ELECTROCARDIOGRAPHY INVESTIGATION OF HEART (ECG).
Separate
myocardial cell electricity.
Potential changes on cell membrane, which is recorded
separately called electro gram. During resting potential membrane charge is
positive. If two electrodes will dispose on membrane surface the voltage
difference is absent and baseline is recorded. In irritation the cell membrane
is depolarized some part firstly. The voltage between two electrodes increased
and electro gram line deflects upward. The positive wave of electro gram is
formed. After depolarizing all the membrane its surface charge becomes
negative. The voltage under both electrodes temporarily becomes the same.
Electro gram line returns downward to baseline level. Then repolarization
begins from the same point as depolarization and in the same order. Polarity of
cell membrane changes and electro gram deflects downward. The negative wave is
formed. After depolarization of hole the membrane
surface the voltage returns to rest potential level and baseline on electro
gram record.
Electrogram
and action potential of one myocardial cell.
During the depolarization phase the rapid Na+ gates open and
inward diffusion of Na+ occur. This event corresponds to formation upward part
of positive wave on electro gram line. The next fast initial repolarization
begins with inward Cl- diffusion. Then electro gram returns to baseline level.
When opening slow Ca2+ gates the potential difference temporarily isn’t
essential and baseline continues. During the next phase outward K+ diffusion
increases and external surface of membrane becomes positive. Voltage
fluctuation leads to deflection of electro gram downward further returning to
baseline level. In rest period all the membrane has positive charge on external
surface and baseline is recorded. Through this period ion pumps restore initial
distribution of ions.
Formation of Electrogram waves
An electrocardiograph is an instrument that measures and
records the electrocardiogram (ECG), the electrical activity generated by the
heart. Electrodes placed on various anatomical sites on the body help conduct
the ECG to the electrocardiograph. The ECG alone is not sufficient to diagnose
all abnormalities possible in the pacing or conduction system of the heart. The
interpretation of the 12-lead ECG provides a differential diagnosis for many
arrhythmias.
The electrical current generated by the heart is conducted
through the pairs of electrodes and leads, and is amplified, recorded, and
processed by the electrocardiograph. The wires connecting the pairs of
electrodes on the surface of the body to the electrocardiograph are called
leads. The different features and modules of a typical electrocardiograph
include the protection circuitry, lead selector, calibration signal,
preamplifier, isolation circuit, driver amplifier, memory system,
microcomputer, and recorder or printer.7
Twelve leads usually comprise a diagnostic ECG recording:
six limb leads (three bipolar and three unipolar), and six unipolar precordial
leads. The instantaneous cardiac scalar voltages resulting from the electrical
activity in the heart is measured in each of the 12 leads. Since the cardiac
vector varies in magnitude with time over a three-dimensional space, it is
important to know its presentation (i.e. appearance or projection) in each of
the 12 leads of the ECG.
Figure on presentation (Fig. 1) shows
the lead placement to acquire the 12-lead ECG. The leads can be categorized
into the frontal leads (I, II, III, aVR, aVL, and aVF), and the transverse
leads (V1, V2, V3, V4, V5, and V6). The frontal leads measure the projection of
the cardiac vector on the frontal plane of the body. The frontal plane is
parallel to the floor when lying supine. The transverse or precordial leads
measure the projection of the cardiac vector on the horizontal plane, (i.e. the
plane that is parallel to the floor when standing).
Leads I, II, and III of the frontal plane are bipolar. They
record the differences between two points on the body. Figure below shows that
lead I is measured between an electrode on the left
arm (the positive electrode) and an electrode on the right arm. The
three-dimensional cardiac vector projects into each of the bipolar leads,
indicating the strength and direction of the instantaneous cardiac vector.
Leads aVR (on the right arm), aVL (on the left arm), and aVF
(on the foot) are unipolar leads. They measure the potential difference on the
limbs with respect to a reference point formed by the two resistors between
limb electrodes (Figure 4.1). For example, lead aVR is measured between an
electrode on the right arm, and a reference point formed via a resistor to the
left arm and another resistor to the left foot. These leads show the cardiac
vector projection on the frontal plane, and are amplified by about 50% (i.e. augmented)
so that their amplitudes are comparable to those of the bipolar leads.
The six precordial leads, V1 to V6, are unipolar and measure
the cardiac vector projection on the horizontal plane. These precordial leads
are measured with respect to the
The 12-lead ECG provides various viewpoints of the three-dimensional
instantaneous cardiac vector that are somewhat redundant, and this is helpful
in providing discriminatory information for diagnosing abnormalities in the
pacing and conduction system of the heart.
The electrical activity due to the specialized cells in the
heart results in an electric potential on the surface of the body. Each cell
can be modeled by a dipole, and the superposition of the potentials from the
dipoles for all of the cells in the myocardium results in a three-dimensional
cardiac vector for the heart at each instant in time. The cardiac vector at
each instant of time represents the net electrical activity in the heart.
Figure The 6-axial system shows the orientation of the
frontal plane leads. During left axis deviation (LAD), the mean axis of the QRS
will be less than –30°. RAD: right axis deviation.
Figure on the left shows the 6-axial reference system which
is used in the diagnosis of certain abnormalities. The 6-axial system shows the
orientation of the frontal-plane leads. The various orientations of each lead
result in a different projection of the cardiac vector onto that particular
lead. A mean electrical axis as a function of time, during the depolarization
and repolarization phases of a cardiac cycle can be calculated. For example,
the electrical axis of ventricular depolarization, QRS, represents the average
of the instantaneous cardiac vectors during ventricular depolarization. The QRS, usually lies between aVL and aVF in Figure 2 It is easy
to diagnose left and right axis deviation, LAD and RAD respectively. In LAD,
lead I is predominantly positive (i.e. R wave is positive) and both leads II
and III are predominantly negative (i.e. R-wave is small or absent). Both II
and III must be predominantly negative, i.e. if in lead II the S wave is
smaller than the R wave, LAD is not present. If lead II is equiphasic (R and S
waves have equal magnitudes), then there is borderline LAD. In RAD, lead I is
predominantly negative and both II and III are predominantly positive (Bennett,
1989).
Figure below shows the Einthoven triangle superimposed on
the locus of points formed in the frontal plane by a normal instantaneous
cardiac vector, the vectorcardiographic loop during one cardiac cycle. The
measured ECG on each lead is a projection of the instantaneous cardiac vector.
The P loop corresponding to atrial contraction, projects onto leads I, II, and
III as an upward deflected wave. However, the S wave is projected onto lead III
remarkably more than in leads I and II. The instantaneous orientation of the
cardiac vector, and the orientation of the lead
determine whether there is a positive going or negative going waveform on the
lead. Thus the different leads of the 12-lead ECG show various projections of
the phases in the cardiac cycle.
Figure For a normal
ECG, the instantaneous cardiac vector projects into lead III with a more
negative S wave than in lead I. The dashed line indicates how the QRS complex
projects onto lead I. Shaded areas represent key segments of the ECG.
ECG correspondence of heart
depolarization
Every cardiac cycle produces ECG waves designated as P, Q,
R, S and T. These waves are not action potentials. They represent potentials
between rested and depolarized or depolarized and repolarized parts of whole
heart. Amplitude and duration of these waves correspond to electrical power
fluctuation in entire heart.
After producing impulse in SA-node depolarization begins at
first in cells of right atrium and ascend part of P wave is recorded. When
depolarization spreads into left atrium, the ECG line returns to baseline
level. Delay of depolarization in AV-node recorded as PQ-interval in baseline.
Then impulse spreads into middle part of septum and heart apex. This event
recorded as descend part of Q wave. In next depolarization of right ventricle
wall ECG line deflexed upward and formation of R wave begins. When impulse
spreads into left ventricle wall, the ECG line returned in contrary side
towards the lowest point of S wave. Depolarization of ventricles basis
afterwards caused formation of S wave, which continues to baseline.
Repolarization of atria is failed to record in ECG because
of greater depolarization of ventricles. Repolarization of ventricles develops
firstly in right part of heart and then in left one. That is why ascend and
descend parts of T wave are formed.
In diastole normally baseline is recorded but U wave may
occur.
ECG leads
a) Bipolar limb leads. The bipolar limb leads record the
voltage between electrodes placed on the wrists and legs. These leads were
proposed by Einthoven in 1913.
I
lead: left arm (+) - right arm (-);
II
lead: left leg (+) -
right arm (-);
III
lead: left arm (+) - left leg (-).
For recording limb leads we put red electrode on right arm,
yellow - on left arm, green - on left leg and black - on right leg. Black
electrode has zero potential (ground).
b) The unipolar limb leads were proposed by Goldberger in
1942. They record voltage between single “exploratory electrode” fro one limb
and zero joined electrode from two other limbs. So there are three leads AVR,
AVL, AVF. In fact zero electrodes records middle voltage of two limbs. Bipolar
limb leads and unipolar limb leads record electrical power in frontal
projection.
c) The unipolar chest leads were suggested in 1934 by
Vilson. One electrode, which is active, situated on the chest in six standard
positions. They labeled V1 - V6. Joined zero electrode
records middle potential of right arm, left arm and left leg. That is
why every chest lead records voltage between active chest electrode and
Vilson’s joined zero electrode.
These standard positions of
active chest electrode are:
V1
- in crossing right IV right intercostal space and parasternal line;
V2
- in crossing left IV intercostal space and parasternal line;
V3
- between V2 and V4;
V4
- in crossing V left intercostal space and medioclavicular line;
V5
- in crossing V left intercostal space and anterior axilar line;
V6
- in crossing V left intercostal space and middle axilar line.
Unipolar chest leads records
changes of heart polarity in horizontal projection.
Algorithm of ECG registration
1) Registration performs fare from electric
motors and other electrical devices.
2)
Tested person may have rest before registration in 10-15 minutes. This
procedure needs 2-hour interval after eating or worm procedures.
3)
For better contact between electrodes and skin use solution NaCl 5-10 % or
special electrode past or electrode gel. Otherwise hindrances in ECG curve may
occur. They will stand in the way of ECG analysis
4)
ECG registration performs in quiet breathing in patients.
5)
Registration begins from standard voltage 1 mV from the electrocardiograph for
regulation of amplitude in ECG. Usually standard voltage amplitude is