An electrocardiogram (ECG or EKG) records the heart’s electrical activity. It provides a visual representation of the heart’s rhythm and electrical impulses, offering insights into its rate and regularity.
Understanding the ECG Grid
ECG recordings are printed on standardized grid paper, featuring small and large squares. This grid provides a consistent framework for measuring the heart’s electrical activity. It has two main axes: a horizontal axis for time and a vertical axis for voltage or amplitude. The paper usually moves at a standard speed of 25 millimeters per second (mm/s). Each large square on the grid measures 5 millimeters by 5 millimeters.
Horizontal Axis: Time Measurements
The horizontal axis of the ECG grid measures time. Each small square, 1 millimeter in width, represents 0.04 seconds (40 milliseconds). Five small squares make up one large square, representing 0.20 seconds (200 milliseconds) horizontally. These time measurements are crucial for evaluating heart rate, rhythm regularity, and the duration of specific electrical events. They help in assessing intervals like the PR interval and the QRS duration, which indicate electrical impulse conduction.
Vertical Axis: Voltage Measurements
The vertical axis of the ECG grid measures the voltage or amplitude of the heart’s electrical signals. Each small square (1 millimeter high) represents 0.1 millivolts (mV). Five small squares vertically form one large square, corresponding to 0.5 mV, while ten small squares (two large squares) represent 1 mV. These voltage measurements are important for assessing the height or depth of electrical waves like the P wave, QRS complex, and T wave. Their amplitude indicates the strength of electrical activity and can suggest conditions like chamber enlargement or other cardiac issues.
Using ECG Grid Measurements
Understanding the time and voltage measurements on the ECG grid is foundational for basic interpretation. Heart rate can be calculated using the large squares between consecutive R waves (the tall spikes on the ECG tracing). The “300 method” divides 300 by the number of large squares between two R waves. The “1500 method” divides 1500 by the number of small squares between R waves, offering a more precise calculation for faster heart rates. Amplitude measurements are used to assess the size of cardiac waves, helping to identify deviations from normal electrical activity. Detailed diagnostic interpretation requires professional medical expertise.