An electrocardiogram (ECG or EKG) is a non-invasive diagnostic tool that records the electrical currents generated by the heart muscle. The process captures the heart’s electrical activity over time, displaying it as a continuous tracing on specialized graph paper or a screen. This visual representation allows clinicians to assess the heart’s rhythm and overall electrical function. Interpreting the tracing begins with understanding the physical layout of the graph, which provides the framework for all subsequent measurements.
Understanding the ECG Graph Grid
The ECG tracing is printed on specialized graph paper marked with a precise grid system. This grid establishes standardized measurements for both time and electrical amplitude. The grid consists of small squares (1 mm by 1 mm) framed by heavier lines into larger 5 mm squares. The horizontal axis measures time, while the vertical axis measures the amplitude, or voltage, of the electrical signal.
At the standard paper speed of 25 mm per second, a single small square horizontally represents 0.04 seconds. A large square, which is five small squares wide, represents a total time duration of 0.20 seconds. Five large squares together equal one second of cardiac activity on the recording strip.
On the vertical axis, the standard calibration is set so that a height of 10 mm, or two large squares, equals 1 millivolt (mV). This means that each small square vertically represents an amplitude of 0.1 mV. Proper calibration is important because an altered amplitude can suggest changes in heart muscle mass or other conditions.
Decoding the Electrical Components
The distinct waves, complexes, and segments on the ECG tracing correspond to the physiological sequence of electrical activation and recovery within the heart muscle. The process begins with the P wave, which is the first positive deflection, representing the depolarization of the atria. This atrial depolarization is the electrical trigger that causes the upper chambers of the heart to contract.
Following the P wave is the PR interval, which measures the time from the start of atrial depolarization to the start of ventricular depolarization. This interval includes the delay as the electrical impulse passes through the atrioventricular (AV) node. A normal PR interval typically spans between 0.12 and 0.20 seconds, or three to five small squares on the grid.
The QRS complex is the most prominent feature, reflecting the rapid depolarization of the ventricles, the heart’s main pumping chambers. This complex is typically composed of a negative Q wave, a positive R wave, and a subsequent negative S wave. The entire QRS complex usually has a narrow duration of less than 0.12 seconds.
The ST segment and the T wave represent the final stages of the electrical cycle. The ST segment is the flat line immediately following the QRS complex, marking the period between the end of ventricular depolarization and the beginning of ventricular repolarization. The T wave represents ventricular repolarization, which is the electrical recovery and resetting of the ventricular muscle cells.
Calculating Heart Rate and Rhythm
Once the grid and the electrical components are understood, the next step is to systematically determine the heart’s rate and rhythm regularity. Heart rate is commonly calculated using the R-R interval, which is the distance between two consecutive R waves on the tracing. For regular rhythms, where R-R intervals are consistent, the 300-method provides a quick estimate.
The 300-Method
This method involves finding an R wave on a heavy grid line and counting the number of large squares until the next R wave. Dividing 300 by this number estimates the heart rate in beats per minute (bpm). For example, if the R-R interval spans four large squares, the heart rate is approximately 75 bpm (300/4).
The 1500-Method and 6-Second Strip
A more precise method for regular rhythms is the 1500-method, where the number of small squares between two R waves is counted and divided into 1500. For irregular rhythms, the 6-second strip technique is used. This requires identifying a 6-second segment (30 large squares), counting the QRS complexes, and multiplying that number by 10 to obtain the average heart rate per minute.
Rhythm determination is assessed by checking the consistency of the R-R intervals and the relationship between the P waves and the QRS complexes. If the distance between each R wave is nearly identical, the rhythm is considered regular. The presence of a P wave before every QRS complex, with a consistent PR interval, confirms normal conduction.