INTRODUCTION
Electrocardiogram (ECG) is nothing but the graphical representation of the electrical activity of the heart during each cardiac cycle. ECG has grown to be one of the most commonly used medical tests in modern medicine. Its utility in the diagnosis of a myriad of cardiac pathologies ranging from myocardial ischemia, arrhythmia, and infarction to syncope has been invaluable to clinicians for decades. The ECG is a tool of extraordinary clinical power, extraordinary both for the ease with which it can be mastered and also for the extraordinary range of situations in which it can provide helpful clinical information. One glance at an ECG can diagnose an evolving myocardial infarction, can predict the possibility of acute coronary syndromes, identify potentially life-threatening arrhythmias, can suggest the possibility of a massive pulmonary embolism or simply provide a measure of reassurance to someone who is worried about his atypical chest symptoms.
While the first electrical current associated with a heartbeat was identified in 1842 by an Italian physicist, it was a British scientist, Augustus Waller, who published the first human ECG in 1887. Adding further, it was not until 1893 that Dutch scientist, Willem Einthoven (21st May 1860 to 29th September 1927) introduced the term “electrocardiogram” and subsequently refined the concept of cardiac electrical conduction, naming the deflections P, Q, R, S, T, and U-waves (Fig. 1.1). The same basic waves are used today to identify and interpret the ECG rhythm strip.1 Willem Einthoven was a doctor and physiologist, he invented the first practical ECG and received the Nobel Prize in Medicine in 1924 for it.1–7
Einthoven's concept was a triangle, an imaginary area on the body, formed by the intersection of the standard bipolar limb leads with the heart at the center (Fig. 1.2). The bipolar leads have a negative and positive pole that captures the direction of the electrical activity of the heart in the various lead configurations (Fig. 1.3). Looking at the heart from the various electrical vectors that are produced helps to determine the way in which the impulse is traveling.3,7
Fig. 1.2: Einthoven's triangle. This triangle is an imaginary formation of three limb leads in a triangle used in electrocardiography. The shape forms an inverted equilateral triangle with the heart at the center that produces zero potential when the voltages are summed.
In resting state, the cardiac cells are electrically polarized; their insides are negatively charged with respect to their outsides. This polarity is maintained by membrane pump system. Cardiac cells lose their internal negativity in a process called depolarization.2
This depolarization is the key electrical event of the heart. This depolarization propagates from cell-to-cell producing a wave, which can be transmitted across the entire heart. These depolarization waves can be detected by surface electrodes in ECG. Once depolarization is complete, the cardiac cells restore their resting polarity through a process called repolarization. Both depolarization and repolarization waves can be sensed and detected in the surface ECG.
An ECG lead consists of two surface electrodes of opposite polarity (one positive and one negative) or one positive surface electrode and a reference point. A lead composed of two electrodes of opposite polarity is called bipolar lead. A lead composed of a single positive electrode and a reference point is a unipolar lead.
The heart consists of three different types of cells:
- Pacemaker cells: Generates impulse
- Electricity conducting cells: Transmits impulse
- Myocardial cells: Maintains contractile function of heart.
The waves those appear on the surface ECG primarily represent the electrical activities of the myocardial cells, which comprise the vast bulk of the heart. Pacemaker activity and transmission by the conducting cells are generally not reflected on the surface ECG, these events do not generate sufficient voltage to be recorded by the surface electrodes. The waves generated by the myocardial depolarization and repolarization are recorded on the ECG graph and have three primary characteristics:
- Duration: Measured in fractions of a second or a millisecond, in a horizontal direction on the ECG paper
- Amplitude: Measured in millivolts (mv), in a vertical direction
- Configuration: Indicates the shape and appearance of a particular wave.
Pacemaker cells are able to depolarize spontaneously, at a particular rate. Each spontaneous depolarization serves as a source of a wave of depolarization that initiates all the electrical activities of each cardiac cycle, cardiac contraction, and relaxation.
Fig. 1.4: Normal anatomy of the conductive system of heart. SA node (sinoatrial node) and AV node (atrioventricular node).
The dominant pacemaker of heart is located high up in the right atrium (RA) and is called sinoatrial node (SA node). During normal sinus rhythm the electrical stimulus is always generated by the SA node (Fig. 1.4). This is a small mass of specialized tissue located in the RA (right upper chamber) of the heart. The SA node generates an electrical stimulus regularly (60–100 times per minute under normal conditions). The atrias are then activated. The electrical stimulus travels down through the conduction pathways and stimulates the ventricles. Anatomic evidence suggests the presence of three intra-atrial pathways: (1) anterior internodal pathway, (2) middle internodal tract, and (3) posterior internodal tract. All these internodal fibers convey the impulse from SA node to atrioventricular (AV) node. Apart from these three internodal pathways there is another pathway called the Bachmann bundle which is a large muscle bundle that appears to conduct the cardiac impulse preferentially from the RA to the left atrium (LA). From SA node the impulse travels to AV node. There, impulses are slowed down for a very short period and then continued down the conduction pathway via the His-Purkinje system into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to the right ventricle (RV) and left ventricle (LV). The distal ends of the His-Purkinje system are called Purkinje fibers. The terminal Purkinje fibers connect with the ends of the bundle branches to form interweaving networks on the endocardial surface of both ventricles, which transmit the cardiac impulse almost simultaneously to the entire right and left ventricular endocardium.7–13 The terminal filaments–Purkinje fibers spread out and terminate beneath the endocardium that lines both left and right ventricular cavities. So, ventricular depolarization begins at the endocardial surface and proceeds toward outer surface, epicardium of the ventricles. In fact, the Purkinje fibers do not penetrate into the myocardium but the terminal ends of the Purkinje fibers subdivide just beneath the endocardium.3
PARTS OF THE ELECTRICAL SYSTEM (FIG. 1.5)
Sinoatrial node: It is known as the heart's natural pacemaker, the SA node has special cells that create the electricity that makes heartbeat.
Atrioventricular node: The AV node is the bridge between the atria and ventricles. Electrical signals pass from the atria down to the ventricles through the AV node.
His-Purkinje system: The His-Purkinje system carries the electrical signals throughout the ventricles to make them contract. The parts of the His-Purkinje system include:
- His bundle (the start of the system)
- Right bundle branch (RBB)
- Left bundle branch (LBB)
- Purkinje fibers (the end of the system).
WAVES OF THE ELECTROCARDIOGRAM
The ECG tracing shows the electrical conduction of the heart as the cells depolarize and repolarize through each beat. Each individual beat is represented on the ECG as the PQRST complex (Fig. 1.6). Each sequential letter represents a sequential discrete part of the complex and each represents a different portion of the cardiac cycle.
The first part of the complex is called the P-wave. The P-wave represents the initiation of depolarization in the sinus node and subsequent atrial contraction. It is relatively small in amplitude due to the thin muscle mass of the atrium.
After P-wave the next part of the ECG tracing is called the QRS complex. The QRS complex represents the conductance and sequential depolarization of the ventricles. It is a much larger wave form than the P-wave due to the large muscle mass of the ventricles. An initial downward (negative) deflection in the QRS is termed the Q wave. Q wave may or may not be present depending on the lead and the presence of any cardiac disease. The R-wave is large and upright (positive) after Q-wave. The S-wave follows the R-wave and is a downward (negative) deflection. The ST segment follows the QRS complex. The ST segment represents the period of time in which the ventricles are isoelectric.
The T-wave follows the ST segment of the PQRST complex. The T-wave represents the repolarization of the ventricles. The T-wave is normally upright in all leads except V1 and aVR (isolated T-wave inversion in III can also be normal). The U-wave is a wave on the ECG that is not always seen. It is typically small, and, by definition, follows the T-wave. U-waves are thought to represent repolarization of the papillary muscles or Purkinje fibers.
ELECTROCARDIOGRAM REPRESENTATION OF SYSTOLE AND DIASTOLE
The period during which mechanical contraction occurs is termed as systole and the remaining periods of time during which the atria or ventricles are relaxing and filling with incoming blood are called diastole. Ventricular systole denotes contraction of the ventricles and it is represented on the surface ECG as the interval between the peak of the R-wave and the end of the T-wave. The ventricular diastole is the interval between the end of the T-wave and the peak of the next R-wave (Fig. 1.7).
SUMMARY
The ECG is a graphic record of the direction and magnitude of the electrical activity generated by the depolarization and repolarization of the atria and ventricles of the heart. This electrical activity is readily detected by electrodes attached to the skin.
The normal electrical conduction in the heart allows the impulse that is generated by the SA node of the heart4 to be propagated to, and stimulate, the cardiac muscle (myocardium). The myocardium contracts after stimulation. It is the ordered, rhythmic stimulation of the myocardium during the cardiac cycle that allows efficient contraction of the heart, thereby allowing blood to be pumped throughout the body. SA node is the normal pacemaker of the heart and the cardiac impulse is generated here.
- Sinoatrial node normally generates the action potential, i.e. the electrical impulse that initiates contraction.
- The SA node excites the RA, and the impulse travels through Bachmann's bundle to excite LA.
- The impulse also travels through internodal pathways in RA to the AV node.
- From the AV node, the impulse then travels through the bundle of His and down the bundle branches, fibers specialized for rapid transmission of electrical impulses, on either side of the interventricular septum.
- Right bundle branch depolarizes the RV.
- Left bundle branch depolarizes the LV and interventricular septum.
- Both bundle branches terminate in Purkinje fibers.
Millions of small Purkinje fibers terminate beneath the endocardium and transmits depolarization wave from endocardium to epicardium. Depolarization wave conducts slowly through the AV node and then conducts rapidly through the His bundle to the right and left bundle branches into the terminal filaments of the Purkinje system, which depolarize the ventricular myocardium (Flowchart 1.1).
Electrocardiogram Tracing
To briefly summarize the components of a normal ECG tracing, it consists of waveform components which indicate electrical events during one heartbeat. These waveforms are labelled as P, Q, R, S, T, and U.
P-wave is the first short upward movement of the ECG tracing. The P-wave in the ECG represents atrial depolarization, which results in atrial contraction.
The PR interval indicates the transit time for the electrical signal to travel from the sinus node to the ventricles.
The QRS complex represents ventricular depolarization.
The ST segment connects the QRS complex and the T-wave.
T-wave represents ventricular repolarization.
The U-wave is a small (0.5 mm) deflection immediately following the T-wave, usually in the same direction as the T-wave.
Ventricular systole: Interval between the peak of the R-wave and the end of the T-wave.
Ventricular diastole: Interval between the end of the T-wave and the peak of the next R-wave.
REFERENCES
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