A guide to electrocardiography
Reading and Applying
Major Confusion!!!
 Reading an ECG and deciphering what it is telling you can be
confusing
 ECG’s dictate the electrical
impulses of the heart
 Electrical impulses are what
make the heart contract
 This contraction is also known as
depolarization
A Little Vocabulary
•Artifact: A wave that arises from sources other
than the heart or brain.
•Cardiac Arrhythmia: Any electrical activity of the
heart that differs from that of a healthy awake
patient. Can be innocuous or life threatening.
•Deflection: movement from one side to the other.
•Depolarization: Contractions as a result of cells
stimulated by electricity.
•Ectopic: occurring in an abnormal position.
•Lead: A pair of electrodes, connected by an axis
that provides a particular view of the of the
electrical activity of the heart.
•Repolarization: Relaxation of cells after
depolarization.
Cables
Placement
 Black and white on front limbs
(B=Left/W=Right)
-Placed at the elbow region
 Green and red on back limbs
(G=Right/R=Left)
-Placed at the stifle region.
*Alcohol, ECG paste or ECG pads can be used as
conduction agents for more accurate readings. If
your patient has long hair, it should be shaved at
the site of cable placement in order to achieve
adequate conduction in addition to the use of a
conduction agent.
Solid Ground
•Placing the patient on a rubber mat
and ensuring that the machine is
plugged into a grounded outlet as
well as checking the ECG machine
for loose wires or cracked
insulation on wires are steps that
can be taken for a good ground.
•Having the ECG
machine well grounded is
an important factor for
obtaining an accurate
reading.
Leads
•Lead I: causes right forelimb to
become a negative pole and the left
forelimb to become a positive pole.
•Lead II: causes right forelimb to
become negative and left rear limb to
become positive
•Lead III: the left hind limb becomes a
positive pole and the left forelimb
becomes a negative pole.
•What happened to the Green cable you
ask? It is only a reference point. Don’t
forget it’s there, but concentrate on the
main three!
•The cables read the electrical activity
of the heart between two points
•Selecting specific leads on the ECG
machine will cause specific cables to
become negative or positive poles.
(mildly complicated)
Oh, no! Humans, who let them in here?
Mind your PQRST’s
 P wave: represents atrial depolarization
 Therefore any changes in the P wave could indicate atrial




dysfunction, or a degree of heart block
QRS wave: represents ventricular depolarization
So, any changes in the QRS wave could indicate ventricular
fibrillation, lack of a QRS wave could indicate a form of heart
block, and a wide QRS wave might indicate Premature
Ventricular Contractions.
T wave: Represents ventricular repolarization
Myocardial hypoxia, and electrolyte disturbances can be
evident with changes in the T wave.
Fill in the Blanks…
•P wave: Created with the depolarization of
the sinoatrial node, which spreads electrical
activity to the atria. This electrical impulse
is spread downwards to the left, in the
direction of the positive pole of lead II.
Because the electrical activity is moving in a
downward deflection away from the
negative pole, the ECG tracing inscribes the
P wave in an upward direction.
Between the Lines…
 The PR interval is inscribed
due to a delay of
depolarization at the
atrioventricular node
(allowing time for the
ventricles to fill) and no wave
is produced.
On your mark….
•Q Wave
•The wave of depolarization moves through the
Bundle of His and it’s branches, and then the
interventricular septum is depolarized. This is
the first part of the ventricular myocardium to
become depolarized. This depolarization occurs
from left to right. The electrical current moves
through the thin layered interventricular septum
in a downward motion, resulting in the Q wave
being inscribed on the ECG. The Q wave is
aimed downward because the current moves
toward the negative pole.
Get ready…
 R wave
 From the downward inscription
of the Q wave on the ECG,
depolarization spreads along the
ventricular conduction system
and depolarization occurs in both
ventricles simultaneously.
Because the Left Ventricle has
greater mass than the right
ventricle, most of the electrical
impulse travels in a left and
downward direction. This
electrical activity moving in an
upward deflection towards the
positive pole of Lead II results in
the inscription of the large R
wave.
Set…
 S wave
 Electrical impulses move
from the apex to the base
of the heart, continuing
ventricular
depolarization. This
results in the ECG
inscription of the S wave.
Almost there….
 ST interval
 After the ventricle is
completely depolarized, few
impulses take place during this
time before repolarization (also
called ventricular relaxation).
This time is inscribed on the
ECG as the ST interval.
 At this time the heart is
susceptible to any electrical
impulse and somewhat
unstable. *This is the point
where defibrillators are placed
during human CPR.
Go!
 T wave
 Ahhhh! Repolarization.
 This occurs from the endocardium
to the epicardium. Because the left
ventricle has more mass than the
right ventricle or atria, electrical
impulses travel downward to the
left. This produces an upward T
wave in Lead II.
 A period of electrical inactivity
follows repolarization, and is
inscribed as a flat line until the SA
node begins all over again!
Panic at the T Wave
 Sometimes the T wave can
deflect downwards
depending on the patient,
so do not be alarmed if it
looks like this on your
reading.
Making sense?
Or do you feel like
this?
So What?
•Now that we know how the ECG
of a normal, healthy heart is
produced and what it should look
like, we can detect abnormalities.
•Excited?
•Some cardiac disturbances which
can be detected are PVC’s,
myocardial hypoxia, electrolyte
disturbances, fibrillation, sinus
bradycardia, atrial dysfunction,
and other arrhythmias.
Interference
 Remember that rubber mat and
how you checked the machine for
any causes of a bad ground to the
machine?
 Interference can be caused by
other machinery such as a pulseox, or BP monitor that is hooked to
the animal, even fluorescent
lighting.
 This is what interference looks like
on the reading. Make a mental
picture of how fuzzy it looks.
Muscle Tremor
•If your patient is not calm and
comfortable, or just really
nervous and shaky… the
reading may look like this. Also
caused by happy, purring feline
friends.
• Reapplying or readjusting the
clips may help.
•A towel or blanket can be
placed on top of patient to help
calm them.
•You can also place a hand on
the chest of your patient. Be
careful not to apply too much
pressure or it will interfere with
the reading.
•Looks much different than interference right?
Wandering Baseline
 Resistance between the patient
and the electrode can cause a
wandering baseline.
 An example would be
respiratory movement.
 Placing the patient in sternal
recumbancy or a standing
position will help to reduce this
resistance.
 Holding the patient’s mouth
closed for a few seconds could
also help reduce resistance.
Atrial Premature Contraction
•The P wave of a Atrial Premature Contraction is different from that
of a normal sinus rhythm.
•The origin of a Atrial Premature Contraction is the sinoatrial node or
displaced locations in the atria, and may or may not conduct to the
ventricles; this depends on when the impulse reaches the AV node.
•If depolarization occurs in the ventricles, the QRS wave will be
normal.
•If depolarization does NOT occur in the ventricles, and reaches the
AV node before repolarization, a premature P waves w/o QRS waves
will read on the ECG.
Ventricular Premature Contraction
 Seen on ECG as wide, bizarre QRS complexes w/o associated P waves. (So
this is going to be a weird and bizarre explanation.)
 This is because the origin of depolarization is a cell-to-cell depolarization
and repolarization from the ventricular myocardium, not the SA node.
 In addition, if the impulse originated in the left ventricle, the wave of
depolarization will be travel up, and the Ventricular Premature Contraction
would be a downward deflection on the ECG.
 However, if the impulse originated from the right ventricle, the Ventricular
Premature Contraction would travel downward and left. The ECG tracing
will read an upward deflection.
Escape Beats
 Escape Beat is a safety mechanism in the
heart which functions when the SA node
fails to fire an impulse.
 They are the result of depolarization in
other locations of the heart which cause
the escape beat.
 When the AV node depolarizes, the
current travels upward into the atria and
then downward to the ventricles along
the normal conduction system.
Escape Beats cont’d
 Depolarization through the ventricles
produces a normal QRS complex.
 Depolarization through the atria causes a
downward tracing of the P wave in lead
II on the ECG. Depending on the speed
of the upward wave of polarization, it
may appear before, after or during the
QRS complex.
 If the AV node fails to depolarize, the
ventricles will take over and resemble a
VPC as a wide, bizarre QRS complex
w/o an associated P wave.
 The difference between the two is that an
escape beat is life-saving, and a VPC
disturbs an otherwise healthy rhythm.
A
B
Tachycardia
 Can result from the use of drugs: ketamine, atropine, or epinepherine.
 Also due to surgical stimulation, but may not mean that anesthetic depth is
too shallow unless accompanied by increased respiration, movement, or
reflex reaction.
 Anesthetic complication?
 Hypoxia, hypotension, or hypercapnia
 Preexisting condition?
 Hyperthyroidism, anemia, circulatory shock, septicemia, cardiac dz,, or
excitement in an awake patient
Direct digital pressure can be applied to the eyeballs, or
propranolol can be administered to slow the heart
Sinus Tachycardia
 Normal sinus rhythm.
 Characterized by a rhythm faster then 160 beats/min in
dogs and faster than 240 beats/min in cats.
Ventricular Tachycardia
 Three or more ectopic ventricular complexes at a rate of 140
beats/min.
 May appear and then disappear during normal rhythm
(paraxysmal ventricular tachycardia) or be present throughout.
 If the arrhythmia is present at all times, it indicates an irritable
ventricular myocardium, and may precede ventricular
fibrillation.
How are you feeling?
A little green?
Atrial Flutter
 Seen as uniform, saw-tooth shaped wavelengths between QRS complexes.
 Can be difficult to identify.
 An atrial flutter is a rapid atrial depolarization which occurs at the rate of 250 -350
beats/min.
 The AV node becomes flooded with depolarizations from the atria.
 The ventricular rate varies from the atrial rate because it takes time for the AV node
to catch up with all the impulses being fired at it by the atria.
 The ventricular rates for this arrhythmia are generally 140-150 beats/min and
bizarre P waves my appear between QRS complexes.
Sinus Bradycardia
 A normal sinus rhythm.
 Originated from the SA node.
 Characterized by a rhythm less then 60 beats/min in dogs and
70-80 beats/min in cats.
 Based on an individual patient basis, and may be normal for
some such as athletic dogs.
Atrial Fibrillation
 Lacking P wave
 Constant irregular rhythm
 Fast rhythm for the atria (350-600 beats/min) and ventricles
(220-240 beats/min)
 Decreased cardiac output due to insufficient atrial contractions
which lead to inadequate filling of the ventricles.
Ventricular Fibrillation
 Chaotic depolarization of the ventricles results in
disorganized contractions
 Inscribed on ECG as small or large undulations of the
baseline, but lacking true QRS complexes
 Ventricular Fibrillation WILL lead to cardiac arrest!!!
Heart Block: Electrical impulse is
not transmitted throughout the heart
 First degree heart block

The interval between the P wave and the QRS complex is
prolonged, but the sequence is normal
 Second Degree heart block

P waves sometimes not followed by QRS complexes
 Third Degree heart block

P wave, QRS complex pattern is irregular
 2nd and 3rd degree heart block lower the ability for the heart to
contract fully. Can be caused by Alpha-2 Agonists, increased
vagal tone, hyperkalemia, and cardiac dz.
Heart Block cont’d
Created by Emily L. Dzek
Fall 2007
With MUCH help from:
McKelvey, Diane and Hollingshead, Wayne K. Veterinary Anesthesia and
Analgesia. 3rd ed. St. Louis: Mosby, 2003. Glaze, Kathy. “Basic
Electrocardiography Part I”. Veterinary Technician 17(1996):661-667. Glaze,
Kathy. “Basic Electrocardiography Part II”. Veterinary Technician
17(1996):719-725. Glaze, Kathy. “basic Electrocardiography Part III”.
Veterinary Technician 18(1997).
Many pictures from online sources
Rhythm strip examples from Veterinary Technician magazine listed above.