An electrocardiogram (ECG or EKG) is a non-invasive diagnostic tool for evaluating heart health. It captures the heart’s electrical activity and translates it into a visual representation, often appearing as a wavy line on paper or a screen. An ECG provides insights into how the heart functions electrically. This article aims to offer a basic understanding of an ECG report.
What an ECG Measures
An ECG measures the electrical signals that govern heart contractions. The heart has an internal electrical conduction system that initiates and coordinates each heartbeat. This system begins with the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right atrium. The SA node generates electrical impulses that spread across the atria, causing them to contract.
These impulses then travel to the atrioventricular (AV) node, situated between the atria and ventricles. The AV node briefly delays the signal, allowing the atria to empty blood into the ventricles before they contract. From the AV node, the impulse moves through specialized pathways, including the bundle of His and Purkinje fibers, which rapidly distribute the signal throughout the ventricles, triggering their contraction.
The ECG machine uses electrodes, typically placed on the chest, arms, and legs, to detect these tiny electrical changes on the skin’s surface. The machine then converts these impulses into a graphical display, showing voltage changes over time. This wavy line tracing shows the heart’s electrical journey with each beat.
Key Components of an ECG Tracing
An ECG tracing has distinct waves, segments, and intervals, each representing a specific electrical event. The P wave is the first small, rounded upward deflection, signifying atrial depolarization (electrical activation of the atria). A normal P wave lasts less than 0.12 seconds and has an amplitude less than 2.5 mm in limb leads.
The QRS complex follows the P wave and represents ventricular depolarization (electrical activation of the ventricles). It comprises a downward Q wave, an upward R wave, and a downward S wave, though not all three may be visible in every ECG lead. A normal QRS complex typically has a duration between 0.08 and 0.10 seconds, not exceeding 0.12 seconds.
The T wave appears after the QRS complex and represents ventricular repolarization (electrical recovery of the ventricles). Normal T waves are generally upright in most leads, rounded, and slightly asymmetrical, with a gradual upslope and a faster downslope. Their height should not exceed 5 mm in limb leads and 10 mm in precordial leads.
Specific intervals and segments provide additional information. The PR interval measures the time from the beginning of the P wave to the start of the QRS complex, representing the electrical impulse’s journey from the atria through the AV node to the ventricles, with a normal duration of 0.12 to 0.20 seconds.
The QT interval spans from the start of the QRS complex to the end of the T wave, reflecting the total time for ventricular depolarization and repolarization. The normal QT interval varies with heart rate but is generally less than or equal to 0.42 seconds. The ST segment is the flat line connecting the end of the QRS complex to the beginning of the T wave, representing the period when the ventricles are fully depolarized before repolarization begins.
The ECG paper is a grid with small squares measuring 1 millimeter (mm) on each side. Each small square horizontally represents 0.04 seconds, and each large square (five small squares) represents 0.20 seconds. Vertically, each small square represents 0.1 millivolts (mV) of voltage. This standardized grid allows for precise measurement of wave durations and amplitudes.
Assessing Heart Rhythm and Rate
Interpreting an ECG requires assessing the heart’s overall rhythm and rate. Heart rate, measured in beats per minute (bpm), can be calculated from an ECG tracing. For a regular rhythm, find an R wave aligned with a thick line. Count the large squares until the next R wave and divide 300 by that number (e.g., 4 squares = 75 bpm). Alternatively, count QRS complexes in a 6-second strip (30 large boxes) and multiply by 10.
A normal resting heart rate for adults is typically 60 to 100 bpm. Athletes or individuals on certain medications may have a lower normal resting heart rate. A heart rate consistently above 100 bpm is tachycardia; below 60 bpm is bradycardia.
Assessing heart rhythm involves observing the regularity of the R-R intervals (spacing between consecutive R waves). A regular rhythm means consistent R-R intervals, indicating an evenly spaced heartbeat. An irregular rhythm shows varying R-R intervals, suggesting an inconsistent beat pattern. “Sinus rhythm” refers to the normal heart rhythm where electrical impulses originate from the SA node and follow the natural conduction pathway. Normal sinus rhythm is characterized by a regular rhythm, a heart rate between 60 and 100 bpm, and a P wave before every QRS complex.
Recognizing Common ECG Variations
While a normal ECG displays a consistent pattern, variations can occur, providing visual cues that may suggest underlying changes in heart function. Tachycardia (a very fast rate) appears as R waves closer together than normal. Bradycardia (a very slow rate) shows R waves spread further apart.
Variations in rhythm are also observable. An irregular rhythm has inconsistent spacing between QRS complexes, where R-R intervals vary significantly. This contrasts with a regular rhythm’s predictable pattern.
Changes in wave and interval appearance can be significant. For example, a missing P wave might indicate the electrical impulse did not originate in the atria as expected, or that atrial activity is not conducted properly. An unusually wide QRS complex (exceeding 0.12 seconds) can suggest inefficient electrical signal travel through the ventricles. An elevated or depressed ST segment (above or below baseline) can point to issues with blood flow to the heart muscle. These deviations can indicate potential cardiac concerns.
When to Consult a Professional
This article offers a foundational understanding of an ECG report. However, accurate interpretation and diagnosis require extensive medical training and expertise. The information provided is for educational purposes and should not be used for self-diagnosis or to replace professional medical advice.
ECG findings must always be considered within the context of a person’s overall health, symptoms, and medical history. Always discuss your ECG results and any related concerns with a qualified healthcare provider. They can provide a comprehensive explanation of your specific report, address questions, and determine appropriate next steps for your health.