A 12-lead electrocardiogram (ECG), also known as an EKG, is a non-invasive diagnostic tool that records the heart’s electrical signals, displayed as a tracing. This test helps healthcare professionals assess heart health by identifying irregular heartbeats or other concerning electrical patterns.
The Electrical Language of the Heart
The heart operates through a sophisticated electrical system, where specialized cells generate and transmit electrical impulses. This process involves two main phases: depolarization and repolarization. Depolarization occurs when heart cells become electrically active, leading to muscle contraction as positively charged ions, primarily sodium and calcium, rush into the cells. Repolarization follows, as the cells return to their resting, negatively charged state, primarily through the outflow of potassium ions, allowing the heart muscle to relax and refill with blood.
An ECG machine captures these electrical changes using “leads,” which are different perspectives of the heart’s electrical activity. While called a “12-lead” ECG, it uses 10 physical electrodes—four on the limbs and six across the chest. These electrodes act like cameras, recording signals from various angles to create 12 distinct graphical representations. Each lead offers a unique spatial view.
Decoding the ECG Tracing
An ECG tracing is typically printed on specialized grid paper, which is crucial for accurate interpretation. The paper is marked with small and large squares; each small square measures 1 millimeter (mm) and represents 0.04 seconds horizontally and 0.1 millivolts (mV) vertically. A larger square consists of five small squares, representing 0.20 seconds horizontally and 0.5 mV vertically. The standard paper speed for an ECG is 25 mm per second.
Specific deflections, or “waves,” on the ECG tracing correspond to distinct electrical events within the heart. The P wave, a small positive deflection, signifies atrial depolarization and contraction. Following the P wave is the QRS complex, usually the most prominent part of the tracing, which represents the rapid depolarization of the ventricles and their subsequent contraction. The T wave, another deflection, indicates ventricular repolarization, the electrical recovery of the ventricles as they relax.
Beyond these waves, intervals on the ECG tracing measure the time electrical impulses travel. The PR interval measures the time from atrial depolarization (P wave) to ventricular depolarization (QRS complex), reflecting conduction through the atrioventricular (AV) node. The QT interval encompasses both ventricular depolarization and repolarization, measured from the beginning of the QRS complex to the end of the T wave.
A Step-by-Step Guide to Basic Interpretation
Interpreting an ECG involves a systematic approach, beginning with assessing the heart rate. For a regular rhythm, you can estimate the heart rate by dividing 300 by the number of large squares between two consecutive R waves. For instance, if there are four large squares between R waves, the rate is 75 beats per minute (bpm). For irregular rhythms, counting the number of R waves in a 6-second strip and multiplying by 10 provides an estimate of the heart rate.
Next, determine the heart’s rhythm by examining the consistency of the R-R intervals, which are the distances between consecutive R waves. If these intervals are consistent, the rhythm is regular; otherwise, it is irregular. Visually “marching out” the R waves by marking them on a piece of paper and sliding it along the tracing can help confirm regularity.
Observe the presence and appearance of the key waveforms. Confirm that a P wave precedes every QRS complex and that each QRS complex is followed by a T wave. The shape and direction of these waves can offer initial insights, though detailed analysis requires further training.
Roughly estimate the duration of the PR and QT intervals. A normal PR interval typically falls between 0.12 and 0.20 seconds (three to five small squares). The QT interval varies with heart rate, but a normal range is generally between 0.36 and 0.44 seconds, though this often requires correction for heart rate for precise evaluation.
What Common Variations Might Indicate
Observing common variations on an ECG can provide clues about heart function, though only a healthcare professional can make a diagnosis. A heart rate below 60 bpm is called bradycardia, while a rate above 100 bpm is tachycardia. These variations can be normal for some individuals, like athletes with bradycardia, or they might suggest an underlying issue that requires medical attention.
An irregularly irregular rhythm, where R-R intervals show no discernible pattern, can be a sign of conditions such as atrial fibrillation. In atrial fibrillation, the atria quiver chaotically instead of contracting effectively, often resulting in the absence of distinct P waves.
Changes in the ST segment, the flat line between the QRS complex and the T wave, can also be significant. An elevated ST segment, for instance, can be a potential indicator of a heart attack. Any findings from an ECG tracing must always be interpreted and diagnosed by a qualified medical professional.