Tachycardia is defined as an abnormally rapid heart rate, generally considered to be over 100 beats per minute (bpm). An electrocardiogram (ECG) captures the electrical activity of the heart over time, displaying it as a series of waves and complexes. Interpreting the visual patterns on the ECG tracing allows clinicians to identify the specific electrical disturbance causing the rapid heart rate. The appearance of the tracing—specifically the duration of the QRS complex and the presence of organized electrical activity—provides the necessary information to distinguish between various forms of tachycardia.
How to Measure Heart Rate on an ECG
The first step in recognizing tachycardia on an ECG is accurately determining the ventricular rate. A standard ECG paper is marked with a grid where each small box represents 0.04 seconds and each large box represents 0.20 seconds.
For rhythms that are regular, the “300-method” offers a quick way to estimate the rate. This technique involves finding an R wave on a thick line and counting the number of large boxes to the next R wave. By dividing 300 by this number, the heart rate in beats per minute is approximated; for example, if the R waves are separated by three large boxes, the rate is 100 bpm (300/3).
When the heart rhythm is irregular, the R-R intervals vary, making the 300-method unreliable. For these rhythms, the “6-second strip method” provides a more accurate average rate. This method utilizes a 6-second segment of the ECG tracing, which corresponds to 30 large boxes. The number of QRS complexes within this strip is counted and then multiplied by ten to yield the rate in beats per minute.
Narrow Complex Tachycardia Rhythms
Narrow complex tachycardias (NCT) are characterized by a QRS complex duration measuring less than 0.12 seconds. This narrow appearance indicates that the electrical impulse is originating above the ventricles (supraventricular origin) and is traveling down the His-Purkinje system efficiently. The defining features of these rhythms relate to the visibility and relationship of the P wave to the QRS complex.
Sinus Tachycardia is an accelerated normal rhythm where the impulse originates in the sinoatrial node. On the ECG, this is identified by a regular rhythm with a rate over 100 bpm, where every QRS complex is preceded by a normal, visible P wave. The P waves have the same morphology as they do during a normal heart rate, just occurring more frequently.
Supraventricular Tachycardia (SVT) often presents with a faster rate, commonly exceeding 150 bpm in adults. The P waves, which represent atrial activation, are often difficult to discern because they may be hidden within the QRS complex or the T wave due to the rapid rate. This rhythm is generally extremely regular, and the absence of clearly defined P waves before each QRS helps distinguish it from Sinus Tachycardia.
Wide Complex Tachycardia Rhythms
Wide complex tachycardias (WCT) are identified by a QRS complex duration measuring 0.12 seconds or greater. This broadened appearance suggests that the impulse is either originating directly in the ventricles or is traveling through the ventricles via an abnormal, slower pathway. The most significant example is Ventricular Tachycardia (VT), where the electrical impulse originates below the bundle of His.
Ventricular Tachycardia is typically a regular rhythm with a rate often between 150 and 200 bpm. The QRS complexes are wide, bizarre, and often uniform in shape, dominating the tracing because the electrical activity spreads slowly through the ventricular muscle. A primary distinguishing feature of VT is Atrioventricular (AV) Dissociation, meaning the atria and ventricles are beating independently.
On the ECG, AV dissociation is seen as P waves, if visible, having no consistent relationship with the QRS complexes. The P waves are often obscured by the rapid, broad QRS complexes. The presence of “fusion beats” (hybrid QRS complexes) or “capture beats” (isolated normal-looking QRS complexes) are also highly suggestive of VT.
When Organization is Lost: Fibrillation Patterns
The visual appearance of a tachycardia changes dramatically when the electrical activity becomes entirely disorganized, characteristic of fibrillation rhythms. These rhythms represent the chaotic end of the electrical spectrum, where coordinated contraction is replaced by a quivering of the heart muscle. The ECG patterns reflect this loss of organized electrical movement, with standard wave components disappearing or becoming erratic.
In Atrial Fibrillation (AFib), the electrical activity in the upper chambers is rapid and disorganized, leading to an absence of discernible P waves. Instead, the baseline often appears chaotic and irregular, showing small, rapid fibrillatory waves. The resulting QRS complexes are typically narrow but occur at an “irregularly irregular” rate, meaning there is no repeating pattern to the R-R intervals.
Ventricular Fibrillation (VFib) is a severe rhythm where the electrical activity in the ventricles is completely chaotic. On the ECG, VFib is visualized as a rapid, wavy, and erratic line that lacks any recognizable QRS complexes. This absence of organized complexes means the ventricles are merely quivering and not pumping blood effectively, making VFib a fatal rhythm requiring immediate intervention.