An electrocardiogram (ECG or EKG) is a non-invasive test that records the electrical activity of the heart, presenting it as a visual tracing on specialized graph paper. This tracing provides a graphical representation of the heart’s electrical impulses, which coordinate the pumping action of the atria and ventricles. Learning to read an ECG involves understanding the standardized grid, recognizing the characteristic electrical waves, and accurately measuring the time and voltage components. This approach offers a fundamental understanding of how the heart’s electrical system is functioning.
Understanding the ECG Grid and Lead Placement
The visual language of the ECG is built upon a standardized grid paper. The horizontal axis of this grid measures time, while the vertical axis measures the amplitude, or voltage, of the electrical signal.
The grid is marked with small squares, each 1 millimeter (mm) by 1 mm. When the paper speed is set to the standard 25 mm per second, each small square horizontally represents 0.04 seconds. Five small squares form a larger, bolder square, representing 0.20 seconds. Vertically, one small square represents 0.1 millivolts (mV). Standard calibration shows a vertical deflection of 10 small squares (two large squares), corresponding to 1.0 mV.
The lines that form the tracing are captured through electrodes placed on the patient’s skin, providing different perspectives on the heart’s electrical flow. These perspectives are called leads. A standard ECG typically utilizes 12 leads, each viewing the heart’s electrical activity from a unique angle. The concept of leads means the same electrical event is recorded differently depending on the placement of the electrodes.
Identifying the Core Electrical Events
A normal cardiac cycle produces a sequence of characteristic waveforms labeled alphabetically: the P wave, the QRS complex, and the T wave. The P wave is the first deflection and represents atrial depolarization, the electrical activation that precedes the contraction of the heart’s upper chambers. Since the atria are relatively small muscle masses, the P wave is usually a small, smooth, and positive (upright) deflection in common monitoring leads.
The QRS complex follows the P wave and is a rapid, high-amplitude sequence representing ventricular depolarization. This electrical signal triggers the contraction of the lower chambers. The QRS is complex because it reflects the swift, simultaneous activation of both ventricles, and its morphology varies between leads. Atrial repolarization occurs during this time but is masked by the stronger ventricular signal.
The final major waveform is the T wave, which represents ventricular repolarization, the electrical recovery of the ventricles before the next beat. A normal T wave is typically broad, slightly asymmetrical, and an upright deflection in most leads. The flat period between the end of the P wave and the start of the QRS complex is the PR segment, reflecting the brief delay as the impulse passes through the atrioventricular (AV) node.
Determining Heart Rate and Rhythm
A systematic reading of the ECG requires differentiating between the heart rate and the heart rhythm. Heart rate is the speed of the heart, measured in beats per minute (BPM), while rhythm refers to the regularity and pattern of the electrical impulses. The rate is calculated by measuring the time interval between successive QRS complexes, specifically the distance between the peaks of the R waves, known as the R-R interval.
For a regular rhythm, where the R-R interval is constant, the “300 method” provides a quick rate estimate. This method uses the fact that there are 300 large squares in one minute of tracing at standard paper speed. To apply this, count the number of large squares between two consecutive R waves and divide 300 by that number. For instance, if R waves are separated by three large squares, the rate is 100 BPM (300/3). Rhythm is assessed by measuring the R-R interval across the strip; if the distance is consistent, the rhythm is regular.
If the rhythm is irregular, the 300 method is inaccurate. An alternative method involves counting the number of R waves in a six-second strip and multiplying that number by ten. A six-second strip is identified as a span of 30 large squares (30 large squares x 0.20 seconds = 6 seconds).
Analyzing Key Intervals and Segments
The time intervals and segments connecting the waveforms reveal the speed of electrical conduction through different parts of the heart. The PR interval is measured from the beginning of the P wave to the start of the QRS complex. This measurement represents the time it takes for the electrical impulse to travel from the atria, through the AV node, and into the ventricles.
The normal PR interval measures between 0.12 and 0.20 seconds (three to five small squares). If the interval is too long, it suggests a conduction delay, often at the AV node. Conversely, a short PR interval indicates the impulse bypassed the normal conduction delay, causing premature ventricular activation.
The QRS duration measures the time required for ventricular depolarization, taken from the beginning to the end of the QRS complex. A normal QRS duration is less than 0.12 seconds, or fewer than three small squares. A prolonged QRS duration suggests a problem with the electrical signal spreading through the ventricles, such as a block in one of the bundle branches.
The ST segment is the flat line that follows the QRS complex and precedes the T wave. It represents the period when the ventricles are fully depolarized before they begin to repolarize. The ST segment should be isoelectric, meaning it lies on the same baseline as the PR segment. Deviations of this segment, such as elevation or depression, are significant findings that point toward conditions like myocardial injury or ischemia.