Electrocardiogram
Wendy Blount, DVM
Nacogdoches TX
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
http://www.vetmed.wsu.edu/deptsVCGL/holter/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
Black – LF
Green – RR (ground)
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 1st negative
deflection
• R wave 1st positive
deflection
• S wave 2nd 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+ LFUnipolar 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
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
– 25 mm/sec 1-1.25 boxes wide
– 50 mm/sec 2-2.5 boxes wide
•
Normal Cat:
<0.2 mV x <0.04 sec
– 2 boxes tall
ECG – P Wave Measurements
•
Wide P wave (Sometimes Notched)
– 25 mm/sec > 1 box wide
– 50 mm/sec > 2 boxes 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
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
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
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
Normal Cat: 0.05-0.09 sec
•
Long PR Interval
– Slow conduction through abnormal AV node
– AV Blocks
ECG – PR Interval
Normal Dog: 0.06-0.13 sec
Normal Cat: 0.05-0.09 sec
50 mm/sec
1st degree AV Block
• Every P wave is followed
by a QRS
• Due to increased vagal tone
• Non-pathogenic
ECG – PR Interval
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)
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
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)
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
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
Prognosis
–
–
Cats – without anesthesia, potentially very good
Dogs – eventual asystole is likely, if no pacemaker implanted
ECG – PR Interval
3rd 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
25 mm/sec 1.25 boxes wide
50 mm/sec 2.5 boxes wide
>40 lbs: <0.06sec x <3.0 mV
•
•
25 mm/ sec 1.5 boxes wide
50 mm/sec 3 boxes wide
Normal Cat:
<0.04sec x <0.9 mV
•
•
•
9 boxes tall
25 mm/sec 1 box wide
50 mm/sec 2 boxes 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 3rd 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 40o to 100o
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 = 105o
+
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
+8
+13
+2
+1
+9.5
-5
-0
-2
-2
-8
-1
-2
+8
+11
0
-7
+8.5
MEA = +120o
Right Axis Shift
ECG – Mean Electrical Axis (MEA)
Normal Canine MEA
Normal Feline MEA
40-110o
0-160o
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