Transcript lecture 5
ELECTROCARDIOGRAM (ECG)
Prof. Sultan Ayoub Meo
MBBS, M.Phil, Ph.D (Pak), PG Dip Med Ed, M Med Ed (Scotland)
FRCP (London), FRCP (Dublin), FRCP (Glasgow), FRCP (Edinburgh)
Professor and Consultant, Department of Physiology,
College of Medicine, King Saud University, Riyadh, Saudi Arabia
LECTURE OBJECTIVES
Define ECG
Identify waves of ECG and the physiological cause of each
Define the normal intervals and segments
Discuss the bipolar and un ipolar leads and their locations
Discuss the bipolar limb lead and the cardiac axis
INTRODUCTION
Cardiac impulse passes through the heart, electrical current spreads
from heart into adjacent tissues surrounding the heart. Current
spreads all the way to the surface of the body. If electrodes are
placed on the chest, opposite sides of the heart, electrical potentials
generated by the current can be recorded; the recording is known as
an electrocardiogram
A record of the waves (impulses) of electrical excitation in the heart
is called ECG.
It helps in the diagnosis of muscle damage or electrical problems in
the heart.
NORMAL IMPULSE CONDUCTION
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers
TYPES OF PATHOLOGY IDENTIFY
AND STUDY FROM ECGS?
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)
NORMAL ECG
The normal ECG is composed of:
P wave
QRS complex
T wave
The separate waves:
Q wave, the R wave, and the S wave.
P wave: Atria depolarize before atrial contraction begins
QRS complex: Ventricles depolarize before contraction. P wave and
the components of the QRS complex are depolarization waves.
The T wave: Ventricles recover from the state of depolarization
This process normally occurs in ventricular muscle 0.25 to 0.35
second after depolarization and the T wave is known as a
repolarization wave
The ECG Paper
P wave:
Atrial depolarization (completed in 0.1 seconds )
• Irregular or absent P waves may indicate arrhythmia.
• The shape of the P waves may indicate atrial problems
approx. 0.44 s
0.12-0.2 s
QT
PR
R
T
Atrial muscle
depolarization
P
Q
S
PR interval: Atrial depolarization & conductive time
Measured from the P wave to the beginning of the QRS complex .
Through the AV node /0.18 seconds (N. R. 0.12 to 0.2 s)
approx. 0.44 s
0.12-0.2 s
QT
PR
R
T
Atrial muscle
depolarization
P
Q
S
QRS complex: Ventrical depolarization
Equal 0.08-0.12 s (max time 0.1seconds)
Very wide and deep Q waves indicate myocardial infarction
approx. 0.44 s
0.12-0.2 s
QT
PR
QRS
T
Atrial muscle
depolarization
P
Q
S
Ventricular muscle
depolarization
QT interval:
Ventricular depolarization & re-polarization (equal . 0.4 seconds
approx. 0.44 s
0.12-0.2 s
QT
PR
R
Atrial muscle
depolarization
T
P
Q
S
Ventricular muscle
depolarization
Ventricular
muscle repolarization
ST segment:
(QT – QRS) Ventricular depolarization (equal . 0.32 seconds )
approx. 0.44 s
0.12-0.2 s
QT
PR
R
Atrial muscle
depolarization
T
P
Q
S
Ventricular muscle
depolarization
Ventricular
muscle
repolarization
ECG LEADS
Leads are electrodes which measure the
difference in electrical potential between
either:
1. Two different points on the body (bipolar leads)
2. One point on the body and a virtual reference point
with zero electrical potential, located in the center of
the heart (unipolar leads)
ECG LEADS
The standard ECG has 12 leads: 3 Standard Limb Leads
3 Augmented Limb Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from
which it looks at the heart.
STANDARD LIMB LEADS
STANDARD LIMB LEADS
AUGMENTED LIMB LEADS
ALL LIMB LEADS
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
ECG
ANATOMICAL REPRESENTATION
(SEPTUM)
ANATOMIC PRESENTATION
(ANTERIOR WALL)
ANATOMICAL PRESENTATION
(LATERAL WALL)
ANATOMICAL PRESENTATION
(INFERIOR WALL)
ANATOMICAL PRESENTATION
(SUMMARY)
DETERMINING THE HEART RATE
1. Rule of 300
2. 10 Second Rule
RULE OF 300
Take the number of “big boxes” between neighboring
QRS complexes, and divide this into 300. The result
will be approximately equal to the heart rate
Although fast, this method only works for regular
rhythms.
WHAT IS THE HEART RATE?
(300 / 6) = 50 bpm
(300 / 4) = 75 bpm
(300 / 1.5) = 200 bpm
10 Second Rule
As most ECGs record 10 seconds of rhythm per
page, one can simply count the number of beats
present on the ECG 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
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)
The QRS Axis •
Clinical Significance of different waves
and segments of ECG
ST Elevation - Acute MI or Angina
ST depression >1 mm - Ischemia/Angina (flat), digoxin (sloping)
Q waves in 2 or more leads - Previous MI (Transmural)
Diffuse ST elevation with PR depression – Pericarditis
T wave inversions and non-specific ST changes - Can be seen
both in normal cases and in many diseases, therefore not useful
for diagnosis.
Tall P waves - Right atrial hypertrophy
Broad (and often bifid) P waves - Left atrial hypertrophy
Peaked T waves or loss of P wave – Hyperkalemia
U waves - Hypokalemia ('Hump' at the end of T wave)
Prolonged QT interval – Hypocalcemia
Shortened QT interval - Hypercalcemi