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 Wilson central terminal which is formed by a three resistor network as shown in Figure 4.1. V1 and V2 are placed on the fourth intercostal space to the right and left, respectively, of the sternum. The V4 electrode is placed on the fifth intercostal space at the left midclavicular line. The V3 electrode lies between V2 and V4. Electrode V5 is placed to the left of V4 on the anterior axillary line, and V6 is placed on the same level as V5 on the midaxillary line. It is important to account for the position of these electrodes when interpreting the ECG on leads V1 through V6. The precordial leads measure the potential between each of V1 through V6 and the Wilson’s central terminal formed as shown in Figure 1

 

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 10 mm. Then continue registration of bipolar limb leads, the next - unipolar limb leads and afterwards - unipolar chest leads.