An electrocardiogram (ECG) captures the electrical activity generated by the heart. This non-invasive diagnostic tool translates the heart’s depolarization and repolarization cycles into a visual tracing on paper or a screen. Analyzing this tracing allows for the identification of the heart’s rhythm, providing valuable information about its function. This guide is intended solely for educational purposes and should not be used as a substitute for professional medical diagnosis or interpretation.
Understanding the ECG Grid and Waveforms
Analyzing an ECG tracing requires understanding the specialized grid paper, which quantifies time and voltage using small and large squares. The horizontal axis represents time: each small square is 0.04 seconds, and a large square (five small squares) is 0.20 seconds. The vertical axis represents voltage (amplitude), calibrated so that ten small squares equal one millivolt (mV). These measurements are used to quantify the duration and height of the waves and intervals.
The electrical activity is represented by three primary waveforms: the P wave, the QRS complex, and the T wave. The P wave reflects the depolarization of the atria. The QRS complex represents the rapid depolarization of the ventricles, leading to their contraction. The T wave signifies the repolarization of the ventricles as they relax.
Two intervals are particularly relevant for rhythm determination. The PR interval measures the time from the start of the P wave to the start of the QRS complex. The QRS duration measures the total time for ventricular depolarization. These segments provide insight into the speed of electrical conduction.
The Systematic Five-Step Rhythm Analysis
Determining the heart rhythm requires a standardized, sequential approach, beginning with the rate, which establishes the speed of the electrical activity.
Step 1: Determine the Rate (How Fast)
Heart rate is measured in beats per minute (bpm) using different methods based on regularity. For regular rhythms, the sequence method offers a rapid estimation: locate an R wave on a heavy line and count subsequent heavy lines using the sequence 300, 150, 100, 75, 60, 50. The rate corresponds to where the next R wave falls.
For irregular rhythms, the 6-second method is used. Count the number of QRS complexes within a 6-second segment (30 large squares) and multiply by ten to estimate the rate per minute.
Step 2: Determine the Regularity (Is it Steady)
The next step is to assess the rhythm’s regularity by measuring the distance between consecutive R waves (the R-R interval). R-R intervals are measured using a ruler or calipers. If the distance between all consecutive R waves is the same, the rhythm is regular; if the distances vary, the rhythm is irregular.
Step 3: Analyze the P Waves (Atrial Activity)
Analyzing P waves identifies the origin of the electrical impulse. Focus on whether a P wave is present before every QRS complex and if they have a consistent, uniform appearance. In a normal rhythm originating from the sinus node, P waves should be smooth, rounded, and upright. If P waves are absent, inverted, or have multiple shapes, the impulse likely originates from an area other than the sinus node, known as an ectopic focus.
Step 4: Measure the PR Interval
The PR interval measures the time for the electrical impulse to travel from the atria, through the atrioventricular (AV) node, and into the ventricles. Measurement is taken from the beginning of the P wave to the beginning of the QRS complex. The normal duration is 0.12 to 0.20 seconds (three to five small squares). A consistently shorter or longer PR interval can indicate conduction issues, such as accessory pathways or AV blocks.
Step 5: Measure the QRS Complex
The final measurement determines the duration of the QRS complex, reflecting the time required for ventricular depolarization. This measurement is taken from the start of the Q wave to the end of the S wave. The normal duration is 0.06 to 0.10 seconds (one and a half to two and a half small squares). A QRS duration consistently widened (greater than 0.12 seconds) suggests a delay in ventricular conduction, such as a bundle branch block or an impulse originating in the ventricles.
Practical Application: Recognizing Key Rhythms
Once the five systematic steps are complete, the resulting data points are combined to identify the specific heart rhythm. This synthesis allows for pattern recognition.
Normal Sinus Rhythm and Rate Variations
Normal Sinus Rhythm (NSR) is the baseline rhythm, exhibiting findings of a healthy electrical system. NSR is defined by a regular rate between 60 and 100 bpm, with a P wave preceding every QRS complex, and all intervals (PR and QRS) within normal ranges.
Rate variations are common and are named Sinus Tachycardia (rate faster than 100 bpm) or Sinus Bradycardia (rate slower than 60 bpm). Both adhere to all other NSR characteristics.
Atrial Fibrillation
Atrial Fibrillation (A-Fib) involves a complete breakdown in organized atrial electrical activity. The rhythm is “irregularly irregular” because R-R intervals show no discernible pattern. A distinguishing feature is the absence of discrete P waves. Instead, the baseline may appear chaotic or wavy due to tiny, rapid electrical impulses (fibrillatory waves). The QRS complexes remain narrow because ventricular conduction usually functions normally.
Ventricular Tachycardia
Ventricular Tachycardia (V-Tach) suggests the electrical impulse originates from the lower chambers of the heart. The rate is fast (100 to 250 bpm) and the rhythm is typically regular. The most notable characteristic is the widened appearance of the QRS complexes. These wide complexes result from the impulse bypassing the normal ventricular conduction system. P waves are often absent or dissociated from the QRS complexes, indicating the atria and ventricles are beating independently.