Transcript Name of presentation

Electrocardiogram
Wendy Blount, DVM
Nacogdoches TX
ECG – What it Detects
Heart chamber enlargement
• Eccentric hypertrophy
– Dilation and growth
– Due to volume overload
• Concentric hypertrophy
– Wall thickening
– Due to pressure overload
Conduction Disturbances
ECG – When to Do
• Pulse deficits detected on exam
• Chaotic heart sounds (arrhythmia)
detected on exam
• Tachycardia
• Bradycardia
• Episodes of weakness or collapse
Intermittent arrhythmias may not be
detected in a single ECG
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
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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
– Atrial depolarization (contraction)
• HS4
– Iternodal tracts (shortcut to AV node)
PR interval (no deflection - baseline)
• Beginning of P wave to beginning of QRS
• AV node (*most of the PR interval is here*)
• Bundle of HIS
• bundle branches (R and 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
• HS1 at beginning of QRS
• HS2 at end of QRS
• Pulse is generated
ECG – The Cardiac Cycle
T wave
• Ventricular repolarization (diastole)
• HS3 if myocardium is stiff
QT interval (no deflection - baseline)
• beginning of QRS to end of T wave
• ventricular depolarization & repolarization
• QRS Contains HS1 and HS2
• HS3 during T wave
• Pulse generated
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 Deflections
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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
• 2 Bic Pens times Ten
• Accurate within 20 beats per minute
Normals
• Giant dogs 60-140
Med-Lg dogs 70-160
• Toy dogs 80-180
Puppies 70-220
• Cats 100-240
ECG – Rhythm
Normal Sinus rhythm
• Regular heart rate
• P, QRS and T waves in each complex
Respiratory Sinus Arrhythmia
• heart rate regularly irregular
• P, QRS and T waves in each complex
• Variable P wave – wandering pacemaker
• Heart rate less than 200
Arrhythmia
ECG – P Wave Measurements
Atrial depolarization (contraction)
Normal Dog: <0.4 mV x <0.04 sec
Normal Cat: <0.2 mV x <0.04 sec
• Wide P wave, Notched P wave
– LA enlargement
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Tall P wave (spiked)
– RA enlargement
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Variable P wave
– “wandering pacemaker” – increased vagal tone
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Lack of P wave
– Atrial standstill
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
• Short PR Interval (tachycardia)
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AV node is bypassed
“Accessory pathway” (Wolff-Parkinson-White)
Congenital or acquired
Treated in people by radioablation of the pathway
Can try calcium channel blockers
ECG – PR Interval
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Long PR Interval
– Slow conduction through abnormal AV node
– AV Blocks
1st degree AV Block
• Every P wave is followed by a QRS
• Due to increased vagal tone
• Non-pathogenic
ECG – PR Interval
2nd degree AV Block
Some P waves not followed by a QRS
• Mobitz type I – PR progressively longer until QRS
dropped
• Mobitz type 2 – no pattern
• Not necessarily pathogenic
3rd degree AV Block (complete AV block)
No relationship between P waves and QRS
• P waves have their own rate (faster)
• QRS has its own rate (slower)
• Treated by pacemaker
ECG – QRS Complex Measurements
Normal Dog: <40 lbs - <0.05sec x <3.0 mV
>40 lbs - <0.06sec x <3.0 mV
Normal Cat: <0.04sec x <0.9 mV
R wave measured from baseline to top
• Tall R wave, wide QRS
– LV enlargement
– Left Bundle branch block
ECG – ST Segment
Late ventricular depolarization
“J point” should be within 0.2 mV of baseline in
dogs
Should be on baseline in cats
• Between QRS and T waves
• Elevation or depression means regional
myocardial hypoxia
• Myocardial infarction not as common in dogs
and cats as in people
– Can see in cats with HCM
ECG – ST Segment
Other Causes of ST segment depression or
elevation
• hypothermia
• hypokalemia
• Digitalis toxicity
• Bundle branch block
ECG – Mean Electrical Axis (MEA)
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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)
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The normal MEA is 40o to 100o (dog)
Lead II is most perpendicular to the normal
MEA
Lead II shows the largest deflections in the dog
with normal MEA
This is why lead II is the most often used lead in
veterinary medicine
aVL is most often the isoelectric lead
ECG – Mean Electrical Axis (MEA)
+1.5 - 5 = -3.5
+10 -1.5 = +8.5
Estimating MEA
• Find the isolectric lead
• MEA is perpendicular to that, in the direction of
net deflection
Calculating MEA by graph
• Graph net deflection in lead I on “x axis”
• Graph net deflection in lead aVF on “y axis”
• Draw the vector between the two (MEA)
ECG – Mean Electrical Axis (MEA)
Right Axis Shift
• Right ventricular enlargement
– RV hypertrophy or dilation
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Right bundle branch block
Left Axis Shift
• HCM in cats
• hyperkalemia
3.5 (MEA)
ECG – Mean Electrical Axis
3.5
17.5
3
J
2.5
5.1
Right Bundle Branch Block (RBBB)
-15.5
2.5Rate
2.8
Heart
– 160 per minute (normal 80-180)
5.2
point
Rhythm – Jnormal
sinus with 2 complex types
Complex 1
Complex 2
P wave 0.05 sec x 0.35 mV
PR interval 0.104 sec
QRS wave 0.056 sec x 1.75 mV(R)
ST segment -0.1 mV depressed
MEA – 85o
P wave 0.05 sec x 0.35 mV
PR interval 0.102 sec
QRS wave 0.06 sec x -1.55 mV(S)
ST segment 0 deviation from PR
MEA -90o to -120o
ECG – Bundle Branch Blocks
Bundle Branch blocks
• Depolarization wave through myocardium rather
than through Purkinje network on affected side
• depolarization takes longer
• depolarization wave “appears bigger” on ECG
• Can be persistent or intermittent
• Intermittent often precipitated by increased
heart rate (delayed refractory period)
• Bilateral BBB looks like 3rd degree AV block
ECG – Bundle Branch Blocks
Right Bundle Branch Block (RBBB)
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primary conduction system disease
Disruption of moderator band
RV enlargement
Congenital (beagles)
ECG
– Deep S save leads I, II, III, aVF
– Wide QRS
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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
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ECG
– Tall R wave
– Wide QRS
– Looks like a VPC, but follows a P wave with normal
PR interval
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
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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)
• Escape rhythm – pacemaker other than
SA node, because it fails to fire