An electrocardiogram (EKG or ECG) is a vital tool that records the electrical activity of the heart. Before diving into interpreting heart rhythms and patterns, it’s essential to understand the EKG paper itself. This guide will help you grasp the basics of the EKG grid, including how time and voltage are measured, so you can accurately read and interpret EKG strips.
The EKG Paper Grid
The EKG paper features a grid of horizontal and vertical lines forming small and large squares. This grid helps measure time on the horizontal axis and voltage on the vertical axis.
The grid is composed of large squares with a bold outline (measuring 5 mm by 5 mm) and each large square is made up of 1mm by 1mm small squares with a lighter outline.
Time Measurement on the Horizontal Axis
The horizontal axis represents time, allowing you to determine the duration of electrical events in the heart.
• Each small square equals 0.04 seconds (40 milliseconds).
• Each large square (5 small squares) equals 0.2 seconds.
• Five large squares equal 1 second of elapsed time.
Example: Counting 10 large squares horizontally would represent 2 seconds of heart activity.
Voltage Measurement on the Vertical Axis
The vertical axis measures voltage, indicating the electrical potential generated by the heart’s activity.
• Each small square equals 0.1 millivolts (mV).
• Each large square equals 0.5 mV.
• Two large squares vertically represent 1 mV of electrical activity.
Calibration and Standard Settings
Before interpreting an EKG, always check the calibration settings, usually indicated at the beginning or end of the recording.
• Calibration Marker: Often, a rectangle measuring 1 cm in height and 5 mm in width is printed at the start of the EKG strip to confirm standard settings. This indicates that the EKG is done with a standard calibration- 1 cm height indicates that on the EKG, a 10mm (or 1cm) deflection reflects a 1 millivolt amplitude. On the other hand, a 5 mm width indicates that the EKG is printed at 25 mm/ second (so, the 25 cm standard EKG reflects the electrical activity of the heart over 10 seconds).
Note: Calibration ensures that time and voltage measurements are accurate. Settings can be adjusted, so always verify calibration to avoid misinterpretation.
Length of a Standard EKG Strip
A typical EKG strip represents 10 seconds of heart activity.
• Length of a standard EKG: Approximately 50 large squares or 250 small squares horizontally.
This length provides enough data to assess the heart’s rhythm and detect many abnormalities.
Conclusion
Understanding the EKG grid is the first step in analyzing the heart’s electrical activity. By knowing how to measure time and voltage using the grid’s squares, you can begin to interpret the rhythms and patterns displayed on an EKG. Stay tuned for our next post, where we’ll explore the different waves, segments, and intervals that make up the EKG tracing.
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