Noninvasive evaluation of patients who are at risk for sudden cardiac death DR.A.YAMINISHARIF Tehran Heart Center

In the United States, coronary disease and advanced left ventricular dysfunction are diagnosed in approximately 400,000 persons each year. Thus routine implantation of defibrillators in this population would be very expensive.

This considerations indicate the benefits that would accrure from effective risk stratification of this population so that ICD therapy could be directed only to those patients who would benefit from it.

Ventricular Late potentials Late potentials are generated by tissue activated later than their usual timing in the cardiac cycle. Most often, this delay is caused by depolarizing tissue within and surrounding infarct regions. Thus the pathophysiologic basis of late potentials is formed by the depolarizing potentials that outlast the normal end of QRS complex.

Signal averaging is a method that improves signal-to-noise ratio when signal are recurrent and the noise is random. In conjunction with appropriate filtering and other methods of noise reduction, signal averaging can detect cardiac signals of few microvolts in amplitude, reducing noise amplitude, such as muscle potentials that are typically 5 to 35mv, to less than 1 mv.

With this method, very low- amplitude electrical potentialsgenerated by the sinus and AV nodes,His bundle, and bundle branches are detectable at the body surface.

One constituent of reentrant ventricular arrhythmias in patients with prior myocardial damage is slow conduction. Direct cardiac mapping techniques can record myocardial activation from damaged areas that occurs after the end of the surface electrocardiographic QRS complex during sinus rhythm.

These delayed signals have very low amplitude that cannot be discerned by routine electrocardiography and correspond to the delayed fragmented conduction in the ventricles recorded with direct mapping techniques. Signal averaging has been applied clinically most often to detect such late ventricular potentials of 1 to 25 mv.

Criteria for late potentials are the following: (1)Filtered QRS complex duration longer than 114 to 120 milliseconds(2) less than 20 mv of root square signals amplitude in the last 40 milliseconds of the filtered QRS complex; (3)terminal filtered QRS complex remains below 40 mv for longer than 39 milliseconds.

These late potentials have been recorded in 70 to 90 percent of patients with spontaneous sustained and inducible VT after myocardial infarction, in only 0 to 6 percent of normal volunteers, and in 7 to 15 percent of patients after myocardial infarction who do not have VT.

Patients with bundle branch block or paced ventricular rhythm already have wide QRS complexes, rendering the technique less useful in these cases. The presence of a late potential is a sensitive, but not specific, marker of arrhythmic risk and its prognostic use is limited.

T-WAVE ALTERNANCE Electrical alternans is defined as beat-to-beat alteration in the shape of electrocardiographic waveforms. Lewis noted that alteration could occur in a normal heart after marked acceleration in heart rate and also in the diseased or intoxicated myocardium

In 1948 Kalter and Schwartz examined the ECGs from 6059 patients and described an association between macroscopic T-wave alternance (TWA) and increased mortality of the affected patients. Assessment of subtle microvolt TWA (MTWA) was first reported in 1982Alteration in action potential duration induced by rapid pacing are not uniform across the myocardium.

Significant part of the ventricular myocardium show sequential lengthening and shortening of the action potential with fluctuation in some region being 180 degrees out of phase with those in other region constituting a phenomenon known as discordance alteration.The resulting spatial gradients in transmembrane potential alternate in magnitude and direction from beat to beat, providing the basis for MTWA in the surface ECG.

Thus, under chronotropic or metabolic stress, discordant alternance lead to repolarization gradient that are large enough to produce unidirectional block and reentry. The molecular mechanisms of cellular alternance similarly remain elusive.However, given the recent advances in molecular and cell biology, answers to these questions are expected in the near future.

Beat –to-beat alternation in the amplitude and /or morphology of the electrocardiographic recording of ventricular repolarization, the ST segment ant T wave has been found in conditions favoring the development of ventricular tachyarrhythmias, such as ischemia and long-QT syndrome and in patients with ventricular arrhythmias.

The electrophysiological basis appears to be the alternation of repolariztion of ventricular myocytes. T wave alternans testing requires exercise or atrial pacing to achieve a heart rate of 100 to 120 beats/min with relatively little atrial or ventricular ectopic activity The test is less useful in patients with wide QRS complex(longer than 120 milliseconds).

Although the predictive value of a positive test varies greatly, depending on the population studied, a ngative test result strongly predicts freedom from VT and VF in all group studies thus far, at least over a short follow –up period.

This has important implications for implantable cardioverter-defibrillator(ICD) use in high-risk patient who have not yet manifested dangerous arrhythmias(primary prevention or prophylactic devices) A significant proportion of these patients will never benefit from these devices.