An electrocardiogram, commonly known as an ECG or EKG, is a non-invasive medical test that provides valuable information about the heart’s electrical activity. By recording the electrical signals produced by the heart, an ECG can help healthcare professionals evaluate heart rhythm and detect potential cardiac issues.
Understanding How an ECG Works
The heart generates electrical impulses that coordinate its contractions. An ECG machine detects these electrical signals through small, adhesive patches called electrodes, which are placed on specific areas of the skin, typically on the chest, arms, and legs. These electrodes act as sensors, picking up the tiny electrical changes that occur as the heart muscle depolarizes and repolarizes during each heartbeat.
The signals captured by the electrodes are then transmitted through wires to the ECG machine. The machine amplifies and filters these signals, converting them into a visual representation of wavy lines on a graph or digital screen. This tracing provides a snapshot of the heart’s electrical activity, offering insights into its rate, rhythm, and the timing of electrical impulses as they travel through the heart’s chambers.
Key Components of an ECG Tracing
A typical ECG tracing is composed of distinct waves, intervals, and segments, each representing specific electrical events within the heart. The P wave is the first positive deflection, signifying the electrical activation (depolarization) of the atria, the heart’s upper chambers. This atrial depolarization leads to their contraction, pushing blood into the ventricles.
Following the P wave is the QRS complex that represents the rapid electrical activation (depolarization) of the ventricles, the heart’s main pumping chambers. This depolarization triggers the powerful contraction of the ventricles. The T wave indicates the electrical recovery (repolarization) of the ventricles, as they prepare for the next heartbeat.
Beyond these primary waves, intervals and segments provide timing information. The PR interval measures the time it takes for an electrical impulse to travel from the atria through the atrioventricular (AV) node to the ventricles. This interval includes the P wave and the PR segment, which is the flat line representing the brief delay at the AV node. The QT interval encompasses the entire duration of ventricular electrical activity, from depolarization through repolarization. The ST segment, the flat line between the QRS complex and the T wave, represents the period when the ventricles are fully depolarized.
Defining Normal ECG Measurements
Normal ECG readings are characterized by specific numerical ranges and patterns for each component. A normal resting heart rate for adults typically falls between 60 and 100 beats per minute (bpm), indicating a regular rhythm.
The P wave, representing atrial depolarization, usually has a duration of less than 0.12 seconds (120 milliseconds) and an amplitude less than 2.5 millimeters. The PR interval, measuring atrial to ventricular conduction time, typically ranges from 0.12 to 0.20 seconds (120-200 milliseconds).
The QRS complex, indicating ventricular depolarization, should have a duration of 0.08 to 0.10 seconds (80-100 milliseconds). A normal QRS complex is generally narrow, signifying efficient ventricular contraction. The ST segment is typically isoelectric, meaning it lies on the baseline, or has a slight upward concavity. Normal ST segment elevation is generally less than 1 mm in limb leads and less than 2 mm in precordial leads.
The QT interval, reflecting total ventricular electrical activity, varies with heart rate, and its corrected value (QTc) is often used. A normal QTc interval is generally less than 450 milliseconds for men and less than 460 milliseconds for women. These measurements serve as general guidelines, and minor variations can occur in healthy individuals.
Factors Influencing ECG Readings
Several individual characteristics and circumstances can influence ECG readings, even within a healthy range. Age is a significant factor, with children often having higher resting heart rates than adults. As people age, their heart rates may naturally vary.
Sex can also play a role; for example, women typically have slightly longer QTc intervals than men. Athletic conditioning is another important consideration, as highly trained athletes often exhibit lower resting heart rates due to their heart’s increased efficiency.
Body position during the test can affect the electrical signals. Certain medications, such as beta-blockers, can intentionally lower heart rate and alter other ECG parameters. Even temporary factors like anxiety or stress can lead to a transient increase in heart rate. Given these potential variations, a healthcare professional is best equipped to interpret ECG results in the context of an individual’s overall health and unique circumstances.