Signal-averaged electrocardiogram
Encyclopedia
Signal-averaged electrocardiography (SAECG) is a special electrocardiographic
technique, in which multiple electric signals from the heart
are averaged to remove interference
and reveal small variations in the QRS complex
, usually the so-called "late potentials". These may represent a predisposition towards potentially dangerous ventricular tachyarrhythmias.
(ECG) is recorded in the supine position
using an ECG machine equipped with SAECG software; this can be done by a physician
, nurse, or medical technician. Unlike standard basal ECG recording, which requires only a few seconds, SAECG recording requires a few minutes (usually about 7-10 minutes), as the machine must record multiple subsequent QRS potentials to remove interference due to skeletal muscle
and to obtain a statistically significant
average trace. For this reason, it is important for the patient to lie as still as possible during the recording.
) in the final portion of the QRS complex (the so-called "late potentials, or more accurately, "late ventricular potentials"). These can be immediately interpreted by comparing results with cut-off values.
myocardium, which may be the substrate for a micro-re-entry mechanism, implying a higher risk of potentially dangerous ventricular tachyarrhythmias. This has been used for the risk stratification of sudden cardiac death
in people who have had a myocardial infarction
, as well as in people with known coronary heart disease
, cardiomyopathies
, or unexplained syncope
.
Still, the real predictive value of these findings is questioned. Late potentials may be found in 0-10% of normal volunteers. When used as a prognostic factor for the development of ventricular tachycardia
, they have a sensitivity of 72% and a specificity of 75%, yielding a positive predictive value
of 20% and a negative predictive value
of 20%.
Electrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
technique, in which multiple electric signals from the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
are averaged to remove interference
Interference
In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have...
and reveal small variations in the QRS complex
QRS complex
The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...
, usually the so-called "late potentials". These may represent a predisposition towards potentially dangerous ventricular tachyarrhythmias.
Procedure
A resting electrocardiogramElectrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
(ECG) is recorded in the supine position
Supine position
The supine position is a position of the body: lying down with the face up, as opposed to the prone position, which is face down, sometimes with the hands behind the head or neck. When used in surgical procedures, it allows access to the peritoneal, thoracic and pericardial regions; as well as the...
using an ECG machine equipped with SAECG software; this can be done by a physician
Physician
A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...
, nurse, or medical technician. Unlike standard basal ECG recording, which requires only a few seconds, SAECG recording requires a few minutes (usually about 7-10 minutes), as the machine must record multiple subsequent QRS potentials to remove interference due to skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
and to obtain a statistically significant
Statistical significance
In statistics, a result is called statistically significant if it is unlikely to have occurred by chance. The phrase test of significance was coined by Ronald Fisher....
average trace. For this reason, it is important for the patient to lie as still as possible during the recording.
Results
SAECG recording yields a single, averaged QRS potential, usually printed in a much larger scale than standard ECGs, upon which the SAECG software performs calculations to reveal small variations (typically 1-25 mVVolt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
) in the final portion of the QRS complex (the so-called "late potentials, or more accurately, "late ventricular potentials"). These can be immediately interpreted by comparing results with cut-off values.
Significance
Late potentials are taken to represent delayed and fragmented depolarisation of the ventricularVentricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
myocardium, which may be the substrate for a micro-re-entry mechanism, implying a higher risk of potentially dangerous ventricular tachyarrhythmias. This has been used for the risk stratification of sudden cardiac death
Sudden Cardiac Death
Sudden cardiac death is natural death from cardiac causes, heralded by abrupt loss of consciousness within one hour of the onset of acute symptoms. Other forms of sudden death may be noncardiac in origin...
in people who have had a myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
, as well as in people with known coronary heart disease
Coronary heart disease
Coronary artery disease is the end result of the accumulation of atheromatous plaques within the walls of the coronary arteries that supply the myocardium with oxygen and nutrients. It is sometimes also called coronary heart disease...
, cardiomyopathies
Cardiomyopathy
Cardiomyopathy, which literally means "heart muscle disease," is the deterioration of the function of the myocardium for any reason. People with cardiomyopathy are often at risk of arrhythmia or sudden cardiac death or both. Cardiomyopathy can often go undetected, making it especially dangerous to...
, or unexplained syncope
Syncope (medicine)
Syncope , the medical term for fainting, is precisely defined as a transient loss of consciousness and postural tone characterized by rapid onset, short duration, and spontaneous recovery due to global cerebral hypoperfusion that most often results from hypotension.Many forms of syncope are...
.
Still, the real predictive value of these findings is questioned. Late potentials may be found in 0-10% of normal volunteers. When used as a prognostic factor for the development of ventricular tachycardia
Ventricular tachycardia
Ventricular tachycardia is a tachycardia, or fast heart rhythm, that originates in one of the ventricles of the heart...
, they have a sensitivity of 72% and a specificity of 75%, yielding a positive predictive value
Positive predictive value
In statistics and diagnostic testing, the positive predictive value, or precision rate is the proportion of subjects with positive test results who are correctly diagnosed. It is a critical measure of the performance of a diagnostic method, as it reflects the probability that a positive test...
of 20% and a negative predictive value
Negative predictive value
In statistics and diagnostic testing, the negative predictive value is a summary statistic used to describe the performance of a diagnostic testing procedure. It is defined as the proportion of subjects with a negative test result who are correctly diagnosed. A high NPV means that when the test...
of 20%.