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84-524: Electrocardiography is the process of producing an electrocardiogram ( ECG or EKG ), a recording of the heart's electrical activity through repeated cardiac cycles . It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in

168-467: A continuously variable signal, in contrast to digital electronics where signals usually take only two levels . The term analogue describes the proportional relationship between a signal and a voltage or current that represents the signal. The word analogue is derived from the Greek word ανάλογος analogos meaning proportional . An analogue signal uses some attribute of the medium to convey

252-449: A medical emergency and CPR should be performed). Ventricular fibrillation produces an ECG but is too dysfunctional to produce a life-sustaining cardiac output. Certain rhythms are known to have good cardiac output and some are known to have bad cardiac output. Ultimately, an echocardiogram or other anatomical imaging modality is useful in assessing the mechanical function of the heart. Like all medical tests, what constitutes "normal"

336-403: A sports physical out of concern for sudden cardiac death . Electrocardiograms are recorded by machines that consist of a set of electrodes connected to a central unit. Early ECG machines were constructed with analog electronics , where the signal drove a motor to print out the signal onto paper. Today, electrocardiographs use analog-to-digital converters to convert the electrical activity of

420-464: A transducer which converts one type of energy into another (e.g. a microphone ). The signals take any value from a given range, and each unique signal value represents different information. Any change in the signal is meaningful, and each level of the signal represents a different level of the phenomenon that it represents. For example, suppose the signal is being used to represent temperature, with one volt representing one degree Celsius . In such

504-412: A 2.5 second tracing of each of the twelve leads. The tracings are most commonly arranged in a grid of four columns and three rows. The first column is the limb leads (I, II, and III), the second column is the augmented limb leads (aVR, aVL, and aVF), and the last two columns are the precordial leads (V 1 to V 6 ). Additionally, a rhythm strip may be included as a fourth or fifth row. The timing across

588-416: A conventional 12-lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest. The overall magnitude of the heart's electrical potential is then measured from twelve different angles ("leads") and is recorded over a period of time (usually ten seconds). In this way, the overall magnitude and direction of the heart's electrical depolarization is captured at each moment throughout

672-442: A new "Start" of the cardiac cycle. Throughout the cardiac cycle, blood pressure increases and decreases. The movements of cardiac muscle are coordinated by a series of electrical impulses produced by specialized pacemaker cells found within the sinoatrial node and the atrioventricular node . Cardiac muscle is composed of myocytes which initiate their internal contractions without receiving signals from external nerves—with

756-427: A normal heart rate is between 60 and 100 bpm (normocardic), whereas it is higher in children. A heart rate below normal is called " bradycardia " (<60 in adults) and above normal is called " tachycardia " (>100 in adults). A complication of this is when the atria and ventricles are not in synchrony and the "heart rate" must be specified as atrial or ventricular (e.g., the ventricular rate in ventricular fibrillation

840-423: A part of the esophagus where the distance to the posterior wall of the left atrium is only approximately 5–6 mm (remaining constant in people of different age and weight). An esophageal lead avails for a more accurate differentiation between certain cardiac arrhythmias, particularly atrial flutter , AV nodal reentrant tachycardia and orthodromic atrioventricular reentrant tachycardia . It can also evaluate

924-404: A signal is, mainly the noise present in the original signal and the noise added by processing (see signal-to-noise ratio ). Fundamental physical limits such as the shot noise in components limits the resolution of analogue signals. In digital electronics additional precision is obtained by using additional digits to represent the signal. The practical limit in the number of digits is determined by

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1008-663: A significant problem and can result in clinical mismanagement. Besides the standard electrocardiograph machine, there are other devices capable of recording ECG signals. Portable devices have existed since the Holter monitor was introduced in 1962. Traditionally, these monitors have used electrodes with patches on the skin to record the ECG, but new devices can stick to the chest as a single patch without need for wires, developed by Zio (Zio XT), TZ Medical (Trident), Philips (BioTel) and BardyDx (CAM) among many others. Implantable devices such as

1092-463: A system, 10 volts would represent 10 degrees, and 10.1 volts would represent 10.1 degrees. Another method of conveying an analogue signal is to use modulation . In this, some base carrier signal has one of its properties altered: amplitude modulation (AM) involves altering the amplitude of a sinusoidal voltage waveform by the source information, frequency modulation (FM) changes the frequency. Other techniques, such as phase modulation or changing

1176-424: A voltage requires two contacts and so, electrically, the unipolar leads are measured from the common lead (negative) and the unipolar lead (positive). This averaging for the common lead and the abstract unipolar lead concept makes for a more challenging understanding and is complicated by sloppy usage of "lead" and "electrode". In fact, instead of being a constant reference, V W has a value that fluctuates throughout

1260-416: A wheel in the coronal plane (vertical), and six precordial leads or chest leads that lie on the perpendicular transverse plane (horizontal). Leads should be placed in standard positions. Exceptions due to emergency or other issues should be recorded to avoid erroneous analysis. The 12 standard ECG leads are listed below. All leads are effectively bipolar, with one positive and one negative electrode;

1344-402: Is 300–600 bpm, whereas the atrial rate can be normal [60–100] or faster [100–150]). In normal resting hearts, the physiologic rhythm of the heart is normal sinus rhythm (NSR). Normal sinus rhythm produces the prototypical pattern of P wave, QRS complex, and T wave. Generally, deviation from normal sinus rhythm is considered a cardiac arrhythmia . Thus, the first question in interpreting an ECG

1428-447: Is a combination of inputs from two limb electrodes, with a different combination for each augmented lead. It is referred to immediately below as "the negative pole". Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system , which is used to calculate the heart's electrical axis in the frontal plane. Older versions of the nodes (VR, VL, VF) use Wilson's central terminal as

1512-642: Is a safe and painless procedure. The machines are powered by mains power but they are designed with several safety features including an earthed (ground) lead. Other features include: Most modern ECG machines include automated interpretation algorithms . This analysis calculates features such as the PR interval , QT interval , corrected QT (QTc) interval , PR axis, QRS axis, rhythm and more. The results from these automated algorithms are considered "preliminary" until verified and/or modified by expert interpretation. Despite recent advances, computer misinterpretation remains

1596-425: Is also reflected from branches in the arterial tree and gives rise to a dicrotic notch in main arteries. The summation of the reflected pulse wave and the systolic wave may increase pulse pressure and help tissue perfusion. With increasing age, the aorta stiffens and can become less elastic which will reduce peak pulse in the periphery. The heart is a four-chambered organ consisting of right and left halves, called

1680-427: Is an anterior lead and V 5 is a lateral lead, they are contiguous because they are next to one another. The study of the conduction system of the heart is called cardiac electrophysiology (EP). An EP study is performed via a right-sided cardiac catheterization : a wire with an electrode at its tip is inserted into the right heart chambers from a peripheral vein, and placed in various positions in close proximity to

1764-415: Is based on population studies . The heartrate range of between 60 and 100 beats per minute (bpm) is considered normal since data shows this to be the usual resting heart rate. Interpretation of the ECG is ultimately that of pattern recognition. In order to understand the patterns found, it is helpful to understand the theory of what ECGs represent. The theory is rooted in electromagnetics and boils down to

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1848-507: Is collected for the next contraction. This period is best viewed at the middle of the Wiggers diagram—see the panel labeled "diastole". Here it shows pressure levels in both atria and ventricles as near-zero during most of the diastole. (See gray and light-blue tracings labeled "atrial pressure" and "ventricular pressure"—Wiggers diagram.) Here also may be seen the red-line tracing of "Ventricular volume", showing an increase in blood volume from

1932-425: Is fundamentally about understanding the electrical conduction system of the heart . Normal conduction starts and propagates in a predictable pattern, and deviation from this pattern can be a normal variation or be pathological . An ECG does not equate with mechanical pumping activity of the heart; for example, pulseless electrical activity produces an ECG that should pump blood but no pulses are felt (and constitutes

2016-608: Is random disturbances or variations, some caused by the random thermal vibrations of atomic particles. Since all variations of an analogue signal are significant, any disturbance is equivalent to a change in the original signal and so appears as noise. As the signal is copied and re-copied, or transmitted over long distances, these random variations become more significant and lead to signal degradation . Other sources of noise may include crosstalk from other signals or poorly designed components. These disturbances are reduced by shielding and by using low-noise amplifiers (LNA). Since

2100-414: Is the isovolumic relaxation , during which pressure within the ventricles begin to fall significantly, and thereafter the atria begin refilling as blood returns to flow into the right atrium (from the vena cavae ) and into the left atrium (from the pulmonary veins ). As the ventricles begin to relax, the mitral and tricuspid valves open again, and the completed cycle returns to ventricular diastole and

2184-410: Is too slow, then it is sinus bradycardia . Cardiac cycle Assuming a healthy heart and a typical rate of 70 to 75 beats per minute, each cardiac cycle, or heartbeat, takes about 0.8 second to complete the cycle. Duration of the cardiac cycle is inversely proportional to the heart rate. There are two atrial and two ventricle chambers of the heart; they are paired as the left heart and

2268-440: Is typically hidden in the much more prominent QRS complex and normally cannot be seen without additional, specialized electrodes. ECGs are normally printed on a grid. The horizontal axis represents time and the vertical axis represents voltage. The standard values on this grid are shown in the adjacent image at 25mm/sec: The "large" box is represented by a heavier line weight than the small boxes. The standard printing speed in

2352-418: Is used to change a digital signal to an analogue signal. A DAC takes a series of binary numbers and converts it to an analogue signal. It is common to find a DAC in the gain-control system of an op-amp which in turn may be used to control digital amplifiers and filters. Analogue circuits are typically harder to design, requiring more skill than comparable digital systems to conceptualize. An analogue circuit

2436-436: Is usually designed by hand because the application is built into the hardware. Digital hardware, on the other hand, has a great deal of commonality across applications and can be mass-produced in a standardised form. Hardware design consists largely of repeated identical blocks and the design process can be highly automated. This is one of the main reasons that digital systems have become more common than analogue devices. However,

2520-462: Is whether or not there is a sinus rhythm. A criterion for sinus rhythm is that P waves and QRS complexes appear 1-to-1, thus implying that the P wave causes the QRS complex. Once sinus rhythm is established, or not, the second question is the rate. For a sinus rhythm, this is either the rate of P waves or QRS complexes since they are 1-to-1. If the rate is too fast, then it is sinus tachycardia , and if it

2604-471: The artificial cardiac pacemaker and implantable cardioverter-defibrillator are capable of measuring a "far field" signal between the leads in the heart and the implanted battery/generator that resembles an ECG signal (technically, the signal recorded in the heart is called an electrogram , which is interpreted differently). Advancement of the Holter monitor became the implantable loop recorder that performs

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2688-523: The cardiac cycle . There are three main components to an ECG: During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial node , spreads throughout the atrium , and passes through the atrioventricular node down into the bundle of His and into the Purkinje fibers , spreading down and to the left throughout the ventricles . This orderly pattern of depolarization gives rise to

2772-410: The right heart and the left heart . The upper two chambers, the left and right atria , are entry points into the heart for blood-flow returning from the circulatory system , while the two lower chambers, the left and right ventricles , perform the contractions that eject the blood from the heart to flow through the circulatory system. Circulation is split into pulmonary circulation —during which

2856-425: The right heart —that is, the left atrium with the left ventricle, the right atrium with the right ventricle—and they work in concert to repeat the cardiac cycle continuously (see cycle diagram at right margin). At the start of the cycle, during ventricular diastole –early , the heart relaxes and expands while receiving blood into both ventricles through both atria; then, near the end of ventricular diastole –late ,

2940-516: The AV valves are forced to close, which stops the blood volumes in the ventricles from flowing in or out; this is known as the isovolumic contraction stage. Due to the contractions of the systole, pressures in the ventricles rise quickly, exceeding the pressures in the trunks of the aorta and the pulmonary arteries and causing the requisite valves (the aortic and pulmonary valves) to open—which results in separated blood volumes being ejected from

3024-533: The United States is 25 mm per sec (5 big boxes per second), but in other countries it can be 50 mm per sec. Faster speeds such as 100 and 200 mm per sec are used during electrophysiology studies. Not all aspects of an ECG rely on precise recordings or having a known scaling of amplitude or time. For example, determining if the tracing is a sinus rhythm only requires feature recognition and matching, and not measurement of amplitudes or times (i.e.,

3108-505: The application of digital hardware is a function of the software / firmware and creating this is still largely a labour-intensive process. Since the early 2000s, there were some platforms that were developed which enabled analogue design to be defined using software - which allows faster prototyping. Furthermore, if a digital electronic device is to interact with the real world, it will always need an analogue interface. For example, every digital radio receiver has an analogue preamplifier as

3192-528: The augmented limb leads and the precordial leads lacks precision." Leads I, II and III are called the limb leads . The electrodes that form these signals are located on the limbs – one on each arm and one on the left leg. The limb leads form the points of what is known as Einthoven's triangle . Leads aVR, aVL, and aVF are the augmented limb leads . They are derived from the same three electrodes as leads I, II, and III, but they use Goldberger's central terminal as their negative pole. Goldberger's central terminal

3276-418: The body surface. Any pair of electrodes can measure the electrical potential difference between the two corresponding locations of attachment. Such a pair forms a lead . However, "leads" can also be formed between a physical electrode and a virtual electrode, known as Wilson's central terminal ( WCT ), whose potential is defined as the average potential measured by three limb electrodes that are attached to

3360-476: The body. The mitral and tricuspid valves, also known as the atrioventricular, or AV valves , open during ventricular diastole to permit filling. Late in the filling period the atria begin to contract (atrial systole) forcing a final crop of blood into the ventricles under pressure—see cycle diagram. Then, prompted by electrical signals from the sinoatrial node , the ventricles start contracting (ventricular systole), and as back-pressure against them increases

3444-424: The cardiac cycle. (See Wiggers diagram: "Ventricular volume" tracing (red), at "Systole" panel.) Cardiac diastole is the period of the cardiac cycle when, after contraction, the heart relaxes and expands while refilling with blood returning from the circulatory system . Both atrioventricular (AV) valves open to facilitate the 'unpressurized' flow of blood directly through the atria into both ventricles, where it

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3528-400: The characteristic ECG tracing. To the trained clinician , an ECG conveys a large amount of information about the structure of the heart and the function of its electrical conduction system. Among other things, an ECG can be used to measure the rate and rhythm of heartbeats, the size and position of the heart chambers , the presence of any damage to the heart's muscle cells or conduction system,

3612-405: The conduction system so that the electrical activity of that system can be recorded. Standard catheter positions for an EP study include "high right atrium" or hRA near the sinus node , a "His" across the septal wall of the tricuspid valve to measure bundle of His , a "coronary sinus" into the coronary sinus , and a "right ventricle" in the apex of the right ventricle. Interpretation of the ECG

3696-436: The diastole immediately before the heart again begins contracting and ejecting blood from the ventricles (ventricular systole) to the aorta and arteries. Ventricular systole is the contractions, following electrical stimulations, of the ventricular syncytium of cardiac muscle cells in the left and right ventricles . Contractions in the right ventricle provide pulmonary circulation by pulsing oxygen-depleted blood through

3780-402: The effects of heart drugs, and the function of implanted pacemakers . The overall goal of performing an ECG is to obtain information about the electrical functioning of the heart. Medical uses for this information are varied and often need to be combined with knowledge of the structure of the heart and physical examination signs to be interpreted. Some indications for performing an ECG include

3864-425: The electrical current before it is conducted below the atria and through the circuits known as the bundle of His and the Purkinje fibers —all which stimulate contractions of both ventricles. The programmed delay at the AV node also provides time for blood volume to flow through the atria and fill the ventricular chambers—just before the return of the systole (contractions), ejecting the new blood volume and completing

3948-407: The end of the diastole, the atria begin contracting, then pump blood into the ventricles; this pressurized delivery during ventricular relaxation (ventricular diastole) is called the atrial systole . The closure of the aortic valve causes a rapid change in pressure in the aorta called the incisura. This short sharp change in pressure is rapidly attenuated down the arterial tree. The pulse wave form

4032-507: The exception of changes in the heart rate due to metabolic demand. In an electrocardiogram , electrical systole initiates the atrial systole at the P wave deflection of a steady signal; and it starts contractions (systole). The cardiac cycle involves four major stages of activity: 1) "isovolumic relaxation", 2) inflow, 3) "isovolumic contraction", 4) "ejection". Stages 1 and 2 together—"isovolumic relaxation" plus inflow (equals "rapid inflow", "diastasis", and "atrial systole")—comprise

4116-601: The first stage in the receive chain. Design of analogue circuits has been greatly eased by the advent of software circuit simulators such as SPICE . IBM developed their own in-house simulator, ASTAP , in the 1970s which used an unusual (compared to other simulators) sparse matrix method of circuit analysis. Analogue circuits can be entirely passive , consisting of resistors , capacitors and inductors . Active circuits also contain active elements such as transistors . Traditional circuits are built from lumped elements – that is, discrete components. However, an alternative

4200-515: The following: ECGs can be recorded as short intermittent tracings or continuous ECG monitoring. Continuous monitoring is used for critically ill patients, patients undergoing general anesthesia, and patients who have an infrequently occurring cardiac arrhythmia that would unlikely be seen on a conventional ten-second ECG. Continuous monitoring can be conducted by using Holter monitors , internal and external defibrillators and pacemakers , and/or biotelemetry . For adults, evidence does not support

4284-400: The four following points: Thus, the overall direction of depolarization and repolarization produces positive or negative deflection on each lead's trace. For example, depolarizing from right to left would produce a positive deflection in lead I because the two vectors point in the same direction. In contrast, that same depolarization would produce minimal deflection in V 1 and V 2 because

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4368-433: The heart cycle. It also does not truly represent the center-of-heart potential due to the body parts the signals travel through. Because voltage is by definition a bipolar measurement between two points, describing an electrocardiographic lead as "unipolar" makes little sense electrically and should be avoided. The American Heart Association states "All leads are effectively 'bipolar,' and the term 'unipolar' in description of

4452-456: The heart to a digital signal . Many ECG machines are now portable and commonly include a screen, keyboard, and printer on a small wheeled cart. Recent advancements in electrocardiography include developing even smaller devices for inclusion in fitness trackers and smart watches . These smaller devices often rely on only two electrodes to deliver a single lead I. Portable twelve-lead devices powered by batteries are also available. Recording an ECG

4536-445: The heart's electrical conduction system, which is the "wiring" of the heart that carries electrical impulses throughout the body of cardiomyocytes , the specialized muscle cells of the heart. These impulses ultimately stimulate heart muscle to contract and thereby to eject blood from the ventricles into the arteries and the cardiac circulatory system ; and they provide a system of intricately timed and persistent signaling that controls

4620-468: The information is quantized , as long as the signal stays inside a range of values, it represents the same information. In digital circuits the signal is regenerated at each logic gate , lessening or removing noise. In analogue circuits, signal loss can be regenerated with amplifiers . However, noise is cumulative throughout the system and the amplifier itself will add to the noise according to its noise figure . A number of factors affect how precise

4704-418: The information is encoded differently in analogue and digital electronics , the way they process a signal is consequently different. All operations that can be performed on an analogue signal such as amplification , filtering , limiting, and others, can also be duplicated in the digital domain. Every digital circuit is also an analogue circuit, in that the behaviour of any digital circuit can be explained using

4788-449: The low plateau of the "isovolumic relaxation" stage to the maximum volume occurring in the "atrial systole" sub-stage. Atrial systole is the contracting of cardiac muscle cells of both atria following electrical stimulation and conduction of electrical currents across the atrial chambers (see above, Physiology ). While nominally a component of the heart's sequence of systolic contraction and ejection, atrial systole actually performs

4872-406: The lower wall of the right heart between the atrium and ventricle. The sinoatrial node, often known as the cardiac pacemaker , is the point of origin for producing a wave of electrical impulses that stimulates atrial contraction by creating an action potential across myocardium cells. Impulses of the wave are delayed upon reaching the AV node, which acts as a gate to slow and to coordinate

4956-457: The negative pole, but the amplitude is too small for the thick lines of old ECG machines. The Goldberger terminals scale up (augments) the Wilson results by 50%, at the cost of sacrificing physical correctness by not having the same negative pole for all three. The precordial leads lie in the transverse (horizontal) plane, perpendicular to the other six leads. The six precordial electrodes act as

5040-409: The normal ECG pattern occur in numerous cardiac abnormalities, including: Traditionally, "ECG" usually means a 12-lead ECG taken while lying down as discussed below. However, other devices can record the electrical activity of the heart such as a Holter monitor but also some models of smartwatch are capable of recording an ECG. ECG signals can be recorded in other contexts with other devices. In

5124-431: The other hand, are not affected at all by the presence of noise until a certain threshold is reached, at which point they fail catastrophically. For digital telecommunications , it is possible to increase the noise threshold with the use of error detection and correction coding schemes and algorithms. Nevertheless, there is still a point at which catastrophic failure of the link occurs. In digital electronics, because

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5208-404: The page is continuous and notes tracings of the 12 leads for the same time period. In other words, if the output were traced by needles on paper, each row would switch which leads as the paper is pulled under the needle. For example, the top row would first trace lead I, then switch to lead aVR, then switch to V 1 , and then switch to V 4 , and so none of these four tracings of the leads are from

5292-437: The performance of the analogue-to-digital converter (ADC), since digital operations can usually be performed without loss of precision. The ADC takes an analogue signal and changes it into a series of binary numbers . The ADC may be used in simple digital display devices, e. g., thermometers or light meters but it may also be used in digital sound recording and in data acquisition. However, a digital-to-analogue converter (DAC)

5376-534: The phase of the carrier signal, are also used. In an analogue sound recording, the variation in pressure of a sound striking a microphone creates a corresponding variation in the current passing through it or voltage across it. An increase in the volume of the sound causes the fluctuation of the current or voltage to increase proportionally while keeping the same waveform or shape. Mechanical , pneumatic , hydraulic , and other systems may also use analogue signals. Analogue systems invariably include noise that

5460-494: The positive poles for the six corresponding precordial leads: (V 1 , V 2 , V 3 , V 4 , V 5 , and V 6 ). Wilson's central terminal is used as the negative pole. Recently, unipolar precordial leads have been used to create bipolar precordial leads that explore the right to left axis in the horizontal plane. Additional electrodes may rarely be placed to generate other leads for specific diagnostic purposes. Right-sided precordial leads may be used to better study pathology of

5544-532: The pulmonary valve then through the pulmonary arteries to the lungs. Simultaneously, contractions of the left ventricular systole provide systemic circulation of oxygenated blood to all body systems by pumping blood through the aortic valve, the aorta, and all the arteries. (Blood pressure is routinely measured in the larger arteries off the left ventricle during the left ventricular systole). Analog electronics Analogue electronics ( American English : analog electronics ) are electronic systems with

5628-400: The rhythmic beating of the heart muscle cells, especially the complex impulse-generation and muscle contractions in the atrial chambers. The rhythmic sequence (or sinus rhythm ) of this signaling across the heart is coordinated by two groups of specialized cells, the sinoatrial (SA) node, which is situated in the upper wall of the right atrium, and the atrioventricular (AV) node located in

5712-435: The right arm, the left arm, and the left foot, respectively. Commonly, 10 electrodes attached to the body are used to form 12 ECG leads, with each lead measuring a specific electrical potential difference (as listed in the table below). Leads are broken down into three types: limb; augmented limb; and precordial or chest. The 12-lead ECG has a total of three limb leads and three augmented limb leads arranged like spokes of

5796-439: The right ventricle or for dextrocardia (and are denoted with an R (e.g., V 5R ). Posterior leads (V 7 to V 9 ) may be used to demonstrate the presence of a posterior myocardial infarction. The Lewis lead or S5-lead (requiring an electrode at the right sternal border in the second intercostal space) can be used to better detect atrial activity in relation to that of the ventricles. An esophageal lead can be inserted to

5880-403: The right ventricle pumps oxygen-depleted blood to the lungs through the pulmonary trunk and arteries; or the systemic circulation —in which the left ventricle pumps/ejects newly oxygenated blood throughout the body via the aorta and all other arteries. In a healthy heart all activities and rests during each individual cardiac cycle, or heartbeat, are initiated and orchestrated by signals of

5964-571: The risk in people with Wolff-Parkinson-White syndrome , as well as terminate supraventricular tachycardia caused by re-entry . An intracardiac electrogram (ICEG) is essentially an ECG with some added intracardiac leads (that is, inside the heart). The standard ECG leads (external leads) are I, II, III, aVL, V 1 , and V 6 . Two to four intracardiac leads are added via cardiac catheterization. The word "electrogram" (EGM) without further specification usually means an intracardiac electrogram. A standard 12-lead ECG report (an electrocardiograph) shows

6048-485: The rules of analogue circuits. The use of microelectronics has made digital devices cheap and widely available. The effect of noise on an analogue circuit is a function of the level of noise. The greater the noise level, the more the analogue signal is disturbed, slowly becoming less usable. Because of this, analogue signals are said to "fail gracefully". Analogue signals can still contain intelligible information with very high levels of noise. Digital circuits, on

6132-402: The same function but in an implantable device with batteries that last on the order of years. Additionally, there are available various Arduino kits with ECG sensor modules and smartwatch devices that are capable of recording an ECG signal as well, such as with the 4th generation Apple Watch , Samsung Galaxy Watch 4 and newer devices. Electrodes are the actual conductive pads attached to

6216-427: The same time period as they are traced in sequence through time. Each of the 12 ECG leads records the electrical activity of the heart from a different angle, and therefore align with different anatomical areas of the heart. Two leads that look at neighboring anatomical areas are said to be contiguous . In addition, any two precordial leads next to one another are considered to be contiguous. For example, though V 4

6300-414: The scale of the grids are irrelevant). An example to the contrary, the voltage requirements of left ventricular hypertrophy require knowing the grid scale. In a normal heart, the heart rate is the rate at which the sinoatrial node depolarizes since it is the source of depolarization of the heart. Heart rate, like other vital signs such as blood pressure and respiratory rate, change with age. In adults,

6384-485: The signal's information. For example, an aneroid barometer uses the angular position of a needle on top of a contracting and expanding box as the signal to convey the information of changes in atmospheric pressure . Electrical signals may represent information by changing their voltage , current , frequency , or total charge . Information is converted from some other physical form (such as sound , light , temperature , pressure , position) to an electrical signal by

6468-413: The skin to the wire and to the electrocardiogram. The common virtual electrode, known as Wilson's central terminal (V W ), is produced by averaging the measurements from the electrodes RA, LA, and LL to give an average potential of the body: In a 12-lead ECG, all leads except the limb leads are assumed to be unipolar (aVR, aVL, aVF, V 1 , V 2 , V 3 , V 4 , V 5 , and V 6 ). The measurement of

6552-502: The term "unipolar" is not useful. Two types of electrodes in common use are a flat paper-thin sticker and a self-adhesive circular pad. The former are typically used in a single ECG recording while the latter are for continuous recordings as they stick longer. Each electrode consists of an electrically conductive electrolyte gel and a silver/silver chloride conductor. The gel typically contains potassium chloride – sometimes silver chloride as well – to permit electron conduction from

6636-449: The two atria begin to contract ( atrial systole ), and each atrium pumps blood into the ventricle below it. During ventricular systole the ventricles contract and vigorously pulse (or eject) two separated blood supplies from the heart—one to the lungs and one to all other body organs and systems—while the two atria relax ( atrial diastole ). This precise coordination ensures that blood is efficiently collected and circulated throughout

6720-438: The two ventricles. This is the ejection stage of the cardiac cycle; it is depicted (see circular diagram) as the ventricular systole–first phase followed by the ventricular systole–second phase . After ventricular pressures fall below their peak(s) and below those in the trunks of the aorta and pulmonary arteries, the aortic and pulmonary valves close again—see, at the right margin, Wiggers diagram , blue-line tracing. Next

6804-528: The use of ECGs among those without symptoms or at low risk of cardiovascular disease as an effort for prevention. This is because an ECG may falsely indicate the existence of a problem, leading to misdiagnosis , the recommendation of invasive procedures, and overtreatment . However, persons employed in certain critical occupations, such as aircraft pilots, may be required to have an ECG as part of their routine health evaluations. Hypertrophic cardiomyopathy screening may also be considered in adolescents as part of

6888-442: The vectors are perpendicular, and this phenomenon is called isoelectric. Normal rhythm produces four entities – a P wave , a QRS complex , a T wave , and a U wave – that each have a fairly unique pattern. Changes in the structure of the heart and its surroundings (including blood composition) change the patterns of these four entities. The U wave is not typically seen and its absence is generally ignored. Atrial repolarization

6972-412: The ventricular diastole period, including atrial systole, during which blood returning to the heart flows through the atria into the relaxed ventricles. Stages 3 and 4 together—"isovolumic contraction" plus "ejection"—are the ventricular systole period, which is the simultaneous pumping of separate blood supplies from the two ventricles, one to the pulmonary artery and one to the aorta. Notably, near

7056-448: The vital role of completing the diastole, which is to finalize the filling of both ventricles with blood while they are relaxed and expanded for that purpose. Atrial systole overlaps the end of the diastole, occurring in the sub-period known as ventricular diastole–late (see cycle diagram). At this point, the atrial systole applies contraction pressure to 'topping-off' the blood volumes sent to both ventricles; this atrial contraction closes

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