Download 5- Normal electrocardiogram

5- Normal electrocardiogram
Objective:
1-Draw an ECG classical waveform and label each component (P, QRS, T).
2-Draw diagrams indicating the 6 standard limb leads (I, II, III, aVR, aVL,
aVF). Objectives:
3-State the relationship between the direction of cardiac vector with the
direction (-ve, +ve) and amplitude of an ECG waves.
4-Draw diagram indicting the axes of limb leads.
The electrocardiogram:
Because the body fluid is good conductors, algebraic sum of the action potentials
of myocardial fibers can be recorded externally. The record of this potential
fluctuation during cardiac cycle is called electrocardiogram (ECG). Most
electrocardiograph machines record these fluctuations on moving strip of paper.
The names of these waves and segments are:
1- P wave represents atrial depolarization. Atrial repolarization is buried in QRS
complex, because it is too slow and diffuse to register. It is about 0.07 – 0.14
seconds and not more than 3 mm.
2- PR segment is due to the delay in conduction through AV node. It is isoelectric
because insulating fibrosis prevents the electrical circuit and no current flow.
3-PR interval is measured from the beginning of the P wave to the beginning of
the QRS complex. This interval corresponds to the time taken for the impulse to
travel from the sinus node to the ventricular muscle. It ranges normally between
0.12- 0.21 seconds.
4- QRS complex represents depolarization of the ventricles. The duration is about
0.06 – 0.10 seconds. Abnormal wide QRS indicate bundle branch block.
5- ST segment is the segment from end of the S wave to the beginning of the T
wave. It is isoelectric, which coincides with the plateau of ventricular action
potential.
6- T wave represents ventricular repolarization. It is positive because
repolarization occurs in reverse sequence to depolarization, producing the upright
T wave. The duration is about 0.25 -0.35 seconds and not taller than 10 mm in
chest leads.
7- QT interval represents the total time from the onset of ventricular
depolarization to the completion of repolarization. It indicates the duration of
ventricular systole i.e. contraction of the ventricle lasts from the beginning of the Q
wave to the end of the T wave. Normally it is about 0.35 seconds; range 0.28 –
0.44 seconds.
8- U wave is not constant finding, due to slow repolarization of papillary muscle.
Figure (17).
Figure (17): Normal waves of ECG (Ganong's review of medical physiology 2010)
ECG leads: The electrocardiograph is a modified galvanometer that measures the
difference in action potential between two points.
1- Bipolar limb leads:
They record the difference between two limbs.
Lead I; between right arm and left arm.
Lead II; between right arm and left leg.
Lead III; between left arm and left leg.
Einthoven stated that Lead II = Lead I + Lead III. This is called Einthoven's law.
Lead II has upright positive deflection. See figure18.
2- Unipolar limb leads:
They measure the action potential difference between that point and zero. They
are VR (right arm), VL (left arm) and VF (left foot). Augmented limb leads
designated by the letter (a) (aVR, aVL, aVF) are generally used. In the aVR The P
wave, QRS complex wave and T wave are all negative (downward) deflection. In
the aVL and AVF deflections are positive. Figure (18).
Figure (18): Unipolar and bipolar limb leads (Guyton & Hall 2006).
3- Unipolar chest leads:
There are 6 uipolar chest leads, designated V1 to V6. The position of these leads
are illustrated in figure 19. Unipolar precordial or chest leads record the absolute
potential at 6 standard points on the anterior chest wall designated as V1 to V6, the
locations of which are as follows:
 V1: At the right margin of the sternum in the 4th right intercostal space.
 V2: At the left margin of the sternum in the 4th left intercostal space.
 V3: Midway between V2 and V4.
 V4: At the left midclavicular line in the 5th intercostal space.
 V5: At the left anterior axillary line in the 5th intercostal space.
 V6: At the left midaxillary line in the 5th intercostal space.
The precordial leads look at the heart in a horizontal plane from the front & left
sides. Leads V1 & V2 look at the right ventricle and reflect its activity, V3 & V4
look at the interventricular septum and reflect its activity, while leads V5 & V6
look at the left ventricle and reflect its activity.
Connections of the electrocardiograph:
By specific electrodes, the electrocardiograph is connected to the 4 limbs and the
chest at the same time. The right leg connection is used to "earth" the subject (to
minimize interference currents).
Figure (19): Position of unipolar chest leads (Fox, 2006).
There are no Q waves in V1 and V2 and the initial potential of QRS complex is
small upward deflection because ventricular depolarization first move across the
midportion of septum from left to right toward the exploring electrode. The wave
of excitation then moves down the septum and into the left ventricle away from
electrode, producing a large S wave, finally it moves back along the ventricular
wall toward the electrode, producing the isoelectric line. In the left ventricular
leads V4-V6, there are an initial small Q wave, large R wave (septal to left
ventricle depolarization), and moderate S wave in V4 and V5. Figure(20).
Figure (20): Unipolar chest leads (Guyton & Hall 2006).
Calibration of the electrocardiograph:
The electrocardiograph is calibrated so that a change of 1 mV upward or
downward produces a deflection of 10 mm amplitude (10 small squares; 2 large
squares). The horizontal lines are time calibration in which duration of each mm
(small square) equals 0.04 second. Each large square (5 small squares) represents
0.2 second.
Speed:
The standard speed is 25 mm/sec. The importance of another speed (50 mm/sec) is
in case of tachycardia (e.g. HR of 180 beat/min), to obtain a proper ECG.
Sensitivity:
The higher the sensitivity of the instrument, the more the deflection and vise versa.
The standard sensitivity is 10 mm/mV (2 large squares), in cardiomegally you must
reduce the sensitivity.
Cardiac vector:
The cardiac vector is the net result of the directions of the spread of depolarization
waves from the SA node to the base of the heart
Bipolar limb leads and cardiac vector:
Because the standard limb leads are records the potential difference between two
points, the deflection in each lead indicates magnitude of direction in the axis of
the lead of electromotive force generated in the heart. Cardiac vector can be
calculated from standard limb leads. If it is assumed that the three electrodes
location from the points of an equilateral triangle (Einthoven's triangle), and that
the heart lies in the center of triangle. Mean QRS vector (electrical axis of the
heart) is plotted by using the average QRS deflection in each lead. The average
QRS deflection should be measured by integrating the QRS complex, measuring
the net differences between the positive and negative peak of QRS. Calculate the
mean QRS of lead I, II, and III. Draw perpendicular lines from the potential L I, II,
III. Figure (21).
Figure (21): Cardiac vector .and Enthoven's triangle (Ganong's review for medical
physiology, 2010).
The normal direction of mean QRS is generally (- 30° to +110°). Left or right
axis deviation – 30 degrees or to the right of + 110 degrees respectively.
Right axis deviation suggest right ventricular hypertrophy, and left axis
deviation may be due to left ventricular hypertrophy. The physiological conditions
change vector of heart are:
I- Expiration shift the vector to left, inspiration shift it to the right.
2-Supine position of body shift it to left.
3- Fatty body shift to left, while non fatty body shift it to right.
Electrical axis of the ventricular QRS:
In relation to the bipolar limb leads (I, II, III), the cardiac vector or axis can be
calculated. The relationship among the six limbs leads, i.e., the axis of each
standard bipolar and unipolar limb leads can be presented in the following
diagram:
Calculation of heart rate from ECG paper:
If the heart rhythm is regular, the heart rate (HR) ran be counted by dividing the
number of large squares between two consecutive R waves into 300 or small
squares into 1500. If the rhythm is irregular, one can multiply the number of
complexes in 6 seconds by 10.
Axis deviation:
Axis deviation occurs if the electric axis of the heart is beyond the normal range
and it may be to the right or to the left. QRS axis further right that of +110
constitutes Right axis deviation (RAD), QRS axis left that of –30 constitutes Left
axis deviation (LAD).
Right axis deviation
This normally occurs in vertical hearts (e.g. in tall slender subjects), but
pathologically, it is common in right ventricular hypertrophy and right bundle
branch block.
Left axis deviation
This normally occurs in horizontal hearts (e.g. in short obese subjects and pregnant
women) but pathologically, it is common in left ventricular hypertrophy and left
bundle branch block.