An electrocardiogram, commonly known as an ECG or EKG, is a non-invasive medical test that records the heart’s electrical activity. It detects the tiny electrical signals produced by the heart as it beats, providing a visual representation over time. This diagnostic tool offers healthcare professionals a quick snapshot of heart health, allowing for assessment of its rhythm and overall function. The ECG serves as a fundamental step in evaluating various cardiac conditions.
The Heart’s Natural Pacemaker
The heart’s electrical activity originates within a specialized network of muscle cells known as the cardiac conduction system. This system initiates and distributes electrical impulses, ensuring coordinated heartbeats. The process begins in the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the upper part of the right atrium.
The SA node spontaneously generates electrical signals. These signals spread rapidly across both atria, causing them to contract and pump blood into the ventricles. The electrical impulse then travels to the atrioventricular (AV) node, situated between the atria and ventricles. Here, the signal briefly delays, allowing the atria to fully empty their blood into the ventricles before the next contraction.
From the AV node, the impulse moves into the bundle of His, which branches into the left and right bundle branches. These branches extend down the interventricular septum, the wall separating the ventricles. Finally, the electrical signal reaches the Purkinje fibers, a network of specialized cells that rapidly distribute the impulse throughout the ventricular muscle walls. This rapid transmission causes the ventricles, the heart’s main pumping chambers, to contract, pushing blood out to the lungs and the rest of the body.
Decoding the Waves and Intervals
The electrical journey through the heart is graphically represented on an ECG as a series of distinct waves and intervals, each corresponding to specific cardiac events. The first visible deflection is the P wave, a small, rounded wave that signifies atrial depolarization. A normal P wave lasts up to 0.11 seconds and has an amplitude less than 2.5 mm.
Following the P wave is the QRS complex, a larger and sharper deflection representing ventricular depolarization. Its greater amplitude reflects the larger muscle mass of the ventricles compared to the atria. The QRS complex has a duration ranging from 0.08 to 0.10 seconds in adults. While all three (Q, R, S) components are named, not every QRS complex will display all three waves, depending on the specific lead being viewed.
The T wave appears after the QRS complex and represents ventricular repolarization. This repolarization allows the ventricular muscle cells to prepare for the next electrical impulse and subsequent contraction. Atrial repolarization also occurs but is obscured by the larger QRS complex on the ECG tracing.
Beyond individual waves, the ECG also displays important time segments called intervals. The PR interval, measured from the beginning of the P wave to the start of the QRS complex, reflects the time it takes for the electrical signal to travel from the atria through the AV node to the ventricles. A normal PR interval ranges between 0.12 and 0.20 seconds. The QT interval, measured from the beginning of the QRS complex to the end of the T wave, represents the total time for ventricular depolarization and repolarization. Its duration varies with heart rate, so a “corrected QT” (QTc) is calculated to standardize this measurement.
How the Signal is Captured
An electrocardiogram test is a straightforward, non-invasive procedure designed to capture the heart’s electrical signals from the body’s surface. During the test, a healthcare professional places small, sticky patches called electrodes onto specific locations on the patient’s chest, arms, and legs. These electrodes act as sensors, detecting the faint electrical impulses generated by the heart and transmitting them through lead wires to an ECG machine.
These electrodes only record electrical activity and do not send any electricity into the body. The machine then processes these signals and displays them as a waveform on a screen or prints them on paper. A standard ECG uses 10 electrodes to derive 12 different “leads” or electrical perspectives of the heart. These multiple viewpoints provide a comprehensive picture of the heart’s electrical activity, aiding in cardiac function assessment. The entire process takes about 10 to 15 minutes to complete.
Interpreting Signal Variations
Healthcare professionals analyze the ECG signal by looking at several characteristics to gain insights into heart function. One aspect is the heart’s rate, which refers to how fast the heart is beating, measured in beats per minute (bpm). A normal resting heart rate falls between 60 and 100 bpm; rates below this range are termed bradycardia, while those above are called tachycardia.
The heart’s rhythm is another significant factor, assessed by examining the regularity of the R-R intervals (the distance between consecutive QRS complexes). A regular rhythm indicates consistent timing between heartbeats, while an irregular rhythm, or arrhythmia, suggests variations in this timing. Identifying whether the rhythm is regularly irregular or irregularly irregular helps pinpoint the source of the electrical issue.
Beyond rate and rhythm, the morphology (shape and size) of the P wave, QRS complex, and T wave offers further diagnostic clues. Deviations from normal wave shapes or durations can indicate underlying cardiac conditions. For instance, changes in the QRS complex might suggest issues with ventricular conduction, while alterations in the T wave could point to problems with ventricular repolarization or muscle damage. Analyzing these characteristics allows medical experts to understand how electrical impulses spread through the heart and identify potential abnormalities.