The signal-averaged ECG (SAECG) is a noninvasive tool for identifying patients at risk for malignant ventricular dysrhythmias, particularly after MI. During the later phase of the QRS complex and ST segment, the myocardium produces high-frequency, low-amplitude signals termed late potentials. These late potentials correlate with delayed activation of certain areas within the myocardium, a condition that predisposes to reentrant forms of ventricular tachycardia (VT).
SAECGs are performed to evaluate the etiology of ventricular dysrhythmias or as a precursor to electrophysiologic studies. Disorders that may produce regions of delayed myocardial conduction include MI, nonischemic dilated cardiomyopathy, left ventricular aneurysm, and some forms of healed ventricular incisions (e.g., scar from tetralogy of Fallot surgical intervention).
The SAECG, which is a modification of the conventional ECG, uses computerized techniques to provide signal averaging, amplification, and filtering of electrical potentials.
Place electrodes on the abdomen and anterior and posterior thorax. The signals received are converted to a digital signal. A typical QRS complex is used as a template against which subsequent cardiac cycles are compared. Typically, several hundred beats are averaged to analyze for late potentials.
Data collection usually takes about 20 minutes. Optimal recordings require that the patient be in a comfortable position and remain quiet, the proper application of electrodes, and elimination of interference from other electrical equipment.
Follow guidelines in Chapter 1 for safe, effective, informed intratest care.
SAECG provides predictive values for potential VT in patients who have a history of MI or coronary artery disease.
Late potentials are stronger predictors of sudden death or sustained VT than are ventricular dysrhythmias from a Holter monitor recording.
Evidence shows that late potentials associated with VT are abolished following successful surgical intervention.
Patients who experience late potentials have a 17% incidence of sustained VT or sudden death, compared with a 1% incidence in patients without late potentials. The incidence is even greater in the presence of decreased ejection fractions (EFs).
SAECG may explain the cause of syncope subsequently identified as VT during electrophysiologic study.
Pretest Patient Care
Explain test purpose and procedure.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Review test results; report and record findings. Modify the nursing care plan as needed.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.
Increased time is required for recording beats in the presence of slow heart rates or frequent ventricular ectopics. Patient movement, talking, and restlessness also delay data procurement.
Bundle branch block can interfere with impulse averaging.
SAECG does not provide information about antiarrhythmic drug effectiveness.
Late potentials do not occur in every patient with VT.
Ventricular pacing prolongs ventricular activation time and obscures late potentials. Conversely, atrial pacing, even at rapid rates, does not alter ventricular late potentials.