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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

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electrocardiogram

Electrocardiogram

Wendy Blount, DVM

Nacogdoches TX

ecg what it detects
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
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
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 do1
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
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 hints1
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
ECG – The Cardiac Cycle

P wave

  • SA node fires
    • Atrial depolarization

(contraction)

      • HS4

2. Iternodal tracts

(shortcut to

AV node)

ecg the cardiac cycle1
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 cycle2
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 cycle3
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 cycle4
ECG – The Cardiac Cycle

T wave

  • Ventricular

repolarization

(diastole)

  • HS3
    • Ventricular filling
    • if myocardium is

stiff

ecg the cardiac cycle5
ECG – The Cardiac Cycle

QT interval

  • beginning of QRS to

end of T wave

  • ventricular depolari-

zation & repolarization

  • HS1, HS2, HS3
  • Pulse generated
ecg the cardiac cycle6
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
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
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

ecg measurements
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
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 rate1
ECG – Heart Rate

Normals

  • Giant dogs 60-140 Med-Lg dogs 70-160
  • Toy dogs 80-180 Puppies 70-220
  • Cats 100-240

Get Baseline heart rates for individuals on every visit

ecg rhythm
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 rhythm1
ECG – Rhythm

Respiratory Sinus Arrhythmia

ecg p wave measurements
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 measurements1
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 measurements2
ECG – P Wave Measurements

Wandering pacemaker

ecg pr interval1
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 interval2
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 interval3
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 interval4
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 interval5
ECG – PR Interval

Normal Dog: 0.06-0.13 sec

Normal Cat: 0.05-0.09 sec

1st degree AV Block

  • Every P wave is followed

by a QRS

  • Due to increased vagal tone
  • Non-pathogenic

50 mm/sec

ecg pr interval7
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 interval8
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
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 pacemakers1
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 interval9
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 interval10
ECG – PR Interval

3rd degree AV Block (complete AV block)

  • Pacemaker above bifurcation of bundle of His
  • Pacemaker left ventricle
ecg qrs complex measurements
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 measurements1
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
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 blocks1
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 blocks2
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
Ventricular Premature Complexes
  • Depolarization wave through myocardium rather than through Purkinje network on affected side
    • takes longer
    • “appears bigger” on ECG
ventricular premature complexes1
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
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
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 mea1
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 mea2
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 mea3
ECG – Mean Electrical Axis (MEA)

+3 - 5 = -2

+10 -1.5 = +8.5

-

+

MEA = 105o

ecg mean electrical axis mea4
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 mea5
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 = +120o

Right Axis Shift

ecg mean electrical axis mea6
ECG – Mean Electrical Axis (MEA)

Normal Canine MEA

40-110o

Normal Feline MEA

0-160o

ecg mean electrical axis mea7
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
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