Electrical Events
of the Cardiac Cycle
Electrophysiology
Dr.s.a.moezzi
Cardiac Cycle
Cardiac Cycle: the electrical, pressure and
volume changes that occur in a functional
heart between successive heart beats.
• Phase of the cardiac cycle when
myocardium is relaxed is termed diastole.
• Phase of the cardiac cycle when the
myocardium contracts is termed systole.
– Atrial systole: when atria contract.
– Ventricular systole: when ventricles contract.
Mechanical Events of the
Cardiac Cycle
1.
2.
3.
4.
Ventricular Filling Period [ventricular
diastole, atrial systole]
Isovolumetric Contraction Period [ventricular
systole]
Ventricular Ejection Period [ventricular
systole]
Isovolumetric Relaxation Period [ventricular
diastole, atrial diastole]
Cardiac Cycle
 Electrical changes in heart tissue cause
mechanical changes, i.e. muscle
contraction.

Thus, changes in electrical membrane
potential of specific parts of the heart
tissue represent mechanical events in
specific areas of the heart tissue.
Electrical System of Heart
Electrocardiography

Two common abbreviations for
electrocardiogram: EKG and ECG.
 EKG comes from German language where
cardiogram is written as kardiogram.
 The ECG records the electrical activity of
the heart.
 Mechanical activity of the heart is sensed by
echocardiography.
Electrophysiology

If an electrode is placed so that wave of
depolarization spreads toward the recording
electrode, the ECG records a positive
(upward) deflection.
 If wave of depolarization spreads away
from recording electrode, a negative
(downward) deflection occurs.
Electrophysiology
Electrophysiology
Electrophysiology
Electrophysiology

When myocardial muscle is completely
polarized or depolarized, the ECG will not
record any electrical potential but rather a
flat line, isoelectric line.
 After depolarization, myocardial cells
undergo repolarization to return to electrical
state at rest.
Electrical Events of the
Cardiac Cycle
•
•
•
Sinoatrial (SA) node is the normal pacemaker
of heart and is located in right atrium.
Depolarization spreads from SA node across
atria and results in the P wave.
Three tracts within atria conduct depolarization
to atrioventricular (AV) node.
• Conduction slows in AV node to allow atria to
empty blood into ventricles before vent. systole.
•
•
Bundle of His connects AV to bundle branches.
Purkinje fibers are terminal bundle branches.
Electrical Conduction System

Sinoatrial node (SA node)
 Intra-atrial Pathways
 Atrioventricular node (AV node)
 Bundle of His
 Left and Right Bundle Branches
 Purkinjie Fibers
The P wave

The first wave form is called a P wave: it
represents Atrial depolarization.
 It is gently rounded, and not larger than 2-3
mm.
 It is usually positive (above the isoelectric
line)
 It should not be large, notched, or peaked
The QRS Complex





The QRS complex represents ventricular
depolarization.
The Q wave is the first negative deflection. It
should not be greater than 1mm wide or larger
than 1/3 height of R wave.
The R wave is the first positive deflection after the
Q wave.
The S wave is the first negative deflection after
the R wave
The normal QRS complex should be < .12 sec.
The T wave

The T wave represents ventricular
repolarization.
 It is usually positive, but can be negative or
biphasic.
 It is usually the same polarity as the QRS
complex.
The PR Interval

The PR interval is measured from the
beginning of the P wave to the beginning of
the QRS complex.
 The PR interal measures the beginning of
atrial depolarization through the beginning
of ventricular depolarization.
 The normal PR interval is from .12-.20
seconds.
The QT Interval

The QT interval is measured from the
beginning of the Q wave to the end of the T
wave. This measures ventricular
depolarization and repolarization.
 Any QT longer than .50 seconds can
predispose certain dangerous arrhythmias.
ST Segment

The ST segment is measured from the end of the S
wave to the beginning of the T wave.
 The ST segment is normally isoelectric, or curves
slightly upwards into the T wave.
 Horizontal or downsloping ST depression of 2 mm
or more is abnormal, indicating ishemia.
 ST segment elevation > 1mm indicates myocardial
infarction
ECG Time & Voltage
•
•
•
•
•
ECG machines can run at 50 or 25 mm/sec.
Major grid lines are 5 mm apart; at standard
25 mm/s, 5 mm corresponds to .20 seconds.
Minor lines are 1 mm apart; at standard 25
mm/s, 1 mm corresponds to .04 seconds.
Voltage is measured on vertical axis.
Standard calibration is 0.1 mV per mm of
deflection.
Components of a NSR
Standard 12-Lead ECG

Usually performed when person is resting in
supine position.
 three bipolar limb leads: I, II, and III;
 three augmented voltage leads: aVR, aVL, aVF;
 six precordial leads: V1 – V6.

All limb leads lie in frontal plane.
 Chest leads circle heart in transverse plane.
ECG Limb Leads
ECG Augmented Limb Leads
ECG Precordial Leads
Standard 12-Lead ECG

Each lead provides a different electrical angle
or picture of the heart.
 Anterior part of heart by looking at V1 – V4.
 Lateral view of heart: I, aVL, V5 and V6.
 Inferior view of heart: II, III, and aVF.
Exercise 12-Lead ECG
12-Lead ECG

Limb lead II shows large
R amplitude because left
ventricle current vector
lies parallel with
electrode placement.
 Chest lead V1 has large S
wave because left
ventricle current vector is
directed away from
electrode.
12-Lead ECG Strip
What types of pathology can we
identify and study from EKGs?

Arrhythmias
 Myocardial ischemia and infarction
 Pericarditis
 Chamber hypertrophy
 Electrolyte disturbances (i.e. hyperkalemia,
hypokalemia)
 Drug toxicity (i.e. digoxin and drugs which
prolong the QT interval)
Interpretation of ECG:
Rate
First measurement to calculate is heart rate.
PQRST waves represent one complete cardiac
cycle.
1. At standard paper speed, divide 1500 by
distance between R to R waves.
2. Find R wave on heavy line. Count off 300,
150, 100, 75, 60 for each following line.
Where next R lands is quick estimate.
3. Multiply number of cycles in 6 second marks
by 10.
The Rule of 300
It may be easiest to memorize the following table:
# of big
boxes
Rate
1
300
2
150
3
100
4
75
5
60
6
50
Interpretation of ECG: Rate
10 Second Rule
As most EKGs record 10 seconds of rhythm per page,
one can simply count the number of beats present on the
EKG and multiply by 6 to get the number of beats per
60 seconds.
This method works well for irregular rhythms.
What is the heart rate?
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
33 x 6 = 198 bpm
Precordial Leads
Summary of Leads
Bipolar
Limb Leads
Precordial Leads
I, II, III
-
(standard limb leads)
Unipolar
aVR, aVL, aVF
(augmented limb leads)
V1-V6
Arrangement of Leads on the EKG
Anatomic Groups
(Septum)
Anatomic Groups
(Anterior Wall)
Anatomic Groups
(Lateral Wall)
Anatomic Groups
(Inferior Wall)
Anatomic Groups
(Summary)
The QRS Axis
The QRS axis represents the net overall direction
of the heart’s electrical activity.
Abnormalities of axis can hint at:
Ventricular enlargement
Conduction blocks (i.e. hemiblocks)
The QRS Axis
By near-consensus, the normal
QRS axis is defined as ranging
from -30° to +90°.
-30° to -90° is referred to as a
left axis deviation (LAD)
+90° to +180° is referred to as a
right axis deviation (RAD)
Quadrant Approach: Example 1
Quadrant Approach: Example 2
Equiphasic Approach: Example 2
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
Interpretation of ECG:
Rhythm
•
Normal heart rhythm has consistent R-R interval.
• Mild variations due to breathing also normal.
Interpretation of ECG: Rhythm
Normal Sinus Rhythm
•
Rate: 60-100 b/min
• Rhythm: regular
• P waves: upright in
leads I, II, aVF
• PR interval: < .20 s
• QRS: < .10 s
Sinus Bradycardia
•
•
Rate: < 60 bpm
Rhythm: regular
Sinus Tachycardia
•
Rate: > 100 bpm
AV Conduction Disturbances
o
Atrioventricular
conduction
disturbances refer to
blockage of electrical
impulse at AV node.
o 1st degree P waves result
in delayed QRS.
o 2nd degree some but not
all P waves have QRS.
Arrhythmias
Arrhythmia: an irregular
heartbeat.
•
•
•
Sinus arrhythmia- P
wave precedes @ QRS
but RR interval varies.
Premature Atrial
Contraction (PAC)
Premature Ventricular
Contraction (PVC)
Arrhythmias
Where is the Conduction
Problem?
1st Degree AV block
2nd Degree AV block
2nd Degree AV block
2nd Degree AV block
3rd Degree AV Block
Atrial Fibrillation

Predominantly of left atrial origin
 High rate of atrial activation (>300bpm)

IRREGULAR VENTRICULAR RESPONSE
– Filtering effect of the AV node
– Protects the ventricle from high atrial rates
– Explains why we “rate control” AF
Atrial Fibrillation
Atrial Fibrillation
Atrial Flutter
Narrow Complex tachycardia
Broad Complex Tachycardia
Left Atrial Abnormality
Right Atrial Abnormality
Prolonged P wave duration >120 msec in lead II Peaked P waves with amplitudes in lead II >0.25
mV (P pulmonale)
Prominent notching of the P wave, usually most
obvious in lead II, with an interval between the
notches of >40 (P mitrale)
Rightward shift of the mean P wave axis to
above +75 degrees
Ratio between the duration of the P wave in lead Increased area under the initial positive portion
II and the duration of the PR segment >1.6
of the P wave in lead V1 to >0.06 mm-sec
Increased duration and depth of the terminal
negative portion of the P wave in lead V1 (the P
terminal force) so that the area subtended by it
exceeds 0.04 mm-sec
Leftward shift of the mean P wave axis to
between -30 and -45 degrees
LV hypertrophy
Parameter
Sokolow-Lyon index
Romhilt-Estes point score system (points)[*]
Criteria
SV1 + (RV5 or RV6) > 3.5 mV
RaVL > 1.1 mV
Any limb lead R wave or S wave ≥2.0 mV (3)
or SV1 or SV2 ≥ 3.0 mV (3)
or RV5 to RV6 ≥ 3.0 mV (3)
ST-T wave abnormality (no digitalis therapy) (3)
ST-T wave abnormality (digitalis therapy) (1)
Left atrial abnormality (3)
Left axis deviation ≤ -30 degrees (2)
QRS duration >90 msec (1)
Intrinsicoid deflection in V5 or V6 > 50 msec (1)
Cornell voltage criteria
SV3 + SaVL ≥ 2.8 mV (for men)
SV3 + SaVL ≥ 2.0 mV (for women)
Cornell voltage-duration measurement
QRS duration × Cornell voltage >2436
QRS duration × sum of voltages in all leads
>17,472
LV hypertrophy
RV hypertrophy
REST
Action potential & Ischemia
Ischemia
Ant . STEMI
ANT .& INF. STEMI
ACUTE PERICARDITIS
Problems with ECG recording

Patient identity
 Lead position
 Paper speed and amplification
 Artifact

Misinterpretation is much more common
than poor recording technique.
Patient Identity
Lead Position
Paper Speed
Signal Amplification
Artifact
Artifact