Electrocardiogram
Dr QAZI IMTIAZ RASOOL
OBJECTIVES
1.
Define ECG and describe the characteristics
(waves & intervals) of normal ECG.
2.
Describe the significance of mean electrical
axis of ventricular QRS.
3.
Describe and explain the causes for
abnormal ECG waves and intervals.
Willem Einthoven
Nobel prize in 1924 for electrocardiogram
(discovered in 1903)
What types of pathology can we identify
and study from EKGs?
1.
Cardiovascular system
2.
Respiratory system
3.
Renal system
4.
GIT system
5.
Nervous system
6.
Endocrines system
(ECG, EKG)
The electrocardiogram (the k is from the German
“kardio” for “heart”)
Electrocardiography –
Procedure of graphic recording the electrical activity
myocardium during cardiac cycle.
Electrocardiogram
is a representation of the electrical events of the cardiac
cycle.
NOTE;-Electrocardiograms must be viewed in the context of demographics health
histories, and other clinical test correlates.
in
BIOPHYSISCS
1.If the direction of current (positive ion) is towards positive pole will
record a positive, if negative wave it will be recorded as negative
deflection
2. If the direction of current is parallel to the lead axis it will
record a maximum deflection and minimum if it is at right angle
Electric field of the heart on the surface of the thorax,
recorded by Augustus Waller (1887).
.
These indicate that
the heart is a dipolar
source having the
positive and negative
poles at (A) and (B),
respectively.
Depolarization and EKG
1. Positive/upward vs. Negative/downward inflection
2. “Wave of depolarization” = “wave of positive charge”
3. Wave of depolarization moving towards positive electrode =
positive inflection
4. 12 leads – different views of the same electrical activity
Waveforms and Intervals
QRS Complex with Interval and Segment Measurements
P wave
1.
2.
3.
4.
5.
6.
The first wave on the ECG
Represents atrial activation(depolarization)
Smooth
Duration is less than 0.12 sec
Amplitude is less than 0.25 mV
Upright inferiorly
◦ Pointy = P pulmonale (RA hypertrophy)
◦ Bifid = P mitrale (LA hypertrophy)
PR interval
1.
Start of P wave to start of QRS
2.
Normal = 0.12-0.2s
3. Too short – can mean WPW syndrome (ie. an
accessory pathway), or normal!
4. Too long –means AV block (heart block) 1st/2nd/3rd degree
QRS
Duration 0.12 seconds
1.
-1st deflection , is negative (downward), is
labeled the Q wave.
2.
-1st positive (upright) deflection is labeled the R
wave,
3.
- 2nd negative deflection following an R wave is
labeled an S wave.
Q wave
1. First negative deflection
2. Normally very small in deflection
3. If prominent, consider infarction
R Waves
1. R wave amplitude increases from V1-V4 or V5
2. Amplitude of 4-5 mm or <
S Waves
1. Large in V1 and V2 then progressively smaller to
V6
QRS complex
1. Should be <0.12s duration
2. >0.12s = BBB (either LBBB or RBBB)
3. ‘Pathological’ Q waves can mean a previous MI
(? territory)
4. >25% size of subsequent complex
5. Q waves are allowed in V1, aVR and III
ST segment
1.
ST depression
1. Downsloping or horizontal = abnormal Ischaemia
(coronary stenosis)
2. If lateral (V4-V6), consider LVH with ‘strain’ or digoxin
(reverse tick sign)
2.
ST elevation
1. Infarction (coronary occlusion)
2. Pericarditis (widespread)
T wave
Rounded symmetric
2. Follow direction of QRS except aVR and
V1
3. Represents myocardial repolarization
1.
4.
5.
6.
Peaked (hyperkalaemia or normal young man)
Inverted/biphasic (ischaemia, previous infarct)
Small (hypokalaemia)
QT interval
1. Start of QRS to end of T wave
2. Needs to be corrected for HR
3. Various formulae
◦ eg. Bazett’s:


Long QT can be genetic (long QT sy.) or secondary eg. drugs
(amiodarone, sotalol)
Associated with risk of sudden death due to Torsades de Pointes
Electrical = Mechanical activity
P wave = atrial contraction
2. PR segment = allows time for blood to pass from atria
to ventricles
3. QRS
1.
1. Shape based on direction/spread of depolarization
2. Duration
4.
T wave = ventricular repolarization
1. Reverse order of depolarization
5.
Atrial repolarization during QRS
Depolarization and EKG
1.
2.
3.
4.
5.
6.
7.
SA node – silent
Atrial DP – P wave
AV node – PR interval
His bundle – PR interval
Purkinje fibers – PR interval
Ventricular depolarization – QRS complex
Ventricular isoelectric period (initial – plateau of
ventricular repolarization) – ST segment
8. Ventricular repolarization – T wave
EKG Leads
Leads are electrodes which measure the
difference in electrical potential between two.
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)
EKG Leads
The standard EKG 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.
II, III and AVF
I and
AVL
V3 & v4
V5 & v6
Orientation of leads
1. Limb Leads:
L2, aVF, L3: Inferior
L1, aVL: Lateral
aVR: Cavity
2. Chest Leads:
V1, V2: Anterior
V3, V4: Septal
V5, V6: Lateral
The Concept of a “Lead”
RA
Leads I, II, and III
RA
-
-
- LA
+ LA
LEAD I
LEAD III
+
LL
Remember, the RL
is always the ground
+LL
LEAD II
Standard Limb Leads
QRS Axis
1. QRS axis represents the net overall direction
of the heart’s electrical activity.
2. Abnormalities of axis can hint at:
Ventricular enlargement
Conduction blocks (i.e. hemiblocks)
Axis
Coronal/frontal plane
Normal axis is about 60 degrees
LAD = axis < -30 degrees
RAD= axis > 90 degrees
QRS Axis
By near-consensus, the normal
QRS axis is defined as ranging
from -30° to +110°.
-30° to -90° is referred to as a
left axis deviation (LAD)
+110° to +180° is referred to as
a right axis deviation (RAD)
Determining the Axis
- Quadrant Approach
- Equiphasic Approach
Predominantly
Positive
Predominantly
Negative
Equiphasic
Axis
LEAD L1
LEAD aVF
NORMAL
POSITIVE
POSITIVE
RAD
NEGATIVE
POSITIVE
LAD
POSITIVE
NEGATIVE
INDETERMINATE
NEGATIVE
NEGATIVE
Equiphasic Approach
1.
Determine which lead contains the most
equiphasic QRS complex.
2.
The fact that the QRS complex in this lead is
equally positive and negative indicates that the
net electrical vector (i.e. overall QRS axis) is
perpendicular to the axis of this particular lead.
EXAMPLES :_ 1. Sinus rhythm
ECGs, Normal and Abnormal
P wave
VPC+APC
Sinus bradycardia (lead III).
Prolonged P-R interval caused
by first degree heart block (lead II)
Complete AV block
Atrial tachycardia with 2:1
conduction
Ventricular preexcitation (WPW)
Arrhythmia: conduction failure at AV node
No pumping action occurs
STOKES ADAMS SYNDROME
 •Seen during acute complete AV block•Ventricles stop
beating due overdrivesuppression•Person faints due
reduced blood supply tobrain•Ventricle recovers after
few seconds &starts generating own impulses•Rxartificial pacemaker
ECG Changes : Ischemia
1.
2.
3.
4.
T-wave inversion ( flipped T)
ST segment depression
T wave flattening
Biphasic T-waves
Baseline
2004 Anna Story
45
ECG Changes: Injury
1.
2.
3.
ST segment elevation of greater than 1mm in at least 2
contiguous leads
Heightened or peaked T waves
Directly related to portions of myocardium rendered
electrically inactive
Baseline
2004 Anna Story
46
ECG Changes: Infarct
1.
2.
3.
Significant Q-wave where none previously existed
1. Why?
1. Impulse traveling away from the positive lead
2. Necrotic tissue is electrically dead
No Q-wave in Subendocardial infarcts
1. Why?
1. Not full thickness dead tissue
2. But will see a ST depression
3. Often a precursor to full thickness MI
Criteria
1. Depth of Q wave should be 25% the height of the R wave
2. Width of Q wave is 0.04 secs
3. Diminished height of the R wave
2004 Anna Story
47
Evolving MI and Hallmarks of AMI
Q wave
ST Elevation
1 year
2004 Anna Story
T wave inversion
48