Electrocardiography

Electrocardiography is the process of producing an electrocardiogram, 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. Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including:

Cardiac rhythm disturbances, such as atrial fibrillation and ventricular tachycardia;
Inadequate coronary artery blood flow, such as myocardial ischemia and myocardial infarction;
and electrolyte disturbances, such as hypokalemia.

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 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 and is recorded over a period of time. In this way, the overall magnitude and direction of the heart's electrical depolarization is captured at each moment throughout the cardiac cycle.
There are three main components to an ECG:

The P wave, which represents depolarization of the atria.
The QRS complex, which represents depolarization of the ventricles.
The T wave, which represents repolarization of the ventricles.

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 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, the effects of heart drugs, and the function of implanted pacemakers.

Medical uses

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 the following:

Chest pain or suspected myocardial infarction, such as ST elevated myocardial infarction or non-ST elevated myocardial infarction
Symptoms such as shortness of breath, murmurs, fainting, seizures, funny turns, or arrhythmias including new onset palpitations or monitoring of known cardiac arrhythmias
Medication monitoring and management of overdose
Electrolyte abnormalities, such as hyperkalemia
Perioperative monitoring in which any form of anesthesia is involved. This includes preoperative assessment and intraoperative and postoperative monitoring.
Cardiac stress testing
Computed tomography angiography and magnetic resonance angiography of the heart
Clinical cardiac electrophysiology, in which a catheter is inserted through the femoral vein and can have several electrodes along its length to record the direction of electrical activity from within the heart.

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.

Screening

For adults, evidence does not support 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 a sports physical out of concern for sudden cardiac death.

Electrocardiograph machines

Mechanical cardiographs, developed in the 19th century, recorded heart movements by transmitting heart or chest wall motions to a spring and air chamber system. A writing lever traced these movements onto a smoked rotating cylinder, producing a cardiogram. Their accuracy was limited, as they captured all body movements, introducing errors.
Modern day electrocardiograms are recorded by machines that consist of a set of electrodes connected to a central unit.
In the late 19th century, scientists discovered the heart's electrical activity, leading to the electrocardiograph's development. Willem Einthoven's 1903 string galvanometer enabled precise measurement of these signals, revolutionizing cardiography. He received the 1924 Nobel Prize for this work.
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 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 is a safe and painless procedure. The machines are powered by mains power but they are designed with several safety features including an earthed lead.
Other features include:

Defibrillation protection: any ECG used in healthcare may be attached to a person who requires defibrillation and the ECG needs to protect itself from this source of energy.
Electrostatic discharge is similar to defibrillation discharge and requires voltage protection up to 18,000 volts.
Additionally, circuitry called the right leg driver can be used to reduce common-mode interference.
ECG voltages measured across the body are very small. This low voltage necessitates a low noise circuit, instrumentation amplifiers, and electromagnetic shielding.
Simultaneous lead recordings: earlier designs recorded each lead sequentially, but current models record multiple leads simultaneously.

Most modern ECG machines include automated interpretation algorithms. This analysis calculates features such as the PR interval, QT interval, corrected QT 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, and can result in clinical mismanagement.

Cardiac monitors

Besides the standard electrocardiograph machine, there are other devices that can record 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, TZ Medical, Philips and BardyDx among many others.
Implantable devices such as 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.
The development of the Holter monitor led to the creation of the implantable loop recorder, which performs the same function but is an implantable device with batteries that last for 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 and leads

Electrodes are the actual conductive pads attached to 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, which is the average of numerous leads. All clinical ECGs use Wilson's central terminal as the virtual electrode from which the precordial leads are measured, whose potential is defined as the average potential measured by the three standard limb leads.
Commonly, 10 electrodes attached to the body are used to form 12 ECG leads, with each lead measuring a specific electrical potential difference.

12-lead ECG

Leads are broken down into three types: limb; augmented limb; and precordial or chest. A 12-lead ECG has a total of three limb leads and three augmented limb leads arranged like spokes of a wheel in the coronal plane, and six precordial leads or chest leads that lie on the perpendicular transverse plane.
Electrodes should be placed in standard positions, with 'left' or 'right' referring to anatomical directions, being the patient's left or right. Exceptions due to emergency or other issues should be recorded to avoid erroneous analysis.
The 12 standard ECG leads and electrodes are listed below. All leads are effectively bipolar, with one positive and one negative electrode; the term "unipolar" is not true, nor useful.

Type	Name	Color	Placement	Name	Color
Limb	RA	White	On the right arm, below the shoulders, avoiding thick muscle.	R	Red
Limb	LA	Black	Symmetrical to the placement of the RA.	L	Yellow
Limb	RL	Green	On the right leg, below the hips.	N	Black
Limb	LL	Red	Symmetrical to the placement of the RL.	F	Green
Precordial	V1	Brown & red	Fourth intercostal space on the right sternal border.	C1	White & red
Precordial	V2	Brown & yellow	Fourth intercostal space on the left sternal border.	C2	White & yellow
Precordial	V3	Brown & green	Halfway between electrodes V2 and V4.	C3	White & green
Precordial	V4	Brown & blue	Fifth intercostal space on the midclavicular line.	C4	White & brown
Precordial	V5	Brown & orange	Left anterior axillary line on the same horizontal plane as V4. If the anterior axillary line is ambiguous, place halfway between V4 and V6.	C5	White & black
Precordial	V6	Brown & purple	Left midaxillary line on the same horizontal plane as V4.	C6	White & purple

Type	Name	Lead view
Limb	I	From the RA to the LA. Along the frontal and horizontal planes at 0°.
Limb	II	From the RA to the LL. Along the frontal plane at 60° clockwise from I.
Limb	III	From the LA to the LL. Along the frontal plane at 120° clockwise from I.
Augmented limb	aVL	From WCT to the LA. Along the frontal plane at -30°.
Augmented limb	aVR	From WCT to the RA. Along the frontal plane at -150°.
Augmented limb	aVF	From WCT to the LL. Along the front plane at 90°.
Precordial	V₁	In the fourth intercostal space just to the right of the sternum
Precordial	V₂	In the fourth intercostal space just to the left of the sternum.
Precordial	V₃	Between leads V₂ and V₄.
Precordial	V₄	In the fifth intercostal space in the mid-clavicular line.
Precordial	V₅	Along the same horizontal line as V₄, in the left anterior axillary line.
Precordial	V₆	Along the same horizontal line as V₄ and V₅ in the mid-axillary line.

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 the skin to the wire and to the electrocardiogram.