Ordinarily, identifying the various types of SVT from the standard ECG is difficult. In many cases, electrophysiologic intracardiac testing is required. Of the major types of SVT—sinus node reentry, intra-atrial reentry, accelerated conduction syndromes with narrow QRS complex, and AV nodal reentry—the latter condition makes up about 60% of the total. If the P waves are inverted in the inferior leads, and when they are present before or immediately after the QRS complex, they allow accurate diagnosis. Unfortunately, many times, the P waves are superimposed in the QRS complex and prevent easy diagnosis. Thus, the standard ECG is only occasionally helpful. Sinus node reentry and AV nodal reentry usually can be terminated by carotid sinus massage.
How does the signal-averaged electrocardiogram aid clinical decision-making?
The signal-averaged ECG (SAECG) amplifies exceedingly weak electrical signals generated by the heart and passes them through an electronic filter. The remaining signals from many cardiac cycles are averaged by a computer and printed out as a single QRS complex. Of particular importance are low-amplitude potentials (usually 40 |iV) occurring late in the QRS complex. To a lesser extent, the magnitude of the QRS vector and the total duration of the QRS complex are also important.
Abnormal SAECG findings have been recorded in many patients with episodes of sustained ventricular tachycardia and may indicate which of those patients, as well as those with unexplained syncope, might benefit from electrophysiologic testing. A potential application of SAECG is in risk stratification of patients after myocardial infarction; there is a high correlation between late potentials and inducible ventricular tachycardia, and the test may pinpoint individuals at high risk of sudden death.
What are the criteria for ventricular hypertrophy?
In left ventricular hypertrophy (LVH), the increased muscle mass generates increased QRS voltage, often with secondary changes in ST and T waves. Because some of these changes are subtle, numerous (at least 25) specific ECG criteria have been proposed to aid the diagnosis of LVH. Voltage criteria appear to be the most helpful: