The document discusses principles of electrocardiography (ECG). It explains that an ECG complex consists of a PQRST waveform. The P wave indicates atrial depolarization while the QRS wave is produced by ventricular depolarization. It provides a brief history of ECG, noting that Willem Einthoven developed the bipolar triaxial lead system still used today. The document also lists common indications for ECG, such as arrhythmias, shock, or murmurs. It discusses the normal cardiac conduction system and rules for interpreting the polarity of deflections in ECG complexes.

1. Principles of ECG
Prof. Dr. Bibek Chandra Sutradhar
An ECG complex consists of a PQRST complex. The
Sinoatrial node (SA) is the pacemaker of the heart and
produces the P wave. The QRS wave is produced by the
Atrioventricular node (AV). The P wave in an ECG complex
indicates atrial depolarization.

2. History of ECG
Dutch physiologist Willem Einthoven in the early 20th century
EINTHOVEN recording his first ECG in 1902
by placing limbs in buckets of conducting solution!
The bipolar triaxial lead system we use today was developed by Willem
Einthoven
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4. • Electrocardiogram
= EKG = ECG
• A recording of the
electrical activity of the
heart from electrodes
placed on the surface of
skin.
• Specific waveforms
represent stages of
myocardial depolarization
and repolarization.
What is ECG???
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5. Electrocardiograph
• Electrocardiograph (ECG machine) is a voltmeter or
galvanometer that records the changing electrical activity of
the heart between a positive and negative electrode.
• Electrocardiography is the process of recording this
electrical changes.
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6. Indications
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7. Indications
The electrocardiogram (ECG) is a valuable diagnostic test in
veterinary medicine and is easy to acquire.
It is the most important test to perform in animals with an
auscultable arrhythmia (other than sinus arrhythmia in
dogs).
The ECG may also yield useful information regarding
chamber dilation and hypertrophy. However, the ECG does
not record cardiac mechanical activity, so it does not yield
information regarding cardiac contractility. It’s also
important to remember that the ECG may be normal even in
the face of advanced cardiovascular disease.
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8. 1. Cardiac arrhythmias.
2. Acute onset of dyspnoea
3. Shock.
4. Fainting or seizures.
5. Cardiac monitoring during and after surgery.
6. Cardiac murmurs.
7. Cardiomegaly found on thoracic radiographs.
Indications
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9. 8. Cyanosis.
8. Pre operatively in older animals.
10. Evaluating the effect of cardiac drugs –
especially
quinidine and propanolol.
11. Electrolyte disturbances, especially potassium
abnormalities.
11. Systemic diseases that affect the heart.
11. Serial electrocardiograms as an aid in the prognosis and
diagnosis of cardiac disease.
Indications
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11. Normal Conduction System
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12. Normal Conduction System
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14. The 1st Rule of ECG
• A current of depolarization
traveling towards the +
electrode is recorded as a
positive deflection
• A current of depolarization
traveling away from the +
electrode is seen as a
negative deflection
• A current of
repolarization traveling
away from the + electrode
is seen as a positive
deflection
Fig: As the positive wave of depolarization within the myocytes flows toward a
positive electrode, there is a positive (upward) deflection recorded on EKG
Positive deflection Negative deflection Biphasic deflection
Isoelectric line
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15. The 2nd Rule of ECG
Fibrotic tissue is electrically inert
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16. The 3rd Rule of ECG
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17. What does the P wave represent
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19. What does the QRS wave
represent
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⚫ S wave
◦ Negative waveform
following the R wave
⚫ Normal duration of
QRS
◦ 0.06 mm – 0.10 mm
⚫ Not all QRS Complexes
have a Q, R and S

24. Positive and Negative Deflection in a lead
A wave of electrical depolarization moves towards
the positive pole of the lead – a +ve deflection
occurs
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25. Limitations
ECG must always be evaluated in conjunction
with clinical findings
öAn animal with CHF may have a normal ECG
and normal animal may show non specific
electrocardiographic abnormalities
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26. Types of ECG
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27. o Single channel recorder
-Has one stylus – records one lead
at a time
o Multiple channel recorder
-Has more than one stylus provide
simultaneous tracings of 3 leads
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30. Position & Restraint
o Lateral recumbency - the standard position for canine and
feline electrocardiography
o No chemical restraint
o Trained attendant or animal owner
o Lead placement
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31. Preparing for the ECG
• Place the patient on a padded surface; don’t let the patient
or the clips touch metal.
• Place the patient in right-lateral recumbency when possible.
Note: This is the only position valid to determine the ECG axis. Other
acceptable positions include standing or sternal, however, we need the
position included in the history if not in right-lateral recumbency.
Try these techniques to minimize panting, shaking or purring:
• For animals that are shaking try placing your hands on either side of
the chest and apply moderate pressure.
• To reduce panting, hold the patient’s mouth closed. Be careful not to
obstruct breathing.
• To stop purring, wave an alcohol-soaked cotton ball under the
patient’s nose.
How to Prepare a Patient for an ECG
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32. • Make sure that the patient’s limbs do not touch each other.
• Make sure cables and electrodes do not touch each other or
anything metal.
• Soak the area to be clipped with alcohol.
• Part the fur for good skin contact and attach clips 1"–2" below
elbows and knees. Clip back dense fur.
• Do not place clips on calluses or joints.
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33. How to get a chest lead
•A chest lead may be helpful if the P wave is not clear or if
the complexes are small.
• Follow the general instructions for using clips, but clip the
black cable to the left side of the chest, between fourth and
fifth ribs, wherever you feel the strongest heartbeat.
• Place the other clips on the limbs, as previously described.
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34. Firmly insert the lead tip of the patient cable to the electrode,
matching the tip color and electrode site. Refer to the table below.
PATIENT CABLE TIP COLOR COATING
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35. Problem solving with cats
Unlike humans and most dogs, the cardiac axis is not aligned
top right to bottom left in cats. The heart has a tendency to lie
more centrally with its apex more ventral than the atria, i.e.,
the heart points downward towards the ground when the
animal stands.
This gives rise to one of the common problems with monitoring cats,
finding the strongest signal to present to the RA and LL electrodes. The
best signal is derived top/bottom axis i.e. Lead II in humans, with RA
looking at the top of the heart and LL at the bottom. In cats, as stated, the
axis may not lie across the body. (see figure 1)
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36. Figure-1
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37. As lead II may not align with the cat’s axis, the signal is
small and sometimes cancels. Therefore, by moving RA
more centrally onto the cat’s body above the top, and LL
onto the cat’s body below the bottom of the heart, a much
larger signal will be obtained. The plane in which the cat’s
heart lies within its body may also vary. The “top” of the
heart may be more dorsal and the “bottom” more ventral. In
this case, we would refer to the base/apex axis (see figure 2)
when the following instructions should be followed. 1. Move
LL to the left apex of the heart. 2. Move RA to the V10
position (over the dorsal spinous process of the seventh
thoracic vertebra) and LL to the V4 position (sixth left
intercostal space at the costochondrail junction). It will be
necessary to annotate the printouts, if any, with actual
configurations used to avoid later confusion.
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39. lead II, a bipolar lead in which the right arm (RA—the right
foreleg in veterinary patients) is negative and the left leg (LL—
or left hind leg), is positive (Figure 1)
Fig: The three bipolar frontal plane leads (I, II
and III) and Einthoven’s triangle (red). In lead I,
the right foreleg (RA) is negative and the left
foreleg (LA) is positive. In lead II the right
foreleg (RA) is negative and the left hind leg (LL)
is positive. In lead III the left foreleg (LA) is
negative and the left hindleg (LL) is positive. The
net depolarization moving through the ventricles
(green arrow) is normally oriented toward the
left hind leg (the positive pole of lead II) in dogs
and cats, and therefore the QRS complex is
predominantly positive in lead II.
With ECG machines that utilize four electrodes,
the electrode placed on the right hind leg is the
ground (it is not part of any of the leads).
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40. Gold standard ECG positioning in a dog 40
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41. For cats it is thought that body positioning may be less important,
and patient compliance to be improved with a more relaxed position (Willis,
2010). If a patient is dyspnoeic, an ECG should be taken in sternal
recumbency and done with minimal stress. Standard electrode configuration
on ECG machines consists of four cables and electrodes, which includes one
earth electrode.
Gold standard ECG positioning in a cat
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43. Electrodes &
Leads
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44. ELECTRODES
Alligator clips or flat contact electrodes
Hair and stern surrounding the electrode should be
moistened with conductive gel or alcohol
Standard paper speed : 50 mm/sec.
Left Arm (LA), Right Arm (RA) and Left Leg (LL)
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Standard leads
Useful for studying
1. Abnormalities in the P-QRS-T
deflection
2. Diagnosing cardiac arrhythmias
3. Determining the mean electrical axis

46. LEAD SYSTEM
BIPOLAR STANDARD LEADS
Lead I : Right arm (-) compared with Left (+) arm
Lead II : Right arm (-) compared with Left (+) leg
Lead III : Left arm (-) compared with Left (+) leg
AUGMENTED UNIPOLAR LIMB LEADS
aVR :Augmented Vector Right
aVL : Augmented Vector Left
aVF : Augmented Vector Front
SPECIAL LEADS
Lead CV5 RL (V2)
Lead CV6 LL (V2)
Lead CV6 LU (V4)
Lead V10
They are derived from the
same three electrodes
as leads I, II, and III, but
they use Goldberger's
central terminal as their
negative pole.
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47. L I, II, III, aVR, aVL, & aVF - Frontal plane leads.
L V1, V2, V3, V4, V5, V6, & V10 – Horizontal
(or transverse) plane leads.
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48. Normal ECG
P
QR
S T
: Atrial depolarisation
: Ventricular depolarisation
: Ventricular repolarisation
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49. QRS Complex T wave
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50. Normal ECG Parameters - Dog & Cat
Heart Rate (beats per minute) :
Canine :
»Adults: 70-160
»Toy Breeds: up to 180
»Puppies: up to 220
Feline :
»160-240
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51. Amplitudes (mV) in lead II
P amp
Canine
< 0.4
Feline
< 0.2
R < 20 kg: < 2.5
> 20 kg: < 3.0 <
0.9
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52. Intervals (seconds)
Canine Feline
P < 0.04 < 0.04
PR 0.06 -0.13 0.05 - 0.09
QRS < 20 kg: < 0.05 < 0.04
> 20 kg: < 0.06
QT 0.15 to 0.25 0.12 to 0.18
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53. Normal ECG Parameters - Dog & Cat
Rhythm : rhythm controlled by sinus node at a rate of 60-100
beats/min; each P wave followed by QRS and each QRS preceded
by a P wave.
Canine :
Normal sinus rhythm
Sinus arrhythmia
Wandering atrial pacemaker
Feline :
Normal sinus rhythm
Sinus tachycardia
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54. Specific Arrhythmias
• Sinus bradycardia
• Sinus tachycardia
• Sick sinus syndrome- disturbance of SA nodal function that results
in a markedly variable rhythm (cycles of bradycardia and
tachycardia).
• Atrial flutter - sinus rate of 250-350 beats/min
• AV nodal blocks - a conduction block within the AV node (or
occasionally in the bundle of His) that impairs impulse conduction
from the atria to the ventricles.
• Ventricular flutter - very rapid ventricular depolarizations
>250/min
• Ventricular fibrillation - uncoordinated ventricular
depolarizations;
leads to death if not quickly corrected
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Methodical approaches to
ECG

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Figure 3. Gold standard ECG positioning for a cat. Always
try to minimise stress when recording an ECG in a cat.
Four electrodes including one earth.

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The six lead ECG system. As seen in this diagram, each lead provides a unique
perspective on the heart.
Gold standard ECG positioning in a dog.

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Electrode
Colour for British
machines
Colour for American
machines
Right forelimb Red White
Left forelimb Yellow Black
Left hindlimb Green Red
Right hindlimb Black Green
Electrode placement for UK and US ECG machines

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An example of electrical artefact. Note the unclear baseline making P wave identification impossible.
ECG machine showing gain and speed.

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Heart rate calculation using an ECG ruler. The ruler has one side for 25 mm/seconds, the
other for 50 mm/second. The ruler should be lined up with lead II as shown, and the distance
from the first R wave to the next R wave equals the heart rate. In this example, the heart rate
is 160 beats/min. If the rate is variable on the ECG trace (if it slows down or speeds up),
repeated measurements might need to be taken along lead II to give a more accurate answer.

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BIC pen method for 25 mm/second or 50 mm/second. Lie the BIC pen alongside lead II.
Measure from the first R wave, count the number of R wave intervals, until the end of the pen.
Depending on the paper speed, multiply by 10 if the paper speed is 25 mm/second or 20 if the
paper speed is 50 mm/second. As with the ruler method, a number of measurements might be
needed if the rate is variable.

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ECG interpretation essentially involves four
main steps:
1. Calculation of the heart rate;
2. Determination of the heart rhythm;
3. Measurement of the complex amplitudes and intervals;
4. MEASUREMENT OF THE MEAN ELECTRICAL AXIS:
A good understanding of the electrical activity of the heart is key to
the accurate interpretations of ECG (Martin, 2002).

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How to measure heart rate
???

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1. Calculation of the average heart rate
• Each large box = 5 × 5 small boxes
• At a paper speed of 25 mm/sec:□ 5 large boxes (a 25-
mm length of paper) pass in 1 second
• In 1 minute, 5 × 60 boxes (300) pass. Therefore each
large box = 0.2 second
• 25 small boxes pass in 1 second
• In 1 minute, 25 × 60 boxes (1500) pass. Therefore
each small box = 0.04 second
Use either of the following methods to calculate heart rate.
(Either method can be used in any species, as long as the heart rhythm
is regular; this method is not reliable if bradycardia with pauses is
present.)

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First, start at a complex on a heavy line (see arrows in figures
following), then go to the next complex.
▪ Count number of large boxes (5 × 5 small boxes) between
consecutive R waves of complexes
-Divide 300 by this number
▪ Count the number of small boxes between consecutive R
waves of complexes
-Divide 1500 by this number
Note: At a paper speed of 50 mm/sec use the same method but substitute
600 for 300 and 3000 for 1500.

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Dog
This dog’s heart rate can be calculated as 300 ÷ 2.4 = 125. An
alternative method is 1500 ÷ 12 = 125.

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Cat
This cat’s heart rate can be calculated as 1500 ÷ 9.5 = 158.
An alternative method is 300 ÷ 1.9 = 158.

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See the Rhythm …Regular
or irregular???

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Rhythm
1. Regular
irregular
2. Irregular
irregular

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How to decide
MEA???

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• The Vector Method: Using leads I, II or III and the frontal
plane diagram, calculate the algebraic sum of the QRS
deflections in any two leads.
• The Isoelectric Method.
• The Largest Net Deflection Method.
There are three common methods of calculating
the MEA in the frontal plane

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Normal ECG

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96. What differentiates a segment from an interval?
A segment is a straight line connecting two waves.
An interval encompasses at least one wave plus the
connecting straight line.
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PQRST Wave
System
P
wave
Width: maximum, 0.04 sec (2
boxes wide)maximum, 0.05 sec (2 ½
boxes wide) in giant breeds.
Height: maximum, 0.4 mV (4
boxes tall).
The P wave is the electrocardiographic representation of electrical
activation of the atrial myocardium.

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Right atrial enlargement - P too high in L II.
Dog: P > 0.4 mv ; Cat: P > 0.2 mv
Biatrial enlargement - P too tall and wide in L II.
Left atrial enlargement - P too wide in L II.
Dog & Cat : P > 0.04 sec

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Rt Atrial enlargement (P Amp )

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What is the PR interval? What is the significance of
the PR interval?
The PR interval is the
electrocardiographic
representation of the delay
in electrical activation of
the ventricles after sino-
atrial nodal discharge.
This interval is mainly a
result of slow conduction
through the atrio-
ventricular node.

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QRS wave

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The QRS complex is the electrocardiographic expression of the
electrical activation of the ventricular myocardium.

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Width: maximum, 0.05 sec (2 ½ boxes wide)
in small breeds. maximum, 0.06 sec (3 boxes)
in large breeds.
Height of R wave*: maximum, 3.0mV
(30 boxes) in large breeds.
maximum, 2.5 mV (25 boxes) in
small breeds.
P-R interval
Width: 0.06 to 0.13 sec (3 to 6 ½ boxes).
QRS
complex

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Wide QRS Complexes
Etiologies:
Aberrant
conduction
(Bundle Branch
Block)
Myocardial
hypoxemia/ischemia
Left ventricular
enlargement

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Low amplitude QRS complexes
Etiologies: Pleural effusion ,Pericardial effusion, Obesity
,Hypothyroidism, Pneumothorax and Diffuse myocardial disease.

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Left Ventricular Enlargement
R wave amplitude in L II increased.
QRS duration in L II increased.

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Lt Ventricular Enlargement (Increased R
Amp)

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Rt. Ventricular enlargement
Presence of an S wave in leads I, II, III.
MEA in the frontal plane shifted to the
right (pointing to the right ventricle): 100°
to – 75°
Deep S wave in lead V3; S = 0.7 mv
Deep Q waves in leads I,II and III and after
aVF greater than 9.5 mV (5 boxes)

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Rt. Ventricular enlargement

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T
wave

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T wave
❖ Can be positive, negative or diphasic
❖Not greater than one fourth amplitude
of R wave; amplitude range + 0.05 – 1.0
mV (1/2 to 10 boxes) in any lead.

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T wave abnormalities
Should not be greater than 1/4 of the R wave.
Sharply pointed (or) Notched – Electrolyte imbalances
Electolyte abnormality

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Peaked T wave

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Q-T interval duration
Dog : 0.15 to 0.25 sec (7 ½ to 12
½ boxes)
Cat: 0.12 to 0.18 sec
Faster the heart rate, shorter the Q-T
interval
Q-T interval should be less than
half the preceding R-R
interval

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Prolongation of Q-T interval
⚫ Myocardial problems,
⚫ Toxicity or Hypoxia
⚫ Hypokalemia
⚫ Hyperkalemia
⚫ Hypocalcemia
⚫ Antiarrhythmic drugs
Shortening of QT interval
⚫ Hypercalcemia

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ST segment abnormalities
ST segment depression - 0.2 mV ;seen in
Myocardial ischemia
Myocardial infarction
Hyper and Hypokalemia
Trauma to the heart.
ST segment elevation - 0.15 mV in Lead I ; seen in
Pericarditis,
Severe ischaemia/infarction (e.g. full wall
thickness).

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Sinus Arrhythmia and Wandering
Atrial Pacemaker in the Dog
Sinus arrhythmia - Irregular ventricular rhythm which is of
sino-atrial origin. On the EKG, the QRS to QRS interval varies
and there is a P wave for every QRS complex.

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Sinus Arrhythmia

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Wandering Atrial Pacemaker in the
Dog

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Sinus rhythm:
A sequence of beats originating from the sino
atrial node forms a rhythm, known as the
sinus rhythm.
There are four common sinus rhythms.

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Sinus Bradycardia in the Dog
1.
Hypothyroidis
m ,
3. Hyperkalemia,
2. Hypothermia,
4.
Hypoglyce
mia
5. Enhanced parasympathetic tone as with:
Increased inspiratory effort, Gastric
irritation, Increased CSF pressure.
Atropine will abolish the bradycardia (atropine - 0.04 mg/kg
IV)

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Sinus bradycardia
In Sinus bradycardia, the SA Node
generates an impulse and depolarization
occurs more slowly than normal. This can
be a normal feature in some giant breed
dogs and in athletically fit animals. The ECG
shows a normal sinus rhythm but at a
slower rate than normal.

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Sinus
bradycardia

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Sinus tachycardia
In Sinus tachycardia, the SA Node
generates an impulse and depolarization
occurs faster than normal. The ECG
shows a normal sinus rhythm but at a
faster rate than normal.

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Sinus tachycardia
• It is a sinus rhythm with an increased ventricular rate.
Dog (<20 kg) with heart rate 180 bpm
Dog (20 kg) with heart rate 160 bpm
Puppies with heart rate 220 bpm
Cat with heart rate 240 bpm
• Etiology: Pain,Fever, Anemia, Reduced cardiac output,
Hyperthyroidism & Excitement.

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Atrial fibrillation
Irregular rhythm and absence of P
waves

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Absence of P Wave (Atrial Fibrillation)

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Ventricular premature contractions
(VPC)
Premature beats. QRS complexes are wide & bizarre.
Common finding in dogs and cats and arise from an ectopic
focus or foci within the ventricular myocardium.

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Ventricular Tachycardia (VT)
Refers to runs of greater than 3 PVC's in
sequence. Markedly reduce cardiac output
(dysynergy of contraction).
Etiology is as for PVC.

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Other abnormalities
Right bundle branch block
Wide S waves
QRS complex greater than 0.08 sec.
Left bundle branch block
QRS complex greater than 0.08 sec
duration
QRS complex wide and +ve.

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Artifacts
Muscle tremor artifact.
Movement artifact.
Electrical interference.
Electrical alternans
Alternation in the size of the QRS
amplitude that occurs nearly every other
beat.

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Artifact

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161. What differentiates a segment from an interval?
A segment is a straight line connecting two waves.
An interval encompasses at least one wave plus the
connecting straight line.
161
Dr. Bibek/CVASU

162. J WAVE AND U WAVE (Abnormal waves) ?
J wave= hypothermic wave (Junction between QRS complex and ST segment)
U wave=hypokalemia/hypercalcemia and thyrotoxicosis (Follows the T wave)
162
Dr. Bibek/CVASU

163. Dr. Bibek/CVASU 163
⚫U wave
◦ Small waveform
◦ Follows T wave
◦ Less than 1.5 mm in
amplitude

164. Dr. Bibek/CVASU 164
Thank U