10.6: Electrocardiogram (ECG)

An electrocardiogram, or ECG, is a recording of the heart’s electrical activity as a graph over a period of time.

Master this section and you'll be able to:

• Describe how ECG electrodes detect the heart’s electrical activity and why a 12-lead ECG is better than a 3 or 5 lead ECG. 
• Identify the main ECG waves, segments, and intervals and relate them to specific cardiac events.
• Interpret normal and abnormal ECG patterns to recognize common arrhythmias.

From Electrodes to Insights: Understanding the ECG

Electrical signals produced by the heart can be captured through the strategic positioning of electrodes on specific areas of the body. This process of tracking the heart's electrical activity is referred to as an electrocardiogram (ECG), also sometimes abbreviated as EKG (with "K" representing "kardiology," the German term for cardiology). Unlike recording signals from individual muscle fibers, a standard ECG showcases the overall fluctuations in electrical current flow, offering a comprehensive view of heart function. 

The ECG graph can show the heart’s rate and rhythm. It can also detect enlargement of the heart, or the presence of current or past heart attacks, making it a key tool for diagnosing cardiac conditions.

The ECG works by detecting and amplifying tiny electrical changes on the skin that occur during heart muscle depolarization. The output for the ECG forms a graph that shows several different waves, each corresponding to a different electrical and mechanical event within the heart. Changes in these waves are used to identify problems with the different phases of heart activity.

Figure \(\PageIndex{1}\): ECG. Illustration of a patient undergoing a 12-lead ECG.
 

A 12-lead ECG records the electrical activity of the heart from 12 different perspectives, or leads, using 10 electrodes placed on specific parts of the body. Despite the name, there are not 12 wires — just 10 electrodes that capture enough information for the ECG machine to calculate 12 unique electrical views of the heart. An ECG can also just use 3 or 5 leads, instead of 12 leads. The level of detail an ECG provides increases with the number of leads used.

What are "leads"?  In ECG terminology, a lead is not a wire — it is an electrical viewpoint of the heart. Each lead measures the heart’s electrical signals from a particular angle. Together, the 12 leads create a complete picture of how electrical impulses travel through the heart, helping to identify arrhythmias (irregular rhythms), heart muscle damage, or myocardial infarctions (heart attacks).

The figure below shows the standard placement of  the 10 ECG electrodes (4 limb electrodes and 6 chest electrodes) in a 12-lead ECG: six electrodes are placed on the chest, and four electrodes are placed on the limbs. Just imagine the purpose to be taking 12 photographs of the heart at once — from different spots around your body — to see how well it is beating and where any problem might be hiding.

Limb electrodes:

• Right arm (RA)

• Left arm (LA)

• Right leg (RL) – serves mainly as a ground or reference

• Left leg (LL)

Chest (precordial) electrodes:

• V1 – right side of sternum, 4th intercostal space

• V2 – left side of sternum, 4th intercostal space

• V3 – midway between V2 and V4

• V4 – 5th intercostal space, midclavicular line

• V5 – level with V4, anterior axillary line

• V6 – level with V5, midaxillary line

Interpreting the ECG Waves and Intervals

When you look at an ECG, you are essentially seeing the heart’s electrical story drawn out on paper (or a screen). Each small bump or spike represents a coordinated wave of electrical activity that triggers specific parts of the heart to contract. The three main features of a normal ECG tracing are the P wave, the QRS complex, and the T wave.

Figure \(\PageIndex{2}\): Electrocardiogram. A normal tracing shows the P wave, QRS complex, and T wave. Also indicated are the PR, QT, QRS, and ST intervals, plus the P-R and S-T segments.
 

The P Wave

The first feature on an ECG is the P wave, a small, rounded upward deflection that represents atrial depolarization — the electrical signal that triggers atrial contraction (atrial systole). This wave appears first because the heartbeat begins in the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right atrium. From there, the impulse travels across the atria. This contraction pushes blood into the ventricles, completing the filling phase of the cardiac cycle.

Because the atria are relatively small and thin-walled, the P wave is gentle and modest in size compared to the tall spike of the QRS complex. However, this small signal carries important diagnostic value. Changes in the height or shape of the P wave can indicate imbalances in potassium ion concentration, which affect the excitability of cardiac muscle cells.

If there is no distinct P wave, but rather there are chaotic "fibrillatory" waves, it may suggest atrial fibrillation, a condition in which the atria quiver instead of contracting rhythmically. In atrial fibrillation, the ventricles do not fill efficiently during diastole, but the condition is usually not immediately life-threatening on its own.

The QRS Complex

Next comes the QRS complex, a sharp, tall spike that represents ventricular depolarization — the electrical event that triggers ventricular contraction (ventricular systole). It is composed of three parts: the small downward Q wave, the tall upward R wave, and the following downward S wave. Because the ventricles contain much more muscle tissue than the atria, this electrical signal is stronger, faster, and more dramatic — truly the star of the ECG show.

The QRS complex corresponds to the movement of action potentials traveling from the AV node, down the bundle of His, through the right and left bundle branches, and out to the Purkinje fibers that spread the impulse through the ventricular walls. This rapid, coordinated wave of depolarization causes the ventricles to contract and pump blood forcefully into the pulmonary trunk and aorta.

Clinically, the QRS complex provides vital information about ventricular function. A wider, taller, or oddly shaped QRS may signal ventricular hypertrophy, conduction abnormalities, or a myocardial infarction (heart attack), all of which alter how the depolarization wave travels through the heart muscle.

The T Wave

After the excitement of ventricular contraction, the T wave appears — a broad, rounded deflection that represents ventricular repolarization, or the electrical “reset” phase that prepares the ventricles for the next heartbeat. During this time, the ventricles relax and refill with blood, readying themselves for the next cycle of depolarization and contraction.

Interestingly, atrial repolarization is not seen as a distinctive wave, since it is hidden within the much larger QRS complex. The strong electrical signal generated by the ventricles effectively masks the smaller recovery wave from the atria, making it invisible on a standard ECG tracing.

Figure \(\PageIndex{3}\): Animation of a Normal ECG Wave. The red lines represent the movement of the electrical signal through the heart.  

Segments and Intervals

Beyond the main waves, the ECG also includes segments and intervals, which help clinicians understand the timing of cardiac events.

• A segment is the flat line between two waves.

• An interval includes one or more waves plus the connecting segment.

For example:

• The P–R segment extends from the end of the P wave to the beginning of the QRS complex and represents the brief pause as the electrical signal travels through the AV node.

• The P–R interval, which includes the P wave and the PR segment, measures the time between the start of atrial depolarization and the beginning of ventricular depolarization—about 0.16 seconds in a healthy heart.

Another key measure is the Q–T interval, lasting roughly 0.35 seconds, which represents the total time for the ventricles to depolarize and then repolarize — that is, the duration of ventricular contraction and recovery.

Timing the Contractions

Electrical activity always comes before the mechanical contraction of heart muscle. About 25 milliseconds after the P wave begins, the atria start to contract. The ventricles follow suit shortly after, beginning their contraction near the peak of the R wave in the QRS complex.

Clinically, the P–R interval is one of the most important measurements on an ECG. It tells doctors how long it takes for an electrical impulse to travel from the atria to the ventricles. A longer or shorter PR interval can indicate problems with conduction — like a heart block or abnormal rhythm.
 

 

Quick Reference: Major ECG Waves and Intervals and their Functional Significance.

Wave / Interval	What It Represents	What’s Happening in the Heart
P wave	Atrial depolarization	The atria contract, pushing blood into the ventricles.
QRS complex	Ventricular depolarization	The ventricles contract powerfully, sending blood to the lungs and body. Atrial repolarization happens here too but is hidden under the strong ventricular signal.
T wave	Ventricular repolarization	The ventricles reset electrically and prepare for the next beat.
P–R segment	Signal delay at AV node	The impulse pauses briefly to allow the ventricles to fill completely before they contract.
P–R interval	Atrial depolarization → start of ventricular depolarization	Measures how long it takes the signal to travel from the atria to the ventricles (about 0.16 seconds).
Q–T interval	Full cycle of ventricular activity	Time from the start of ventricular contraction to the end of repolarization (about 0.35 seconds).

 

Interactive Element

 

Decoding Disturbances: ECG Patterns of Arrhythmia

As mentioned above, ECGs provide a window into the electrical activity of the heart, allowing clinicians and scientists to identify disturbances in its normal rhythm known as arrhythmias. These disruptions arise when the heart’s conduction system fails to generate or transmit impulses properly, resulting in beats that are too fast, too slow, or irregular. While some arrhythmias cause minimal physiological impact, others — such as ventricular fibrillation — can rapidly lead to circulatory collapse and cardiac arrest. Understanding how to interpret ECG patterns associated with different types of arrhythmias is therefore essential for diagnosing underlying conduction abnormalities and guiding appropriate clinical intervention.

Ventricular Fibrillation

When an ECG shows no identifiable P waves, QRS complexes, or T waves, it indicates ventricular fibrillation (VF) — a severe, life-threatening arrhythmia. During VF, the ventricles quiver rapidly and irregularly instead of contracting in a coordinated way. As a result, the heart cannot pump blood effectively and essentially acts as a mass of disorganized muscle activity.

Ventricular fibrillation leads to sudden cardiac death within minutes unless electrical resuscitation with an automated external defibrillator (AED) is performed immediately. VF often occurs in association with myocardial infarction or heart failure and is thought to result from re-entry of action potentials from the ventricular muscle back into the conduction pathway (such as the AV node), triggering rapid, chaotic impulses that prevent effective pumping.

 

Each tracing in the figure below shows a different type of abnormal rhythm. Some, like partial or complete heart block, interfere with the conduction of signals between the atria and ventricles. Others, such as atrial or ventricular fibrillation, involve chaotic electrical activity that prevents coordinated contraction. By examining how the P waves, QRS complexes, and T waves appear — or disappear — clinicians can identify the type of arrhythmia and predict how it affects the heart rate and the ability of the heart to pump blood effectively.

 

Figure \(\PageIndex{4}\): Electrocardiogram (ECG) Tracings demonstrating Five Distinct Types of Cardiac Arrhythmias. Review the accompanying descriptions and verify that each explanation corresponds appropriately to the pattern displayed.