ECG Basics
The Normal Conduction System
What is an ECG?
The electrocardiogram (ECG) is a
representation of the electrical events of the
cardiac cycle.
Each event has a distinctive waveform, the
study of which can lead to greater insight
into a patient’s cardiac pathophysiology.
What types of pathology can we
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)
Parts of the ECG
(Waveforms and Intervals)
Parts of the ECG
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 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.
The Standard Leads
Lead I is obtained by
measuring the voltage
between the left arm and right
arm.
The left arm is the positive pole.
An electrical wave moving
towards the left arm will cause
an upward deflection of the
ECG machine stylus on the
paper.
Lead I is most useful for
seeing electrical activity
moving in a horizontal direction.
The Standard Leads
Lead II connects
the right arm to the
leg, and therefore
best sees electricity
moving down and
leftward.
The Standard Leads
Lead III compares
voltage in left arm and
the leg, and will
measure electricity
moving down and
rightward.
Lead II and Lead III
are positive at the foot
Standard Limb Leads
The Augmented Leads
Three additional limb leads can be
obtained by mixing combinations of
electrodes. These are leads R, L, and F.
To create these limb leads, two electrodes
are connected together to create an
“average” electrode, then connected
through the ECG machine to the
remaining electrode.
The Augmented Leads
Lead F is created by
connecting the two arms
together to create an
“average” electrode. To
the ECG machine, this
combination looks like a
single electrode midway
between the two arms —
directly in the center of
the body above the heart.
This “average” electrode
is connected through the
ECG machine to the foot
electrode
The Augmented Leads
Lead L is created by
connecting the right arm
and the leg together, then
comparing this “average”
electrode to the left arm
electrode. The left arm
electrode is positive,
meaning that electricity
moving to the left will
cause an upward motion
of the ECG stylus.
The Augmented Leads
To create lead R, the left arm
and the foot electrodes are
connected together, then the
voltage of this “average”
electrode is compared to the
right arm lead.
Because lead R “aims” in the
opposite direction from the other
limb leads, its ECG waveforms
will usually be “upside down” by
comparison.
This means that every
electrical wave in lead R
should have a net negative
direction, unless pathology is
present.
Augmented Limb Leads
All Limb Leads
The Chest (Precordial) Leads
Precordial Leads
The precordial (chest) leads start with:
Lead V1 is placed in the 4th intercostal space, near
by right edge of sternum
Lead V2 is opposite Lead V1 at the left side of the
sternum.
Lead V3 is halfway to lead V4, which is placed
below rib 5 on the midclavicular line.
Lead V5 is straight around the chest from Lead
V4, on the anterior axillary line.
Lead V6 is directly around from Lead V5, straight
down from the middle of the armpit (the mid-axillary
line)
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
Anatomic Groups
(Septum)
Anatomic Groups
(Anterior Wall)
Anatomic Groups
(Lateral Wall)
Anatomic Groups
(Inferior Wall)
Anatomic Groups
(Summary)
The ECG Grid
The paper on which the
ECG is drawn is divided up
into 1 millimeter lines
horizontally and vertically.
The vertical lines
represent passage of
time. Because the paper
moves at a rate of 25 mm
per second, each 1 mm
line represents 0.04
seconds of time. Every fifth
line is darkened to help
with counting. The time
between large boxes
(darkened lines) is 0.2
seconds, and five large
boxes equals one second.
The ECG Grid
The vertical direction
represents the strength
of electrical voltage.
Positive voltage moves the
stylus up, negative voltage
moves it downward. Each
millimeter vertically
represents 0.1 millivolt.
Ten vertical boxes is one
millivolt.
The horizontal direction
represents passage of
time
Parts of the ECG
The P Wave
The P wave represents the spread of
electrical activity over the atrium. The
normal depolarization begins at the
sinoatrial (SA) node near the top of the
atrium. It’s normally largest in lead II. The
normal P wave is upright in all leads
except R.
The P wave normally lasts less than 0.11
seconds (just less than three small
boxes). An abnormally long P wave
occurs whenever it takes extra time for
the electrical wave to reach the entire
atrium. This occurs in left atrial
enlargement.
The height of the P wave is normally less
than 2.5 small boxes (less than 0.25
millivolts). An abnormally tall P wave is
seen when larger amounts of electricity
are moving over the atrium. This usually
indicates hypertrophy of the right atrium.
The P wave may be decreased in height
by hyperkalemia.
Parts of the ECG
The PR Interval:
Following the P wave is the PR segment. (NOTE:
the PR segment and the PR interval are NOT the
same thing.) The PR segment is not routinely
measured, but may be commented on if it is
depressed or elevated. During the PR segment,
the electrical wave moves slowly through the
atrioventricular (AV) node. This activity is not
seen on the ECG.
The PR interval is the time from the beginning
of the P wave until the beginning of the QRS
complex. It is normally between 0.12 and 0.2
seconds (three to five small boxes) in length.
The PR interval may be prolonged when
conduction of the electrical wave through the AV
node is slow. This may be seen with
degenerative disease of the node, or with digoxin,
hyperkalemia, hypercalcemia, or hypothermia.
The PR interval may be unusually short when
conduction is rapid. A mildly short PR interval
may be seen with hypokalemia or hypocalcemia.
An artificially-short PR interval occurs when the
QRS complex begins early, as happens with an
extra conducting bundle — Wolff-ParkinsonWhite Syndrome (WPW).
Parts of the ECG
The QRS Complex:
The QRS complex represents
activation of the ventricle.
The QRS complex is normally
less than 0.10 seconds in
length — two and a half boxes.
Lengthening of the QRS
indicates some blockage of the
electrical action in the
conducting system. This may
be due to ischemia, necrosis of
the conducting tissue,
electrolyte abnormality, or
hypothermia.
Parts of the ECG
Q wave
If the first deflection of the QRS is
downward, it’s called a Q wave. The Q
wave represents activation of the
ventricular septum. The electricity
spreads from right to left through the
septum.
Q waves may be normal. For example in
lead I, a Q less than 1/4 of the R height,
and less than one box wide, is
considered normal. This is the early
activation of the septum. This activation
goes left — away from lead I — and is
therefore negative on the ECG. “Septal
Qs” are normal in I, F, V5 and V6. Qs are
also generally innocent in lead III and
lead V1 if no other abnormality is seen.
Q waves are “significant” if they are
greater than 1 box in width (longer than
0.04 msec) OR are larger than 1/4 of the
R wave. Significant Q waves indicate
either myocardial infarction or obstructive
septal hypertrophy (IHSS).
Parts of the ECG
R wave
The first upward
deflection of the QRS is
called the R wave. Most
of the ventricle is
activated during the R
wave. The R wave may
be prolonged if the
ventricle is enlarged, and
may be abnormally high
(indicating strong voltage)
if the ventricular muscle
tissue is hypertrophied.
Parts of the ECG
S wave
The S wave is any
downward deflection
following the R wave.
Like the R wave, an
abnormally large S
wave may indicate
hypertrophy of the
ventricle.
Parts of the ECG
The ST Segment
The ST segment is the portion of the
tracing falling between the QRS complex
and the T wave. During this time, the
ventricle is contracting, but no electricity is
flowing. The ST segment is therefore
usually even with the baseline.
The length of the ST segment shortens
with increasing heart rate. Abnormality of
electrolytes may also affect the ST
segment length, however measurement of
the length of the ST segment alone is
usually not of any clinical use.
Upward or downward shifts in the ST
segment are extremely important.
Deviation of the ST segment from
baseline can indicate infarction or
ischemia, pericarditis, electrolyte
abnormality, or ventricular strain. ST
segment elevation or depression is
generally measured at a point two boxes
beyond the QRS complex.
Parts of the ECG
The T wave
The T wave represents the wave of
repolarization, as the ventricle prepares to fire
again. The T wave is normally upright in leads
I, II, and V3-V6. It is normally inverted in lead
R. Ts are variable in the other leads (III, L, F,
and V1-V2).
T wave abnormalities may be seen with, or
without ST segment abnormality. Tall T waves
may be seen in hyperkalemia or very early
myocardial infarction. Flat T waves occur in
many conditions. Inverted T waves may be
seen in both ischemia and infarction, late in
pericarditis, ventricular hypertrophy, bundle
branch block, and cerebral disease.
In young children, T waves may be inverted in
the right precordial leads (V1 to V3).
Occasionally, these T inversions persist in
young adults.
Parts of the ECG
The U Wave
A second wave
following the T wave
is called a U wave.
Large U waves may
be seen in electrolyte
abnormality (such as
hypokalemia), or with
drug effects.
Parts of the ECG
The QT Interval
The QT interval is the time from the
beginning of the QRS complex until the
end of the T wave. The “normal” QT
length varies with heart rate. Very fast
rates shorten the QT length.
At normal heart rates, QT length is
abnormal if it’s greater than 0.40 sec
(10 boxes) for males and 0.44 sec (11
boxes) for females. Extreme QT
prolongations (greater than 0.60 sec —
15 small boxes) predispose the patient
to arrhythmias.
The QT interval may be prolonged with
electrolyte abnormality, such as
hypokalemia, hypocalcemia, or
hypomagnesemia. Myocardial
ischemia may also prolong the QT
interval.
Sequence of ECG analysis
Exclusion of technical faults
2. Estimation: the strength of electrical voltage
3. What is the rhythm? Is the rhythm regular or irregular ?
6. What is the rate?
7. Determining QRS axis.
7. Check the waves, intervals, complexes
9. ECG conclusion
10. Compare ECG data with
- age and body habitus of patient
- physiologic features (e.g. pregnancy)
- clinical findings
- treatment
1.
Where to look
Look for P waves. Are they all the same shape? Are there any
inverted Ps other than in lead R? Does the PR interval vary? Are
there any non-conducted P waves? Is the P wave abnormally wide
or high?
Check the PR interval. Is there first degree AV block? Is the PR
abnormally short?
Look at the QRS complex in each lead. Is the QRS axis normal? Is
the QRS width normal? Do the wave forms suggest conduction
block? Are there significant Q waves? Is the precordial R wave
pattern normal? Are the QRS complexes too small or too large?
Look at the ST segments. Is there an abnormality? Is the
abnormality diagnostic of ischemia, infarction, or ventricular strain?
Check the T waves. Is the shape normal? Are there inverted Ts in I,
II, or V3-V6?
Look at the QT interval. Is it over half the R-R distance, or over 10
boxes in length?
Once you spot an abnormality, check for other findings that firm up
the diagnosis. For example, when you spot a large R in V1, you
check for axis deviation, ST depression, and the orientation of the P
wave in V1.
The sinus rhythm
P-wave precedes
QRS complex
The regular rhythm
RR intervals are
equal
Determining the Heart Rate
Rule of 300
10 Second Rule
Cardio- tables
Cardio- tables
Cardio-ruler
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 rate
Although fast, this method only works for
regular rhythms.
What is the heart rate?
(300 / 6) = 50 bpm
What is the heart rate?
(300 / ~ 4) = ~ 75 bpm
What is the heart rate?
(300 / 1.5) = 200 bpm
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
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?
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)
Axis Deviation
Normal QRS axis is from
around -30 to +90 degrees.
More negative than -30 is
called left axis deviation.
More positive than +90 is
called right axis deviation.
Axis Deviation
Right axis deviation is seen on the ECG when more electrical
forces are moving to the right than normal. This is usually due to
hypertrophy of the right ventricle (RVH). Causes of right axis deviation
include COPD, pulmonary emboli, valvular disease, septal defects, and
pulmonary hypertension.
An axis of +90 is common in persons with emphysema. This so-called
“vertical heart” reflects both the rotation of the heart downward as the
diaphragm position drops due to air trapping, and some degree of
hypertrophy of the right ventricle.
Left axis deviation occurs when additional electrical forces move to
the left (hypertrophy), or when the time required for the electrical activity
to move over the ventricle is prolonged (LBBB, left ventricular dilation).
Causes of left axis deviation include hypertension, aortic stenosis or
regurgitation, subaortic stenosis, mitral regurgitation, and left ventricular
conduction defects.
The QRS axis may shift during the respiratory cycle if elevation of the
diaphragm changes the physical position of the heart. Beat-to-beat
variation in QRS axis (an every-other-beat change in QRS shape) is
called “electrical alternans.” This is thought to be caused by the heart
physically swinging back and forth in a pericardial effusion.
Determining QRS axis by inspection
lead I positive, lead III positive = normal
axis
lead I negative (+/- R positive) = RIGHT axis
lead III negative, lead II negative = LEFT
axis
Determining QRS axis by
inspection
Normal axis
Determining QRS axis by
inspection
Vertical axis
Determining QRS axis by
inspection
Horizontal axis
Determining QRS axis by
inspection
Right axis deviation
Determining QRS axis by
inspection
Left axis deviation
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)
Chamber Enlargement
Right atrial enlargement
Right atrial enlargement (RAE) is
diagnosed by the presence of a P
wave 2.5 millimeters or greater in
height (often called “P
pulmonale.” ). The P wave often
has a sharp, peaked appearance.
This increased voltage is caused
by hypertrophy or acute strain of
right atrial tissue.
Causes of right atrial enlargement
include COPD, mitral stenosis,
mitral regurgitation, or pulmonary
emboli.
Chamber Enlargement
Left atrial enlargement
Dilation or hypertrophy of the
left atrium may increase the
DURATION of the P wave.
P wave longer than 0.11
milliseconds (called “P
mitrale”) is diagnostic of left
atrial enlargement (LAE).
LAE often occurs in mitral
valve disease (either
stenosis or insufficiency).
Chamber Enlargement
Right ventricular hypertrophy
RVH Criteria
R in V1 > 7 mm or > S wave
T in V1 inverted
Right axis deviation
S waves in V5-V6
Chamber Enlargement
Left ventricular hypertrophy
Summary of LVH Criteria
1) R-I + S-III >25 mm
2) S-V1 + R-V5 >35 mm
3) ST-Ts in left leads
4) R-L >11 mm
5) LAE + other criteria
Positive Criteria: 1=possible 2=probable 3=definite