Transcript Ventricular Late potentials

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.