Electrocardiography

An electrocardiography is a way to look at the heart’s electrical activity by using a special device called an electrocardiograph. This device makes a recording called an electrocardiogram, often known as an ECG or EKG. It shows a line graph of the heart’s electrical changes during each heartbeat. These changes happen because of the heart’s muscles getting ready to contract and then resting afterward.

Use of real time monitoring of the heart in an intensive care unit in a German hospital (2015), the monitoring screen above the patient displaying an electrocardiogram and various values of parameters of the heart like heart rate and blood pressure

To make an ECG, small pads called electrodes are placed on the skin. These electrodes pick up the tiny electrical signals made by the heart. In a common type of ECG called a 12-lead ECG, ten electrodes are placed on the limbs and chest. The device records the heart’s electrical activity from twelve different angles over a short time.

The ECG has three main parts: the P wave, which shows the upper chambers of the heart getting ready to beat; the QRS complex, which shows the lower chambers getting ready to beat; and the T wave, which shows the lower chambers resting after beating. These patterns help doctors learn important information about the heart’s health, like how fast and regular the heartbeats are and if there is any damage to the heart.

Medical uses

Normal 12-lead ECG

An ECG is a test that helps doctors learn about how the heart works by looking at its electrical signals. Doctors use this information together with other heart tests to understand what might be wrong. Some reasons doctors might do an ECG include:

A 12-lead ECG of a 26-year-old male with an incomplete right bundle branch block (RBBB)

When someone has chest pain or might be having a heart attack
If they feel short of breath, have strange heart sounds, faint, shake, or have irregular heartbeats
To check how certain medicines are affecting the heart
To monitor the heart during surgeries or other medical procedures
To prepare for tests that look closely at the heart

ECGs can be quick recordings or continuous monitoring for people who are very sick or having unusual heart problems.

A patient undergoing an ECG

Screening

For most adults without symptoms or risk of heart disease, regular ECGs are not needed because they might show false problems. However, some jobs, like airplane pilots, may require ECGs as part of health checks. Sometimes ECGs are also used in teens who play sports to help keep them safe.

Electrocardiograph machines

Old machines for recording the heart's activity used a spring and air system to track heart movements. These early tools could pick up many body movements, which made them less accurate.

An ECG electrode

Today, machines called electrocardiograms use electrodes connected to a central unit to record the heart’s electrical activity. In the late 1800s, scientists learned about the heart’s electrical signals, which led to better tools for measuring them. A scientist named Willem Einthoven created a device in 1903 that helped make these measurements more exact, and he won a special prize called the Nobel Prize in 1924 for his work.

Early machines used parts that moved to record the heart’s signals on paper. Now, modern machines change these signals into digital information that computers can understand. Many of today’s machines are small and can be moved easily. Some new devices, like those in fitness trackers and smartwatches, are even smaller and can be worn on the wrist. These newer devices often use just two points to get a basic reading of the heart.

Getting an ECG reading is safe and doesn’t hurt. The machines are made to be safe, even if someone needs other medical help like a shock from a defibrillator. They also have parts that reduce unwanted signals and protect the tiny voltages from the heart. Modern machines can look at many readings at once and use computer programs to help understand the results, but a doctor still needs to check the information.

Cardiac monitors

Main article: Cardiac monitoring

Besides the usual ECG machines, there are other tools that can record the heart’s activity. Portable devices have been used since 1962, when the Holter monitor was created. These monitors usually use patches on the skin, but newer ones can stick to the chest without wires. Some devices can even be placed inside the body, like pacemakers, to record heart signals. There are also kits and smartwatch devices, such as the Apple Watch and Samsung Galaxy Watch, that can record ECG signals.

Electrodes and leads

Electrodes are special pads stuck to the body that help doctors see how the heart works. These pads can measure tiny changes in electricity made by the heart.

A 12-lead ECG uses 10 of these electrodes to create 12 different views of the heart’s activity. These views help doctors understand how well the heart is beating. The electrodes are placed in special spots on the chest and arms to get the best pictures of the heart’s electricity.

ECG electrodes and positioning
Type	Name (AHA)	Color (AHA)	Placement	Name (IEC)	Color (IEC)
Limb	RA (Right arm)	White	On the right arm, below the shoulders, avoiding thick muscle.	R (Right)	Red
	LA (Left arm)	Black	Symmetrical to the placement of the RA.	L (Left)	Yellow
	RL (Right leg)	Green	On the right leg, below the hips.	N (Neutral)	Black
	LL (Left leg)	Red	Symmetrical to the placement of the RL.	F (Foot)	Green
Precordial	V1	Brown & red	Fourth intercostal space on the right sternal border.	C1	White & red
	V2	Brown & yellow	Fourth intercostal space on the left sternal border (symmetrical to V1).	C2	White & yellow
	V3	Brown & green	Halfway between electrodes V2 and V4 (in a straight line).	C3	White & green
	V4	Brown & blue	Fifth intercostal space on the midclavicular line.	C4	White & brown
	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
	V6	Brown & purple	Left midaxillary line on the same horizontal plane as V4.	C6	White & purple

ECG leads and views
Type	Name	Lead view
Limb	I	From the RA to the LA. Along the frontal and horizontal planes at 0° (directly to the left).
	II	From the RA to the LL. Along the frontal plane at 60° clockwise from I.
	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° (which is 330° clockwise from I).
	aVR	From WCT to the RA. Along the frontal plane at -150° (which is 210° from I).
	aVF	From WCT to the LL. Along the front plane at 90°.
Precordial	V₁	In the fourth intercostal space (between ribs 4 and 5) just to the right of the sternum (breastbone)
	V₂	In the fourth intercostal space (between ribs 4 and 5) just to the left of the sternum.
	V₃	Between leads V₂ and V₄.
	V₄	In the fifth intercostal space (between ribs 5 and 6) in the mid-clavicular line.
	V₅	Along the same horizontal line as V₄, in the left anterior axillary line.
	V₆	Along the same horizontal line as V₄ and V₅ in the mid-axillary line.

Category	Leads	Activity
Inferior leads	Leads II, III and aVF	Look at electrical activity from the vantage point of the inferior surface (diaphragmatic surface of heart)
Lateral leads	I, aVL, V₅ and V₆	Look at the electrical activity from the vantage point of the lateral wall of left ventricle
Septal leads	V₁ and V₂	Look at electrical activity from the vantage point of the septal surface of the heart (interventricular septum)
Anterior leads	V₃ and V₄	Look at electrical activity from the vantage point of the anterior wall of the right and left ventricles (Sternocostal surface of heart)

Electrophysiology

Main article: Cardiac electrophysiology

The study of how the heart’s electrical system works is called cardiac electrophysiology. Doctors can look at this system closely using a special test. They place a thin wire with a tiny sensor at the end into the heart through a vein. This helps them record the heart’s electrical activity from very close to its pathways.

Common places they check include near the top right part of the heart, across a wall near a heart valve, inside a small tube in the heart, and at the bottom tip of the right ventricle.

Interpretation

The electrical conduction system of the heart helps us understand how an electrocardiogram (ECG) works. A normal heart sends signals in a predictable way, but changes can show if something is unusual. An ECG doesn’t show how the heart pumps blood; for example, pulseless electrical activity shows an ECG that looks like it should pump blood, but it doesn’t. Ventricular fibrillation shows an ECG but can’t pump blood well.

What’s “normal” for an ECG is based on studies of many people. A resting heart rate between 60 and 100 beats per minute is usually normal.

Theory

Reading an ECG is about recognizing patterns. These patterns come from the heart’s electrical activity, which follows certain rules:

When the heart’s electrical activity moves toward a positive electrode, it makes a positive line on the ECG.
When it moves away, it makes a negative line.
The heart’s activity also changes when it resets itself, creating different lines on the ECG.

Normal heart rhythms create four main waves — a P wave, a QRS complex, a T wave, and a U wave — each with their own shape.

Background grid

ECGs are printed on paper with lines. The bottom shows time, and the side shows voltage. Small boxes are 0.1 mV by 0.04 seconds, and bigger boxes are 0.5 mV by 0.20 seconds.

In the United States, ECGs usually print at 25 mm per second, but other places might use 50 mm per second.

Rate and rhythm

The heart’s rate depends on signals from the sinoatrial node. Normal adult heart rates are between 60 and 100 beats per minute. Slower rates are called “bradycardia.”

A normal rhythm is called “normal sinus rhythm,” with a clear pattern of P wave, QRS complex, and T wave. Changes in this pattern can show different heart rhythms.

Some rhythm changes to know:

No P waves with irregular QRS complexes can mean atrial fibrillation.
A “saw tooth” pattern with QRS complexes can mean atrial flutter.
A sine wave pattern can mean ventricular flutter.
No P waves with wide QRS complexes and a fast rate can mean ventricular tachycardia.

Axis

The heart’s electrical axis usually points down and to the left. Changes in this axis can show problems with the heart’s shape or electrical system.

Amplitudes and intervals

All waves and spaces between them on an ECG have normal sizes and shapes. Changes can be important for doctors.

For easy measuring, ECGs are printed on graph paper where each small box is 40 milliseconds of time and 0.1 millivolts of voltage.

Time-frequency analysis in ECG signal processing

In ECG processing, time-frequency analysis helps see how frequency changes over time, especially in irregular heart rhythms.

Common methods

Steps

Step 1: Preprocessing

Remove noise from the signal.
Separate the signal into heartbeat parts.

Step 2: Choose a method to see the signal over time and frequency.

Step 3: Calculate the time-frequency picture.

Step 4: Find important features.

Step 5: Use computer programs to recognize or classify heart events.

Application scenarios

Heart Rate Variability Analysis (HRV):

Helps understand heart rate changes.

Atrial Fibrillation Detection:

Looks at changes in heart activity.

Ventricular Fibrillation Analysis:

Finds unusual high-frequency changes.

Limb leads and electrical conduction through the heart

The way electricity moves through the heart creates the waves we see on an ECG. When the electricity moves toward a positive electrode, the ECG line goes up. When it moves away, the line goes down. The bigger the movement, the bigger the wave on the ECG.

Ischemia and infarction

Problems with blood flow to the heart can change the ECG. Low blood flow might show as small dips or changes in certain waves. Blocked blood flow shows changes over time, like special waves or long dips.

Artifacts

ECGs can be affected by movement or outside electricity, creating wrong signals. Doctors have ways to tell real heart signals from these false ones.

Interpretation

There are rules to help read a normal ECG. Breaking these rules can show problems. Some rules include:

Rule 1: All waves in aVR are negative.

Rule 2: The flat part after the waves starts at the baseline.

Rule 3: A certain time space should be between two waves.

Rule 4: Another time space should not be too long.

Rule 5: Two big waves usually point the same way.

Rule 6: A wave grows in size from one place to another on the ECG.

Rule 7: Two waves are usually upward.

Rule 8: A small wave is usually upward in certain places.

Rule 9: There shouldn’t be a big dip or only a small one.

Rule 12: Is there a special wave?

Rule 13: Is there a certain wave?

Rule 14: Is there a special wave shape?

Rule 15: Are there patterns that show serious heart problems?

Classification	Angle
Normal	−30° to 105°
Left axis deviation	−30° to −90°
Right axis deviation	+105° to +180°
Indeterminate axis	+180° to −90°

Feature	Description	Pathology	Duration
P wave	The P wave represents depolarization of the atria. Atrial depolarization spreads from the SA node towards the AV node, and from the right atrium to the left atrium.	The P wave is typically upright in most leads except for aVR; an unusual P wave axis (inverted in other leads) can indicate an ectopic atrial pacemaker. If the P wave is of unusually long duration, it may represent atrial enlargement. Typically a large right atrium gives a tall, peaked P wave while a large left atrium gives a two-humped bifid P wave.
PR interval	The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. This interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node.	A PR interval shorter than 120 ms suggests that the electrical impulse is bypassing the AV node, as in Wolff-Parkinson-White syndrome. A PR interval consistently longer than 200 ms diagnoses first degree atrioventricular block. The PR segment (the portion of the tracing after the P wave and before the QRS complex) is typically completely flat, but may be depressed in pericarditis.	120 to 200 ms
QRS complex	The QRS complex represents the rapid depolarization of the right and left ventricles. The ventricles have a greater muscle mass proportion compared to the atria, hence the QRS complex usually has a much larger amplitude than the P wave.	If the QRS complex is wide (longer than 120 ms) it suggests disruption of the heart's conduction system, such as in LBBB, RBBB, or ventricular rhythms such as ventricular tachycardia. Metabolic issues such as severe hyperkalemia, or tricyclic antidepressant overdose can also widen the QRS complex. An unusually tall QRS complex may represent left ventricular hypertrophy while a very low-amplitude QRS complex may represent a pericardial effusion or infiltrative myocardial disease.	80 to 100 ms
J-point	The J-point is the point at which the QRS complex finishes and the ST segment begins.	The J-point may be elevated as a normal variant. The appearance of a separate J wave or Osborn wave at the J-point is pathognomonic of hypothermia or hypercalcemia.
ST segment	The ST segment connects the QRS complex and the T wave; it represents the period when the ventricles are depolarized.	It is usually isoelectric, but may be depressed or elevated with myocardial infarction or ischemia. ST depression can also be caused by LVH or digoxin. ST elevation can also be caused by pericarditis, Brugada syndrome, or can be a normal variant (J-point elevation).
T wave	The T wave represents the repolarization of the ventricles. It is generally upright in all leads except aVR and lead V1.	Inverted T waves can be a sign of myocardial ischemia, left ventricular hypertrophy, high intracranial pressure, or metabolic abnormalities. Peaked T waves can be a sign of hyperkalemia or very early myocardial infarction.	160 ms
Corrected QT interval (QTc)	The QT interval is measured from the beginning of the QRS complex to the end of the T wave. Acceptable ranges vary with heart rate, so it must be corrected to the QTc by dividing by the square root of the RR interval.	A prolonged QTc interval is a risk factor for ventricular tachyarrhythmias and sudden death. Long QT can arise as a genetic syndrome, or as a side effect of certain medications. An unusually short QTc can be seen in severe hypercalcemia.
U wave	The U wave is hypothesized to be caused by the repolarization of the interventricular septum. It normally has a low amplitude, and even more often is completely absent.	A very prominent U wave can be a sign of hypokalemia, hypercalcemia or hyperthyroidism.

Method	Advantage	Disadvantage	Example
Short-time Fourier transform	Simple to implement, suitable for analyzing steady or near-steady heart rhythms and easy to perform using Fast Fourier Transform (FFT).	Time and frequency resolution are affected by window length, making it difficult to efficiently capture both short-term and long-term variations simultaneously.	Monitoring short-term heart rate variability.
Wavelet transform	Provides multi-resolution analysis, making it suitable for processing non-stationary signals.	Computationally intensive.	Local feature extraction of P-wave or T-wave and frequency analysis of atrial fibrillation signals.
Hilbert–Huang transform	Suitable for fully non-stationary and nonlinear signals. Provides instantaneous frequency distribution.	Susceptible to mode mixing issues.	Detection of transient heart rate variability.

Diagnosis

Doctors can learn a lot about a person's heart by looking at an electrocardiogram (ECG or EKG). The patterns on the graph can show different heart problems. For example, certain patterns might suggest a fast or irregular heartbeat. Sometimes, more tests are needed to be sure.

ECGs can help find issues like abnormal heart rhythms, problems with how electrical signals move through the heart, and changes caused by things like minerals in the blood. They are useful tools, but they must be used along with other information about the patient to make the right diagnosis.

History

An early commercial ECG device (1911)

In 1872, Alexander Muirhead connected wires to a patient's wrist to record their heartbeat electronically.
In 1887, Augustus Waller created an early ECG machine that used a special tool to project the heartbeat onto photographic film.
In 1901, Willem Einthoven from Leiden, the Netherlands, used a new device called the string galvanometer, which was much better at recording heartbeats.
In 1924, Einthoven won the Nobel Prize in Medicine for his important work on ECG machines.
In 1942, a doctor named Emanuel Goldberger improved some ECG methods, leading to the 12-lead ECG used today.
In the late 1940s, Rune Elmqvist created a new device called the Mingograf that printed ECGs on paper.