Golwalla’s Electrocardiography for Medical Students and General Practitioners Sharukh A Golwalla
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IntroductionCHAPTER 1

 
BASIC PRINCIPLES
When the heart contracts, electric currents are produced and distributed throughout the body to the skin. Two electrodes can be applied to any two parts of the body to lead the heart current to a recording galvanometer. The graphic representation of these electric currents is called an electrocardiogram.
 
BASIC ELECTROPHYSIOLOGY
The changes in the electrical potential with each heart beat can be understood by considering the electrical behavior of a single cell. The surface of the resting cell will be electrically positive compared with the interior of the cell which is electrically negative. A cell in this condition is said to be in the ‘polarized’ state (Fig. 1A) and the exterior and interior of the cell can be compared to the two poles of a battery. When the cell is stimulated, the positive ions migrate into the cell and the negative ions migrate out of the cell. With this reversal of polarity, the cell is said to be ‘depolarized’ (Fig. 1B). When the effect of excitation has passed off and the cell has returned to its former resting state, the positive charge outside and negative charge inside are restored, the cell is ‘repolarized’ (Fig. 1C).
When an excitatory (depolarization) process flows towards a unipolar electrode, the galvanometer will record a positive or upward deflection, and when it flows away from the electrode, a negative or downward deflection (Fig. 2):
Thus, the excitation and subsequent recovery of the muscle strip have given rise to two electrical currents or deflections of opposite directions. The currents of repolarization (during recovery) are weaker and extend over a longer period of time than those of depolarization (during excitation).2
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Figs. 1A to C: Illustrates: (A) Polarized; (B) Depolarized; and (C) Repolarized cell.
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Fig. 2: Illustrates the electrical behavior of a single cell or strip of muscle, indicated by a rectangle from the two ends of which the potentials are led off to a galvanometer.
Applying this to the electrical changes produced by the heart beats, the same fundamental principle holds but with some modifications. This is because the heart consists of a multitude of intercommunicating muscle fibers and has four chambers which are activated in sequence more complicated than the simple spread of excitation through a muscle strip.
 
NORMAL SEQUENCE OF CARDIAC DEPOLARIZATION AND REPOLARIZATION
The normal process of activation begins in the sinoatrial (SA) node and spreads through the atria in a lateral and downward direction (Fig. 3, arrow 1). Since the atria are thin-walled structures, little electrical activity results from their depolarization—P wave. An electrode placed on the left side of the body will record an upright P wave, on the right side a negative P wave.3
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Fig. 3: Sequence of electrical activation in the heart.
The wave of depolarization then activates the atrioventricular (AV) node where there is a 1/10 second delay to allow the ventricles to fill prior to ventricular systole. During this time, electrical activity moves very slowly through the AV node and then into the ventricles through the proximal portion of the ventricular conducting system, the bundle of His and the bundle branches, the septum being activated from, left to right. All these structures are so small that electrical activity within them is not detected and on the ECG no movement of the base line is seen—the isoelectric PR interval.
Activation spreads into the main mass of ventricular muscle from the subendocardial region outwards. Electrocardiographically the ventricles are made up of three muscle groups—right ventricle (RV), interventricular septum (which behaves as a left ventricular structure) and the left ventricle (LV). The first portion of ventricular depolarization in the ECG results from septal depolarization from left to right (Fig. 3, arrow 2). Since the septum is smaller than the bulk of the myocardium, this initial deflection is relatively small—q wave.
The depolarization then spreads outwards simultaneously through the free ventricular walls from endocardial to epicardial surface (Fig. 3, arrow 3). The thick left ventricular electrical force counteracts the smaller right ventricular force. A large upright deflection R is thus produced in a left-sided electrode. Sometimes, late activation of an upper part of the right ventricle (Fig. 3, arrow 4) produces a 4late negative deflection S. The QRS pattern recorded by a left-sided electrode is mirrored by an rsr’ pattern in an electrode on the right side of the chest (Fig. 3).
After the ventricle has been totally depolarized, there is no electrical activity for a brief period until repolarization begins—ST segment.
Repolarization, that is the return of myocardial cells to their resulting negative potential then proceeds from endocardium to epicardium. Ventricular repolarization produces—T wave. The recovery process (Fig. 3, arrow 5) is much slower than the activation and the—T wave is generally a broad deflection in a similar direction as a rule to the main wave of the QRS complex (upright in left-sided electrodes).
After the conclusion of repolarization, there is again a period of electrical inactivity and the base line of the ECG remains isoelectric until the next impulse originates producing the next series of P-QRS-T complexes.
 
ECG REGISTRATION—TYPES OF LEADS
The term lead is used to denote the connection of the galvanometer by wires to the electrodes and also for the actual tracing obtained. Although two electrodes can be attached to any part of the body to lead the heart current to the galvanometer, it is customary to make use of the forearms, the left leg and the precordium.
Each chamber of the heart produces a characteristic electrocardiographic pattern. Since the electrical potentials over the various areas of the heart differ, the recorded tracings from each limb vary accordingly.
 
Bipolar Limb Leads or Standard Leads
Here two electrodes are placed on two extremities and both record simultaneously the particular electrical pattern of the heart facing these extremities (Fig. 4).
Lead 1: Records the potential between the left arm electrode (positive pole) and right arm electrode. Thus when an electric current moves through the heart from right arm to left arm a positive deflection is recorded in lead I.5
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Fig. 4: Limb leads.
Lead II: Left leg (positive pole) and right arm. An electric current moving from the right arm diagonally downward to the left leg causes an upward deflection in lead II.
Lead III: Positive pole is at left leg and negative pole at left arm. A current flowing from left arm to left leg records a positive deflection in lead III.
Although electrodes are attached to both legs, the right leg electrode is used as spare or ground since recordings obtained from the other leg are identical.
 
Unipolar Leads
Each standard limb lead, it will be obvious, is in reality a combination of two tracings. It would be however an advantage to obtain a final 6pattern of the electrocardiogram which would represent the unaltered potential of one area of the heart. This can be achieved with the unipolar technique.
Unipolar leads are obtained by placing one electrode (exploring electrode) in close proximity to the heart, while the other indifferent electrode is far removed from it so that its potential is more or less reduced to zero.
All unipolar leads are designated by the letter V. There are two types of unipolar leads — unipolar limb leads and unipolar chest leads.
 
Unipolar Limb Leads
These are registered by a recording system in which one electrode is placed in turn, over one of the three extremities used in recording standard leads, while the other is connected to the central terminal. With this technique however, the amplitude of the deflections is so small that their interpretation is difficult. The amplitude can be increased by 50% if the exploring electrode is attached to either the right arm, left arm, or left leg and connected to one pole of the galvanometer, whilst the other pole is connected to an indifferent lead point—a central terminal connecting the remaining three extremities not being explored. They are therefore called augmented limb or extremity leads and the letter ‘a’ is used as prefix to denote augmentation.
From Figure 5, it will be seen that the electrical picture present at any particular extremity covers a variable surface of the heart. Since the electrical potentials over various areas of the heart differ, the recorded patterns from each limb will also vary.
Lead aVR: The electrode of the right forearm, conveys the electrical picture of the heart as it presents itself at the right shoulder and reflects the potential variations of the atrial and ventricular cavities.
Lead aVL: Conveys the electrical potentials from the left lateral cardiac wall.
Lead aVF: Which corresponds to the center of the left lower limb with the trunk, reflects the potential of the diaphragmatic or inferior surface of the heart.
The basic information obtained from these leads is the same as bipolar limb leads. The six limb leads provide a round the clock view of cardiac electrical activity in the frontal plane of the body.7
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Fig. 5: Augmented limb leads, and precordial leads V1–V6. Each augmented limb lead is visualized as a solid cone projecting on to the surface of the heart.
 
Unipolar Precordial or Chest Leads (Fig. 5)
Direct leads from the various points on the heart itself present the most detailed information regarding the spread of the excitation wave through the heart and abnormalities thereof. The best substitute for such direct leads is precordial leads with the exploring electrode placed on the skin over the part of the heart which it is desired to study.
A series of six positions across the precordium (leads Vl to V6) have been chosen for standard recording giving a round the clock view of the horizontal plane of the heart (Fig. 5):
V1
:
4th intercostal space right sternal border.
V2
:
4th intercostal space left sternal border.
V3
:
Midway between V2 and V4.
V4
:
5th intercostal space in midclavicular line.
V5
:
5th intercostal space in anterior axillary line
V6
:
Midaxillary line at level of V4.8
Additional leads sometimes recorded are:
Ve
:
Inferior border of the sternum, slightly to the left of the ensiform process.
VSc
:
Below the inner end of the left clavicle.
V3R, V4R
:
Position on right anterior chest corresponding to V3,V4.
V7
:
Posterior axillary line at level of V4.
V8
:
Posterior scapular line at level of V4.
Lead Classification, for the purpose of analysis the 12 leads that are usually recorded fall into three groups:
  1. Left-sided leads — I, aVL, V5 and V6.
  2. Right-sided leads — aVR, Vi and V2.
  3. Inferior (diaphragmatic) leads — II, III, aVF
Mid-precordial leads V3 and V4 depict a transition between right- and left-sided patterns.