Electrocardiogram
Wendy Blount, DVM
Nacogdoches TX
Dr. Callan Video

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

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

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

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
• Can get interpretation of Holter Recording http:// www.vetmed.wsu.edu/deptsHolter/requestform.aspx

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 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
P wave
• SA node fires
1. Atrial depolarization
(contraction)
• HS4
2. Iternodal tracts
(shortcut to
AV node)

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

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

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

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

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

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

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
1. Heart Rate and Rhythm
2. 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 – Measurements
• Take 3-5 measurements and average
• All measurements done in lead II
• Use calipers
• Measure from the center of the line
• 50mm/sec

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

ECG – Heart Rate
Normals
• Giant dogs 60-140
• Toy dogs 80-180
• Cats 100-240
Med-Lg dogs 70-160
Puppies 70-220
Get Baseline heart rates for individuals on every visit

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

ECG – Rhythm
Respiratory Sinus Arrhythmia

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 2-2.5 boxes wide
– 25 mm/sec 1-1.25 boxes wide
• Normal Cat:
<0.2 mV x <0.04 sec
– 2 boxes tall

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

ECG – P Wave Measurements
Wandering pacemaker

ECG – PR Interval
Conduction from atria to ventricles (AV node)
Establishes the ECG baseline
Normal Dog: 0.06-0.13 sec
• 50mm/sec – 3-6.5 boxes
• 25mm/sec – 1.5-3.25 boxes
Normal Cat: 0.05-0.09 sec
• 50mm/sec – 2.5-4.5 boxes
• 25mm/sec – 1.25-2.25 boxes

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

ECG – PR Interval
Normal Dog: 0.06-0.13 sec (3-6.5 boxes)
Normal Cat: 0.05-0.09 sec (2.5-4.5 boxes)
• Long PR Interval
– Slow conduction through abnormal AV node
– AV Blocks

ECG – PR Interval
Normal Dog: 0.06-0.13 sec
• 3-6.5 boxes
Normal Cat: 0.05-0.09 sec
• 2.5-4.5 boxes
1 st degree AV Block
• Every P wave is followed by a QRS
• Due to increased vagal tone
• Non-pathogenic
50 mm/sec

ECG – PR Interval

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

ECG – PR Interval
2 nd 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 (escape beat)
• 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)

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

ECG – PR Interval
3 rd 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

ECG – PR Interval
3 rd 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
• 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

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

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 3 rd degree AV block

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

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 normal PR interval

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

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

ECG – ST Segment
ST segment depression or elevation
• >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

ECG – Mean Electrical Axis (MEA)
• The normal MEA is 40 o to 100 o 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

ECG – Mean Electrical Axis (MEA)
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)

ECG – Mean Electrical Axis (MEA)
+3 - 5 = -2
+10 -1.5 = +8.5
-
+
MEA = 105 o

ECG – Mean Electrical Axis (MEA)
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

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

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

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