment depression
 >7 mmol/L: decreased P wave amplitude and increased
duration, prolonged QRS, PR, and QT interval
 >8.5 mmol/L: P waves disappear
 >10 mmol/L: QRS widen more, then ventricular flutter,
then fibrillation, then asystole
ATRIAL STANDSTILL
Pre Post
DSH; Urethral Obstruction HR ~160
Fox, Sisson Moise
Dog with Addison’s K+: 8.4
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
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HYPERKALEMIA ATRIAL STANDSTILL
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 PACEMAKER
 Atrial myopathy
 Progressive disease
 Develop CHF even with PM
implantation
PERSISTENT ATRIAL STANDSTILL = SILENT ATRIUM
2yo English Setter 50 mm/sec, HR ~30 www.vmth.ucdavis.edu/cardio/cases
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
10/16/2018CE FALL 2017/ECH FOR TECHS

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 Sinus tachycardia
 Appropriate or inappropriate?
 Hemodynamic stability
 Blood pressure
 Hemorrhage
 Pericardial effusion
 Anxiety
 Pain
 Fever
 Drugs
 Endocrine disease
 Hyperthyroidism
 Toxicity
 Methylxanthines, cocaine, thyroxine
over-supplementation, Ventolin
TACHYARRHYTHMIAS
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
10/16/2018CE FALL 2017/ECH FOR TECHS

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 Atrial Premature Complexes
 Premature P wave with different appearance
 Followed by QRS that looks like sinus QRS
 Generally very well tolerated unless very frequent
SUPRAVENTRICULAR ARRHYTHMIAS
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 Narrow complex (QRS complexes are narrow)
 May see electrical alternans
 R-wave amplitude alternates
 Rapid: ~240-300 bpm
 Could be higher in cats
 Abrupt start and finish
 May be slight “warm-up” and/or “cool-down”
 P-waves can be buried in the preceding T wave
 Paroxysmal (intermittent) versus sustained
SUPRAVENTRICULAR TACHYCARDIA
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SUPRAVENTRICULAR/ATRIAL TACHYCARDIA
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
10/16/2018CE FALL 2017/ECH FOR TECHS

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 Rapid
 Irregularly irregular
 No P-waves
 “sneakers in a clothes dryer” on auscultation
 Most commonly due to structural heart disease
 “ Lone atrial fibrillation”
 Large breed dogs (ex Irish Wolfhound), HR not as fast
 Anesthesia induced – in dogs, due to high vagal tone
 Can give lidocaine 2 mg/kg IV
ATRIAL FIBRILLATION
www.vetgo.com
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ATRIAL FIBRILLATION
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 Ventricular premature complexes
 Inappropriate impulse from an abnormal site in the ventricle(s)
 Premature, wide and bizarre (cell to cell conduction)
 Causes – five broad categories
Cardiac disease: structural (DCM) or electrical (ARVC)
Metabolic/endocrine disease: Electrolytes (Ca, K, Mg),
Hyperthyroidism
Drugs/Toxins: Digoxin overdose, Thiopental, iatrogenic
hyperthyroidism, asthma inhaler
Usual Suspects: Splenic disease, pancreatitis, sepsis, DIC trauma
(HBC), GDV, severe anemia…
Autonomic imbalance (high sympathetic tone)
 Stress, anxiety, pheochromocytoma
VENTRICULAR ARRHYTHMIAS
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VENTRICULAR PREMATURE COMPLEXES
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 Alternating normal sinus beat and VPC
VENTRICULAR BIGEMINY
50 mm/sec, HR ~160
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 Presence of >3 VPCs in sequence
VENTRICULAR TACHYCARDIA
P-waves seen “marching through” but not associated with QRS complexes
ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM
10/16/2018CE FALL 2017/ECH FOR TECHS

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POLYMORPHIC/MULTIFORM VENTRICULAR PREMATURE
COMPLEXES
**R-on-T**
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 “Slow ventricular tachycardia”
 May see fusion beats (combination of normal and ventricular beats)
 GDV, splenectomy, digoxin toxicity
 Rarely needs treatment, often self-limiting
ACCELERATED IDIOVENTRICULAR RHYTHM
www.vmth.ucdavis.edu/cardio/cases
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f

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 Need defibrillator!
VENTRICULAR FIBRILLATION
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ECG courtesy of Dr. Brian Scansen, DVM, MS, DACVIM

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 Don’t be fooled…this is not ventricular tachycardia
 Look for consistent P-waves with consistent PR interval
 Left bundle branch block - positive in lead II
 Right bundle branch block -negative
in lead II
BUNDLE BRANCH BLOCK
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ELECTRICAL ALTERNANS
•Alternating QRS amplitude
•Can see with severe pericardial effusion
•Pendulous movement of the heart within the
pericardial fluid
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 Next…various ECGs
QUESTIONS SO FAR?
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WHAT IS THE RHYTHM AND HEART RATE?
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8 year old MC Doberman, dilated cardiomyopathy
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10 year old FS mixed breed dog with degenerative valve disease
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11 year old MC Boston Terrier, degenerative valve disease
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11 year old MC mixed breed dog
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6 year old Boxer
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12 year old Miniature Schnauzer with syncope
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5 year old cat, severe HCM
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11 year old mixed breed dog
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1 year old male German Shepherd
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10 year old Boxer
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12 year old fox terrier

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25 mm/s

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10 year old Cocker Spaniel

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8 year old Labrador Retriever

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9 year old cat

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Elderly Bulldog

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12 year old cat

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10 year old cat

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Dog