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 .