1. Wendy Blount, DVM Nacogdoches TX
Electrocardiogram
Wendy Blount, DVM
Nacogdoches TX
Dr. Callan Video

2. ECG – 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

3. ECG – 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
Holter Monitor is more thorough

4. ECG – 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

5. ECG – When to Do Event Recorders
Owner/witness starts recording during an event
Holter Monitors
Continuously record ECG for 24 hours
Can rent various places
NCSU – Kate Meurs, DACVIM Cardiology – Vets only
$ hours, $ hours, $ hours; recheck $150
Pet Cardiology.com – breeders and vets
(form)
Alba Medical – breeders and vets
Sell Holter Monitors, interpretation $40-80 (form)

6. 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 touching the dog can move while the ECG is being recorded
Enhance lead contact with gel or alcohol
Alcohol is FLAMMABLE!!

7. 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

8. ECG – The Cardiac Cycle P wave SA node fires Atrial depolarization
(contraction)
HS4
2. Iternodal tracts
(shortcut to
AV node)

9. ECG – The Cardiac Cycle 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

10. ECG – The Cardiac Cycle QRS complex ventricular depolarization
(systole)
Q wave 1st negative
deflection
R wave 1st positive
S wave 2nd negative

11. ECG – The Cardiac Cycle QRS complex HS1 HS2 Pulse is generated
AV valves closing
beginning of QRS
HS2
Semilunar valves
closing (AoV, PV)
end of QRS
Pulse is generated

12. ECG – The Cardiac Cycle T wave Ventricular repolarization (diastole)
HS3
Ventricular filling
if myocardium is
stiff

13. ECG – The Cardiac Cycle QT interval beginning of QRS to end of T wave
ventricular depolari-
zation & repolarization
HS1, HS2, HS3
Pulse generated

14. ECG – The Cardiac Cycle 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

15. 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)-

16. ECG – Systematic Interpretation
Heart Rate and Rhythm
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
Mean Electrical Axis
Form

17. ECG – Measurements Take 3-5 measurements and average
All measurements done in lead II
Use calipers
Measure from the
center of the line
50mm/sec

18. 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
A Bic Pen times Twenty
Accurate within 20 beats per minute

19. Get Baseline heart rates for individuals on every visit
ECG – Heart Rate
Normals
Giant dogs Med-Lg dogs
Toy dogs Puppies
Cats
Get Baseline heart rates for individuals on every visit

20. ECG – 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)
Variable P wave – wandering pacemaker
Heart rate in normal range
Arrhythmia

21. Respiratory Sinus Arrhythmia – 10 mm/sec
ECG – Rhythm
Respiratory Sinus Arrhythmia – 10 mm/sec

22. ECG – P Wave Measurements
Atrial depolarization
(contraction)
Normal Dog:
<0.4 mV x <0.04 sec
<0.5 sec in giant breeds
4 boxes tall (10mm=1mV)
50 mm/sec boxes wide
25 mm/sec boxes wide
Normal Cat:
<0.2 mV x <0.04 sec
2 boxes tall

23. ECG – P Wave Measurements
Wide P wave (Sometimes Notched)
50 mm/sec > 2.5 boxes wide
25 mm/sec > 1.25 box 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

24. ECG – P Wave Measurements
Wandering pacemaker

25. ECG – PR Interval Conduction from atria to ventricles (AV node)
Establishes the ECG baseline
Normal Dog: sec
50mm/sec – boxes
25mm/sec – boxes
Normal Cat: sec
50mm/sec – boxes
25mm/sec – boxes

26. ECG – PR Interval 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

27. ECG – PR Interval Short PR Interval (tachycardia)
AV node is bypassed by abnormal myocardium that passes through the fibrous tissue that normally insulates the atria from the ventricles
“Accessory pathway” (Wolff-Parkinson-White)
Congenital or acquired
Sudden onset of tachycardia in a dog

28. ECG – PR Interval Short PR Interval (tachycardia)
AV node is bypassed by abnormal myocardium that passes through the fibrous tissue that normally insulates the atria from the ventricles
“Accessory pathway” (Wolff-Parkinson-White)
Congenital or acquired
Sudden onset of tachycardia in a dog

29. ECG – PR Interval WPW Syndrome - ECG shows SVT Fused T and P wave
Heart rate bpm+
Normal QRS

30. ECG – PR Interval Exam Treatment Generalized weakness
Heart rate often too fast to count
Pale mucous membranes
Weak pulses, pulse deficits
No heart murmur, normal bloodwork, x-rays and echo
Treatment
Can try calcium channel blockers (Class 2 or 4 antiarrhythmics)
Diltiazem SR 3-5 mg/kg PO BID
If you don’t treat right away, the myocardium will poop out & rapidly progressive CHF will ensue
Refer for as an emergency radioablation of the pathway

31. ECG – PR Interval
Types of SVT – all have high heart rate with narrow/normal QRS
Sinus tachycardia (normal P waves)
Atrial tachycardia (bizarre P waves negative in I & aVF)
Junctional tachycardia (unrelated P wave)
Atrial flutter (coarse atrial fibrillation)
Atrial fibrillation (chaotic baseline)
Wolff-Parkinson-White Syndrome (SVT + re-entry rhythm)

32. ECG – PR Interval Normal Dog: 0.06-0.13 sec (3-6.5 boxes)
Normal Cat: sec ( boxes)
Long PR Interval
Slow conduction through abnormal AV node
AV Blocks

33. ECG – PR Interval Normal Dog: 0.06-0.13 sec Normal Cat: 0.05-0.09 sec
3-6.5 boxes
Normal Cat: sec
boxes
1st degree AV Block
Every P wave is followed
by a QRS
Due to increased vagal tone
Non-pathogenic
50 mm/sec

34. ECG – PR Interval 2nd degree AV Block
Some P waves not followed by a QRS
Mobitz type I – PR progressively longer until QRS dropped (Wenkebach Phenomenon)

35. ECG – PR Interval 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

36. ECG – PR Interval

37. Physiology - Cardiac Pacemakers
Automatic cells in the heart
Depolarize on their own during phase 4 of the cardiac cycle (escape beat)
Rate of depolarization affected by autonomic nervous system
SA node ( beats/min dog) ( bpm cat)
AV node (40-60 beats/min dog) ( bpm cat)
Purkinje fibers, Bundle of HIS, Ventricular myocytes (20-40 beats/min) (60-80 bpm cat)

38. Physiology - Cardiac Pacemakers
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

39. ECG – PR Interval 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, if escape rhythm rate doesn’t support normal activity
Prognosis
Cats – without anesthesia, potentially very good, as they usually escape from the AV node
Dogs – eventual asystole is likely, if no pacemaker implanted, as they escape from Purkinje fibers, HIS or ventricles

40. ECG – PR Interval 3rd degree AV Block (complete AV block)
Pacemaker above bifurcation of bundle of His
Pacemaker left ventricle

41. ECG – QRS Complex Measurements
Normal Dog:
<40 lbs: <0.05sec x <3.0 mV
30 boxes tall
50 mm/sec 2.5 boxes wide
25 mm/sec 1.25 boxes wide
>40 lbs: <0.06sec x <3.0 mV
50 mm/sec 3 boxes wide
25 mm/ sec 1.5 boxes wide
Normal Cat:
<0.04sec x <0.9 mV
9 boxes tall
50 mm/sec 2 boxes wide
25 mm/sec 1 box wide

42. ECG – QRS Complex Measurements
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

43. ECG – 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

44. ECG – Bundle Branch Blocks
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

45. ECG – Bundle Branch Blocks
Left Bundle Branch Block (LBBB)
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

46. ECG – Bundle Branch Blocks

47. Ventricular Premature Complexes
Depolarization wave through myocardium rather than through Purkinje network on affected side
takes longer
“appears bigger” on ECG

48. Ventricular Premature Complexes
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

49. ECG – ST Segment ST segment depression or elevation
>0.2mV (2 boxes) between
baseline and ST segment
hypothermia
hypokalemia
Digitalis toxicity
Bundle branch block
Myocardial infarction
Rare in dogs
Can be seen in feline HCM

50. 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

51. ECG – Mean Electrical Axis (MEA)
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
aVL + II +

52. ECG – Mean Electrical Axis (MEA)
Calculating MEA by graph
Calculate the net deflection in lead I
Graph on “x axis”
Calculate net deflection in lead aVF
Graph on “y axis”
Draw the vector between the two (MEA)

53. ECG – Mean Electrical Axis (MEA)
= -2
= +8.5 - +
MEA = 105o

54. ECG – Mean Electrical Axis (MEA)
Estimating MEA
Find the isoelectric lead
NOT necessarily the lead with smallest deflections
Lead with smallest NET DEFLECTION
MEA is perpendicular to that, in the direction of net deflection

55. ECG – Mean Electrical Axis (MEA)
Estimating MEA
Isoelectric lead = aVR +3 -5 -2 +8 -0
+13 -2
+11 +2 -2 +1 -8 -7
+9.5 -1
+8.5
MEA = +120o
Right Axis Shift

56. ECG – Mean Electrical Axis (MEA)
Normal Canine MEA
40-110o
Normal Feline MEA
0-160o

57. ECG – Mean Electrical Axis (MEA)
Right Axis Shift
Right ventricular enlargement
RV hypertrophy or dilation
Right bundle branch block
Left Axis Shift
Left ventricular enlargement or dilation
LV hypertrophy or dilation
Hyperkalemia
Left Bundle Branch Block

58. Acknowledgements
Open.osmosis.com (WPW Video)
Kittleson, Mark, DACVIM - Cardiology
Small Animal Cardiovascular Medicine, Veterinary Information Network. Chapter 5 – Electrocardiography
Edwards J
Bolton's Handbook of Canine & Feline Electrocardiography

59. Acknowledgements
Larry Tilley, DACVIM - Cardiology
Essentials of Canine and Feline Electrocardiography
Tilley L, Miller M, Smith FWK
Canine and Feline Cardiac Arrhythmias – A Self Assessment