Signal-averaged electrocardiogram

Signal-averaged electrocardiogram (SAECG) is a form of electrocardiography (ECG) that involves recording the electrical signals of the heart over a prolonged period and averaging them to analyze microvolt-level changes. This technique enhances the detection of subtle cardiac abnormalities that are not visible on a standard ECG, particularly those related to the heart's electrical conduction system. It is primarily used in the diagnosis and risk stratification of patients at risk for ventricular arrhythmias and sudden cardiac death.

Overview

The heart's electrical activity is responsible for initiating and coordinating the contraction of the heart muscle, which pumps blood throughout the body. An ECG records these electrical signals through electrodes placed on the skin's surface. However, the standard ECG can sometimes miss small, abnormal signals that may indicate a higher risk of dangerous arrhythmias. The signal-averaged ECG addresses this limitation by recording the heart's electrical activity over a longer period—typically 15 to 20 minutes—and averaging the data to improve the signal-to-noise ratio.

Indications

Signal-averaged ECG is indicated for patients who:

• Have survived a myocardial infarction (heart attack) and are at risk of developing ventricular arrhythmias.
• Exhibit symptoms suggestive of arrhythmias, such as palpitations, fainting (syncope), or near-fainting spells.
• Require risk stratification for sudden cardiac death due to conditions like dilated cardiomyopathy or hypertrophic cardiomyopathy.

Procedure

During a SAECG, the patient is at rest, and electrodes are placed on the chest, similar to a standard ECG. The equipment used for SAECG is capable of recording and averaging multiple ECG signals to highlight the heart's late potentials—very low amplitude signals that occur at the end of the QRS complex and are associated with arrhythmic risk.

Interpretation

The analysis of a signal-averaged ECG focuses on three main parameters:

• QRS duration: Prolonged QRS duration can indicate slowed conduction in the ventricles, which is associated with an increased risk of arrhythmias.
• RMS voltage of the last 40 milliseconds: Low RMS voltage suggests reduced electrical activity at the end of the QRS complex, potentially indicating areas of scarred or damaged heart tissue that can predispose to arrhythmias.
• LAS40 (Late Activation Signals over 40 milliseconds): The presence of late activation signals is a marker for fragmented and delayed conduction, increasing the risk of ventricular arrhythmias.

Clinical Significance

The primary clinical significance of SAECG lies in its ability to identify patients at increased risk for ventricular arrhythmias and sudden cardiac death, particularly among those with a previous myocardial infarction or cardiomyopathy. It can guide the need for further testing, closer monitoring, or interventions such as the implantation of a cardiac defibrillator.

Limitations

While SAECG is a valuable diagnostic tool, it has limitations. It may not detect all patients at risk, and false positives can occur, leading to unnecessary anxiety or interventions. The interpretation of SAECG results should always be considered in the context of the patient's overall clinical picture.

See Also

• Electrocardiography
• Ventricular arrhythmia
• Sudden cardiac death
• Cardiomyopathy