An electrocardiogram (ECG or EKG) is a non-invasive test that measures the electrical activity of the heart. This activity is recorded as a line tracing on paper or a screen, providing a visual representation of the heart’s rhythm and function. The electrical signals detected by electrodes placed on the body allow healthcare professionals to diagnose various conditions, including heart damage, reduced blood flow, and disturbances in the heart’s rhythm. This guide provides a foundational approach to reading the cardiac strip by examining its structure, characteristic waveforms, heart rate, and time intervals.
The Foundation of the Cardiac Strip
The cardiac strip is printed on specialized grid paper that establishes standardized measurements for both time and voltage. The paper is marked with a grid consisting of small squares, each measuring 1 millimeter by 1 millimeter. These small squares are grouped into larger squares, with each large square encompassing five small squares vertically and five small squares horizontally.
The horizontal axis of the paper measures time, which is standardized so that the paper moves at a speed of 25 millimeters per second. At this speed, one small square horizontally represents 0.04 seconds, meaning that a single large square represents 0.20 seconds.
The vertical axis of the grid measures the amplitude, or voltage, of the electrical signal. By convention, 10 millimeters of vertical deflection (equivalent to two large squares) represents 1 millivolt of electrical potential. Therefore, each small square vertically represents 0.1 millivolts.
Anatomy of the Normal Waveform
The tracing of a single heartbeat is composed of three primary electrical components: the P wave, the QRS complex, and the T wave. These waves correspond to the coordinated depolarization and repolarization of the heart’s chambers. The P wave represents the electrical activation, or depolarization, of the atria as the signal spreads across the upper chambers.
Following the P wave is the QRS complex, which signifies the rapid depolarization of the ventricles. Because the ventricular muscle mass is larger than the atria, the QRS complex appears as a taller, more prominent deflection. Atrial repolarization occurs during this time but is usually concealed within the larger QRS complex.
The T wave represents the electrical recovery, or repolarization, of the ventricles, allowing them to relax and prepare for the next beat. The entire sequence—P wave, QRS complex, and T wave—represents one complete cardiac cycle.
Calculating Heart Rate and Assessing Regularity
A systematic reading of the cardiac strip begins by determining the heart rate and assessing the consistency of the rhythm. Regularity is determined by measuring the distance between consecutive R waves (the tall, upright spikes of the QRS complex). If the distance between each R-R interval is identical or nearly constant, the rhythm is considered regular.
For regular rhythms, the most straightforward way to estimate the rate is by using the 300 method. This involves locating an R wave on a thick line and counting the number of large squares to the next R wave. The heart rate is estimated by dividing 300 by that number (e.g., 300, 150, 100, 75, 60, and 50 beats per minute (bpm) for one through six large squares). A normal resting heart rate typically falls between 60 and 100 bpm.
When the rhythm is irregular, the R-R intervals vary, making the 300 method inaccurate. In these cases, the 6-second method is used: the interpreter counts the number of QRS complexes within a 6-second strip (30 large boxes) and multiplies that number by ten to get the estimated average heart rate.
Measuring and Interpreting Time Intervals
Measuring the specific time intervals between the waveforms provides details about the heart’s electrical conduction system. The PR interval is the first measurement, representing the time from the start of the P wave to the beginning of the QRS complex. This interval reflects the time it takes for the electrical impulse to travel from the atria through the atrioventricular (AV) node to the ventricles.
A normal PR interval measures between 0.12 and 0.20 seconds, which corresponds to three to five small squares on the grid paper. If the PR interval is consistently longer than 0.20 seconds, it suggests a delay in conduction, often referred to as a first-degree AV block. Conversely, a shorter interval may indicate an accessory pathway that bypasses the normal conduction delay in the AV node.
The QRS duration is the next measurement, indicating the total time required for ventricular depolarization. This measurement is taken from the beginning to the end of the QRS complex, and a normal duration is less than 0.12 seconds (under three small squares). A QRS duration of 0.12 seconds or more suggests the electrical impulse is traveling slowly through the ventricles, which can be a sign of a conduction defect.
The QT interval represents the total time for both ventricular depolarization and repolarization, measured from the start of the QRS complex to the end of the T wave. Because this interval changes with heart rate, it is often “corrected” (QTc) to provide a standardized value. A prolonged QTc (greater than 450 milliseconds in men and 460 milliseconds in women) increases the risk of serious ventricular arrhythmias.