Biomedical Engineering Reference
In-Depth Information
entire running ATP sequence. Upon application of shock
therapy, the ICD monitors the heart rhythm after each shock
and makes use of ventricular detection windows and post-
shock detection enhancing functions to identify the end of
arrhythmia. While charging the ICD capacitor, the ICD con-
tinues sensing arrhythmia. A shock therapy continues until
redetection recognizes that the therapy has been successful
(the episode ends), all ventricular shocks available in the
respective zone have been applied, or a transition to a lower
zone occurred.
interpreted as VT. To reduce the number of shock therapies,
the onset function must be programmed to a higher numeri-
cal value.
Stability analysis function distinguishes between irregu-
lar and regular ventricular rhythms. The evaluation is based
on measuring the variability of the RR interval during tachy-
cardia. Certain tracked supraventricular tachycardia may
lead to ventricular acceleration up to the detection zone.
These tachycardia relating to irregular heart rate have higher
variability of the RR interval when compared with real mon-
omorphic VT, which is stable. The function may also be used
to distinguish monomorphic and polymorphic VT. The sta-
bility algorithm calculates differences in the length of the RR
interval. These differences are determined throughout the
duration period and, finally, an average difference is calcu-
lated. Then, rhythm stability is evaluated by comparing the
average difference with a programmed stability threshold.
Another system classifies an interval as instable if the differ-
ence between the interval and any other of three previous
intervals is bigger than the programmed interval for stability.
If the difference exceeds the programmed threshold value,
the ventricular action is not stable and the rhythm is reported
as instable (supraventricular tachycardia). To reduce the
number of shock therapies, the stability function must be
programmed to a lower numerical value.
Onset and stability functions may be combined in some
manufacturers' ICDs to ensure higher specificity of tachy-
cardia classifications. The function combination may be
programmed so that a therapy is indicated at concurrent
validity of onset and stability functions or the validity of only
one of them.
10.2.3 Detection-Enhancing Functions
For the purpose of restricting false-positive shocks, when a
shock therapy would be applied - even for arrhythmias not
originating in heart ventricles -ICD systems usually include
algorithms extending the initial detection. These enhancing
functions evaluate detected tachycardia as ventricular or, for
example, tracked atrial arrhythmia. Sensitivity to VT of
100 % and maximum specificity is required from the func-
tions. The absolute sensitivity requirement means that all VT
episodes are detected, despite their cause. The specificity
means that only real VT and not tracked supraventricular
tachycardia are treated.
There are several functions, and each manufacturer imple-
ments their own approach. In general, algorithms may be
classified according to how they evaluate arrhythmias. The
evaluation may be based on a more detailed analysis of
sensed tachycardia time intervals, comparison of atrial and
ventricular rhythm, or the evaluation of electrogram mor-
phology. These functions are usually not available in the
highest detection zone.
10.2.3.2 Morphological Algorithms
These functions work on the basis of mathematical analysis
of intracardial signals sensed at tachycardia or comparison of
the sensed rhythm characteristics with a stored supraventric-
ular tachycardia template. The description of two such detec-
tion-enhancing functions may serve as an example; according
to data from clinical studies, the functions show high
speci fi city [ 76 ] while maintaining maximum sensitivity.
The algorithm of one manufacturer makes use of a sensed
electrogram wavelet transformation for the purpose of
dynamically comparing tachycardia morphology with a pre-
viously stored sinus rhythm template, which is updated con-
tinuously. The algorithm stores the template of sensed QRS
complexes of the intrinsic rhythm and creates a template
comprising 48 electrogram samples taken every 4 ms using
HaaR wavelet. The template morphology transformation is
compared in real time with transformation of electrogram
morphology during tachycardia. This comparison is
expressed as a percentage agreement describing the level of
morphological similarity of tachycardia and sinus rhythm
electrogram. Sensed QRS complexes with agreement under
10.2.3.1 Algorithms Based on a Time Interval
Analysis
A classic approach available in the majority of ICDs is the
use of onset and stability parameters analyzing cardiac cycle
intervals [ 70, 74, 75 ] . The onset parameter identi fi es onset of
tachycardia as sudden or gradual; stability quantifies the
course of tachycardia, that is, the variability of the RR
interval.
The onset function distinguishes between sinus tachycar-
dia starting slowly and pathologic ventricular tachycardia
starting suddenly. It measures the rate of change from a slow
ventricular rhythm to tachycardia. The onset function may
be programmed as a percentage of cycle length or as the
interval length. The onset value represents a minimum dif-
ference between intervals (or a group of intervals). If the
onset parameter is programmed to a value lower as a percent-
age, the ICD will need greater acceleration of cardiac action
to identify the rhythm as VT. If the rate increases gradually,
then the onset function avoids sensed intervals from being
