An electrocardiogram (ECG) is a non-invasive test that records the heart’s electrical activity, producing a graph that reflects the cardiac cycle. An arrhythmia is an irregular heartbeat where the heart may beat too fast, too slow, or with an erratic pattern. The ECG is the primary tool used to identify and classify these irregularities by allowing a direct analysis of the heart’s electrical function and waveforms.
Decoding the Normal Heartbeat on an ECG
To identify an abnormal heartbeat, one must first understand the components of a normal one. An ECG tracing for a single beat consists of three main parts: the P wave, the QRS complex, and the T wave. Each waveform corresponds to a specific electrical event within the cardiac cycle.
The first deflection, the P wave, represents the depolarization of the atria, the heart’s two upper chambers. This electrical signal triggers the atria to contract, pushing blood into the ventricles. In a healthy heart, each P wave is followed by a QRS complex, indicating the atrial impulse was successfully transmitted.
Following the P wave is the QRS complex, the most prominent feature on the ECG. This larger set of waves signifies the depolarization of the ventricles, the lower chambers of the heart. The contraction of the ventricles is responsible for pumping blood to the lungs and the rest of the body.
The final wave is the T wave, which represents ventricular repolarization. This is the electrical “reset” of the ventricles as they prepare for the next heartbeat.
Classification by Heart Rate and Rhythm
The most direct way to classify arrhythmias is by analyzing the heart’s rate and rhythm. An arrhythmia can be categorized as a tachycardia, a resting heart rate faster than 100 beats per minute, or a bradycardia, a resting heart rate slower than 60 beats per minute.
Beyond speed, the regularity of the heartbeat is a defining characteristic. A normal rhythm is consistent, and on an ECG, this is assessed by measuring the R-R interval, the time between the peaks of two consecutive QRS complexes. Consistent intervals indicate a regular rhythm, while varying intervals mean the rhythm is irregular.
An example of an arrhythmia classified by rate is sinus tachycardia, where the heart rate exceeds 100 beats per minute but the ECG waveform remains normal. Conversely, sinus bradycardia is characterized by a heart rate below 60 beats per minute with an otherwise normal ECG pattern. In both conditions, the rhythm is regular, but the rate is outside the normal range.
Classification by Origin in the Heart
Arrhythmias can also be classified by where the abnormal electrical signals originate. They are divided into two groups: supraventricular arrhythmias, which start above the ventricles, and ventricular arrhythmias, which begin within the ventricles. This distinction is relevant because the origin often correlates with the arrhythmia’s potential severity.
Supraventricular arrhythmias originate in the atria or the atrioventricular (AV) node, the electrical bridge between the chambers. Atrial fibrillation (AFib) is a common example, where chaotic signals cause the atria to quiver instead of contracting effectively. On an ECG, AFib is identified by the absence of clear P waves and an “irregularly irregular” rhythm with unpredictable R-R intervals.
Ventricular arrhythmias arise from the lower chambers and can be more serious as they can compromise the heart’s ability to pump blood. Ventricular Tachycardia (VTach) is a form of this arrhythmia where the ventricles beat very fast. Its presentation on an ECG is a series of wide and abnormally shaped QRS complexes, which look different from the narrow complexes of a normal rhythm. This wide appearance is due to the inefficient spread of the electrical impulse.
Distinguishing Other Common Irregularities
Some rhythm disturbances are identified as distinct events rather than by sustained rate or origin. These include ectopic beats and conduction blocks, which represent different electrical system malfunctions.
Ectopic beats are extra or premature heartbeats that originate from a focus in the heart muscle. They are classified as Premature Atrial Contractions (PACs) if they start in the atria, or Premature Ventricular Contractions (PVCs) if they start in the ventricles. On an ECG, these appear as single, out-of-place beats that disrupt the regular cadence.
Conduction blocks are problems with the transmission of the electrical signal as it travels from the atria to the ventricles. This can cause a delay or a complete blockage of the impulse, leading to a “dropped” beat where a P wave appears on the ECG without a subsequent QRS complex.