3. SPECIFIC OBJECTIVES
After the session, students are able to,
❖Review electrophysiology of heart.
❖Reiterate conduction system.
❖Interpret ECG grid and different leads.
❖Enlist the precautions to be taken to take ECG.
❖Identify the waves and complexes.
❖Calculate rate, identify normal rhythm, axis deviations.
5. ECG GRID
ECGs are normally printed on a grid. The horizontal axis represents
time and the vertical axis represents voltage.
A small box is 1 mm × 1 mm and represents 0.1 mV × 0.04
A large box is 5 mm × 5 mm and represents 0.5 mV × 0.20 seconds.
In one minute the ECG paper moves by 300 thick lines 0r 1500 mm
The "large" box is represented by a heavier line weight than the
7. ECG LEADS
Bipolar limb leads:
The three standard bipolar limb leads (I,II,III), selected by Einthoven,
detect variation in the electric potential between two points in the
frontal plane of the body. Electrodes are applied to the right arm
(RA), left arm(LA) and left leg (LL) anywhere on the extremity, usually
they are applied just above the wrist and ankles. If the extremity has
been amputated it may be applied on the amputation stump. Thus
the current will flow from RA to LA (lead1), RA to LL (lead II) and LA
to LL (lead III) (negative electrode to positive electrode).
8. EINTHOVEN’S TRIANGLE:
The heart is situated at the center of its electrical field, the intensity of which
decreases with the distance from its center. With distances greater than 15cm
from the heart, this decrease in intensity is negligible and all electrodes at a
distance greater than 15cm from the heart may be considered equidistant.
Since the three standard electrodes are placed on the limbs beyond 15cms
from the center they are considered equidistant from the heart and form a
triangle, with the heart at its center.
These three leads can be drawn from a center, to facilitate graphic
The axis of the three unipolar limb leads form a hypothetical line drawn from
that limb(right shoulder, left shoulder and left hip) to the center of the
10. Relation between the bipolar limb leads:
Kirchhoff's law states that the algebraic sum of all the
potential differences in a closed circuit equals zero.
11. If the polarity of lead II was LL to RA instead of RA to LL, the three bipolar
leads would have formed a closed circuit and leads I+ II+ III=0; however, the
polarity of lead II was reversed by Einthoven, probably to get an upright
deflection in all the three leads. Thus, I+II+III=0.
Einthoven’s equation is: II= I+III
12. UNIPOLAR AUGMENTED
In ‘unipolar lead’ two
employed. By an electric
recorded by one
electrode) are rendered
negligible so that only
the electrical activity of
the other electrode
(exploring electrode) is
13. The indifferent electrode terminal is RA+LA+LL=0. Now if the exploring
electrode is placed on the right arm, a potential difference between RA and
the indifferent electrode is recorded. Since the potential of the indifferent
electrode is negligible, it is taken as zero. Hence the potential recorded is the
actual potential of RA.
In a similar manner, actual potentials of LA and LL are recorded by placing
the exploring electrode over the left arm and the left leg respectively. These
three leads are designated VR, VL, and VF. By a minor change in the
technique, the voltage of these leads can be increased by 50% and therefore
these leads are called augmented leads by aVR, aVL and aVF.
15. OTHER UNIPOLAR PRECORDIAL LEADS:
In addition to the above six precordial leads, precordial leads taken at
other points on the chest walls may be useful in case of dextrocardia.
V7,V8, and V9 are recorded in the same plane as V4 but in the posterior
axillary, posterior scapular and posterior left border of spine respectively.
VE is recorded at the place of the ensiform cartilage. V3R to V9R
recorded on the right side in the same position as V3—V9 respectively.
18. ORIENTATION OF LEADS
INFERIOR LEADS: Leads II, III and aVF,
Look at electrical activity from the vantage point of the inferior surface (diaphragmatic
surface of heart).
ST elevation, T wave inversion and Q wave depression in these leads indicate postero-
inferior wall MI.
LATERAL LEAD : I, aVL, V5 and V6
Look at the electrical activity from the vantage point of the lateral wall of left ventricle.
ST elevation, T wave inversion and Q wave depression in these leads indicate MI
19. SEPTAL LEADS: V1 and V2
Look at electrical activity from the vantage point of the septal surface of the
heart (interventricular septum).
ST elevation, T wave inversion and Q wave depression in these leads indicate
ANTERIOR LEADS: V3 and V4
Look at electrical activity from the vantage point of the anterior wall of the
right and left ventricles (Sternocostal surface of heart).
ST elevation, T wave inversion and Q wave depression in these leads indicate
apical & anteroseptal MI.
20. CALIBRATION OF THE ECG MACHINE:
The machine must be properly standardized so that 1mV produces a
deflection of 1cm vertically.
Speed of the paper should be 25mm/sec.
The patient and the machine must be properly grounded to avoid interference
from alternating currents.
➢ The patient must be placed comfortably on a bed and should be relaxed.
The procedure must be explained to an apprehensive patient to relieve his
anxiety. Any muscular twitching by the patient alter the tracing.
➢ ECG jelly, soap solution or even plain water should be applied over the areas
of skin over which the electrode is to be placed.
➢ The electrodes must be in proper contact with the skin.
➢ The electrodes must be washed after each use and kept free of dirt and
➢ The connections must be checked properly as wrong connections can
completely alter the recording.
22. NORMAL WAVE FORM:
The ECG machine is arranged so that an
electrode facing a wave of depolarization
will record a positive or upward deflection,
whereas an electrode on the side
where the wave is receding will form
a negative or downward deflection.
The P wave is the first positive wave and
represents atrial depolarization.
The P wave
is best visualized in lead II.
23. Criteria for normal P wave:
1.Duration not more than 0.12 sec(3 small squares horizontally).
2.Amplitude not more than 0.25mV (2.5small squares vertically).
3.Upright in all leads except in aVR
4.Smooth and rounded in contour.
5.May be upright or biphasic in leads V1 and V2. If biphasic, the terminal
negative deflection should not be greater than 1mm depth and 0.03sec
24. QRS complex:
The QRS complex is produced
by ventricular activation or
through the heart in many
directions. It’s the main spike
seen on an ECG line. The QRS
complex comprises of the Q,
R, and S waves.
The Q, R, and S waves occur
in rapid succession, do not all
appear in all leads, and
reflect a single event and
thus are usually considered
25. Q - wave:
The Q wave is the negative deflection which precedes the R wave. It denotes
depolarization of the ventricular septum from left to right. For this reason, they
are referred to as septal Q waves and can be appreciated in the lateral leads I,
aVL, V5 and V6.
Pathologic Q waves occur when the electrical signal passes through stunned or
scarred heart muscle; as such, they are usually markers of previous myocardial
infarctions, with subsequent fibrosis. A pathologic Q wave is defined as having
a deflection amplitude of 25% or more of the subsequent R wave, or being >
0.04 s (40 ms) in width and > 2 mm in amplitude. However, diagnosis requires
the presence of this pattern in more than one corresponding lead.
26. R - wave:
The R wave is the positive deflection of the QRS complex. It denotes
depolarization of the ventricles; at first the anteroseptal portion followed by
the major ventricular muscle mass.
Accurate R peak detection is essential in signal processing equipment for
heart rate measurement and it is the main feature used for arrhythmia
Poor R wave progression is commonly attributed to anterior myocardial
infarction, but it may also be caused by left bundle branch block, Wolff–
Parkinson–White syndrome, right and left ventricular hypertrophy, or a faulty
ECG recording technique.
27. S - wave:
The S wave is the negative deflection of the QRS complex that follows the R
wave. It occurs due to depolarization of the postero basal part of the left
ventricle, pulmonary conus and the upper most part of the inter ventricular
The point where the QRS complex meets the ST segment is the J-point. J point
elevation can be seen in early repolarization. At times J point elevation can be
ischemic, however this is somewhat rare.
28. T wave:
The T wave is produced by ventricular repolarization. It is a smooth dome shaped wave with
two limbs asymmetrical, the peak being nearer the end than the beginning. The T wave can
be described by its symmetry, skewness, slope of ascending and descending limbs,
29. Criteria for normal T waves: Limb leads:
1.Upright T wave if QRS complex is positive and inverted T waves if QRS
complex is negative.
2.Always upright in leads I and II.
3.Always inverted in a VR
4.In lead V1-normally upright.
5.In lead V2- normally upright
6.Must be upright in leads V3 to V6
7.Height should not be >2/3 and not <1/8 of height of preceding R wave in
8.Angle between mean frontal plane QRS axis and mean frontal plane T axis
must be less than 45°.
30. U wave:
The U wave probably represents the slow repolarizationof the purkinje’s fibres,
the papillary muscles or the ventricular septum or slow ventricular
repolarization. It follows the T wave and precedes the P wave of the next cycle.
It is the successor of the 'T' wave and may not always be observed as a result
of its small size.
It is evident in the following cases,
Delayed repolarizationof Purkinje fibers
Prolonged re-polarisation of mid-myocardial M-cells
After-potentials resulting from mechanical forces in the ventricular wall.
31. P-P and R-R interval:
When there is sinus rhythm, the P-P and R-R intervals are equal. They are used
to calculate the heart rate. The P-P interval denotes the atrial rate and the R-R
interval denotes the ventricular rate.
32. Calculation of heart rate: GRID METHOD
Take the duration between two identical points of consecutive ECG waveforms
such as the R-R duration.
Take this duration and divide it into 60.
The resulting equation would be:
Rate = 60/(R-R interval)
A quicker way to obtain an approximate rate is to go by RR or PP interval.
If it is 1 big box (0.2 secs) then the rate is 60/0.2 = 300 bpm.
The rest of the sequence would be as follows.
1 big box = 300 beats/min (duration = 0.2 sec)
2 big boxes = 150 beats/min (duration = 0.4 sec)
3 big boxes = 100 beats/min (duration = 0.6 sec)
4 big boxes = 75 beats/min (duration = 0.8 sec)
5 big boxes = 60 beats/min (duration = 1.0 sec)
33. In 1 minute, with the normal speed, 1500 small squares are covered,
Thus the heart rate can be calculated by dividing 1500 by the number of
small squares between two consecutive P or R waves.
34. For irregular rhythm:
Count the number of RR intervals(QRS complex) between 6 seconds ie; 30
small squares(6 large squares) in the rhythm strip and multiply by 10 to
get the bpm. This method is more effective when the rhythm is irregular.
35. PR interval:
The PR interval is measured from the beginning of the P wave to the
beginning of the QRS complex and hence the term PQ interval is more
accurate. It represents the time interval between atrial and ventricular
depolarization and hence include the time taken for atrial depolarization,
atrial repolarization and the delay of excitation in the AV node.
Normal range from 0.12 - 0.20seconds.
The QRS interval is the time taken for ventricular depolarization. It is
measured from the beginning of the Q wave to the end of the S wave.
The upper limit of a normal QRS interval is 0.12seconds.
36. Ventricular activation time(VAT):
The VAT is the time taken by an impulse to traverse the myocardium from the
endocardium to the epicardium. It is measured from the beginning of the Q
wave to the peak of the R wave.
The upper limit of normal VAT is 0.03 sec in V1& V2 and 0.05 sec in V5& V6.
A longer VAT indicates diastolic dysfunction.
The PR segment is from the end of the P wave to the beginning of QRS
complex. It is normally isoelectric.
PR segment depression can be a signal for pericarditis or atrial infarction. PR
segment elevation occurs in lead aVR in the setting of pericarditis.
37. ST junction:
The ST junction is the point at which the QRS complex ends and the ST
The ST segment is measured from the ST junction to the beginning of the T
wave. It is usually isoelectric but may be slightly depressed (0.5mm) or
elevated (0.2mm) in precordial leads. It represents the time between
ventricular depolarization and repolarization. It has great significance in
diagnosing myocardial infarction.
38. ELECTRICAL AXIS OF THE HEART:
The axis of the ECG is the major direction of the overall electrical activity of
the heart. It can be normal, leftward (left axis deviation, or LAD), rightward
(right axis deviation, or RAD) or indeterminate (northwest axis). The QRS axis
is the most important to determine.
To determine the QRS axis, the limb leads (not the precordial leads) need to
39. Note that lead I is at zero
degrees, lead II is at +60
degrees, and lead III is at +120
degrees. Lead aVL (L for left arm)
is at -30 degrees and lead aVF (F
for foot) is at +90 degrees. The
negative of lead aVR (R for right
arm) is at +30 degrees; the
positive of lead aVR is actually at
40. AXIS DEVIATION:
The normal QRS axis should be
between -30 and +90 degrees. Left axis
deviation is defined as the major QRS
vector, falling between -30 and -90
degrees. Right axis deviation occurs
with the QRS axis and is between +90
and +180 degrees. Indeterminate axis
is between +/- 180 and -90 degrees.
The fastest non-specific method to
determine the QRS axis is to find the
major direction of the QRS complex —
positive or negative — in leads I and
41. How to interpret axis deviation from ECG:
If the QRS complex is upright (positive) in both lead I and lead aVF, then the
axis is normal. The image below demonstrates this example, with the electrical
vector heading towards the positive of lead I and the positive of lead aVF, as
indicated by the arrows. The QRS axis is thus between these two arrows, which
falls within the normal range.
42. LEFT AXIS DEVIATION:
If the QRS is upright in lead I (positive) and downward in lead aVF (negative), then the axis is
between 0 and -90 degrees. However, recalling that left axis deviation is defined as between
-30 and -90, this scenario is not always technically left axis deviation. In this scenario, the
QRS axis could fall between 0 and -30, which is within normal limits. To further distinguish
normal from left axis deviation in this setting, look at lead II.
If lead II is downward (negative), then the axis is
more towards -120, and left axis deviation is present. If the QRS complex in lead II is upright
(positive), then the axis is more
towards +60 degrees, and the QRS axis is normal.
43. Right Axis Deviation:
If the QRS is predominantly negative in lead I and positive in lead aVF, then the
axis is rightward (right axis deviation).
44. Indeterminate Axis:
If the QRS is downward (negative) in lead I and downward (negative) in lead
aVF, then the axis is indeterminate and sometimes referred to as
“northwestern axis.” This finding is uncommon and usually from ventricular
rhythms; however, it can also be from paced rhythms, lead misplacement and
certain congenital heart diseases.
45. NORMAL ECG: (SUMMARY)
Rate: 60-120 bmt
Axis: between -30°-- +100°
P wave: amplitude is 0.25mV and not more than 0.12 secs.
PR interval: 0.12—0.20 secs
QRS complex: In standard leads there is dominant R wave. In lead V1 there is a
small ‘r’ wave and big ‘S’ wave. Gradually the R wave increases and S wave
decreases in amplitude from V1-V6.
QRS duration: The normal duration is 0.04-0.12 secs
ST segment: Normal ST segment is isoelectric.
T wave: Normally, T waves are upright in all leads, except aVR, aVL, III and V1
U wave: It may not be evident in a normal ecg due to its negligible size.
50. ATRIAL FIBRILLATION & FLUTTER
❖ Irregularly irregular rhythm. No P waves. Absence of an isoelectric baseline.
❖ Variable ventricular rate.
❖ QRS complexes usually < 120 ms unless pre-existing bundle branch block, accessory
pathway, or rate related aberrant conduction.
52. VENTICULAR TACHYCARDIA
❖Very broad QRS duration (> 160 bpm).
❖Positive concordance in the precordial leads (dominant R waves in V1-
❖Brugada's sign – time from onset of QRS to nadir of S wave > 100 ms;
best seen in leads aVR and aVL.
53. VENTRICULAR FIBRILLATION
❖ Chaotic irregular deflections of varying amplitude.
❖ No identifiable P waves, QRS complexes, or T waves.
❖ Rate 150 to 500 per minute.
❖ Amplitude decreases with duration (coarse VF -> fine VF)