Biomedical Engineering Reference
In-Depth Information
by some enhancing functions. These may be applied at the
initial and postshock detection of VT for the purpose of dis-
tinguishing VT from supraventricular tachycardia. The func-
tions extend the detection specificity beyond the limits of the
basic parameter (i.e., the zone threshold rate) because apply-
ing shock treatment in case of supraventricular tachycardia it
is not necessary or even apposite.
The ICD triggers a VT therapy if detection conditions are
met. It happens upon the concurrence of all of the following
situations:
The condition of a zone detection window is met and
programmed detection enhancing function, and a therapy is
started. On the condition that the last detected interval is not
in the respective zone, a therapy is not delivered. Each fol-
lowing interval will be checked until an interval falling into
the original zone is identified or until the window condition
is met. A separate duration time parameter is programmed
for each tachycardia detection zone. The value of the dura-
tion time parameter programmed in lower detection zones
must be equal to or higher than the value in higher ventricular
zones. Longer durations may be used to prevent delivering a
therapy at inconstant tachycardia.
Duration timers count independently of one another in
their respective detection zones. If arrhythmia is detected in
the highest zone, its timer gains priority over timers in lower
zone. If the duration parameter time elapses and the detec-
tion condition is met, a therapy corresponding to the respec-
tive zone is applied regardless of whether the timer in the
lower zone has finished counting. If the condition of a higher
zone detection window ceases to be met, lower detection
zone timers are taken into account again.
•
remains met throughout the period determined by the
duration parameter.
The zone duration time determined by the duration param-
•
eter elapses.
The condition of a higher zone detection window is not
•
met.
Detection-enhancing functions indicate a therapy.
•
•
The last sensed interval is in the tachycardia zone.
If the above-listed criteria are not met, the therapy is not
delivered, and the ICD continues evaluating intervals.
A detection time window is used to ensure appropriate
treatment application. Each tachycardia zone has a detection
window comprising a certain number of RR intervals last
measured by the ICD. Upon measurement, each new interval
is compared with a programmed zone threshold rate and
classified as fast or slow (i.e., above or below the zone thresh-
old rate, expressed in milliseconds) within each detection
window. The ICD gets prepared for a potential episode if it
counts a defined number of consecutive fast intervals. The
condition of a detection window remains met if certain por-
tion of the intervals (e.g., six of ten) remains classified as
fast. If the number of fast intervals drops below this limit, the
condition of a zone detection window ceases to be met - it
would be considered to be met only if the defined number of
intervals was classified as fast again. Since a higher zone
threshold must be programmed to a value higher than a lower
zone threshold, the interval classified as fast in a higher zone
will also be classified as fast in all lower zones [70, 74]. Some
defibrillation systems make use of a timer duration parame-
ter, measuring the period of time for which the rhythm in
individual zones must be constant before a therapy is applied.
The duration timer is started if the condition of a respective
zone detection window is met. Programmed duration time
and its passing are evaluated after each cardiac cycle. As the
timer counts synchronously with the cardiac cycle, the pro-
grammed duration parameter may be exceeded by one com-
plete cardiac cycle. If the condition of a zone detection
window remains met, the timer continues counting. If the last
detected interval after the lapse of the duration time is still in
the respective detection zone, detection conditions are con-
sidered to be met, the application is not suppressed by another
10.2.2 Redetection
After application of therapy, the ICD continues evaluating
the heart rhythm to identify the need for another therapy.
Redetection criteria are applied as for the initial detection. If
the redetection criteria are met, the therapy to be applied is
selected. Parameters for duration of ventricular redetection
duration and duration aftershock are applied to identify
tachycardia during ventricular redetection. To minimize the
time until possible application of the therapy, it is recom-
mended that the redetection duration in VT-1 and VT zones
in multiple-zone configurations be programmed to
5 s.
Redetection can be started by both the previous ther-
apy application and a therapy suspended because of
reconfirmation, a therapy suspended manually from a pro-
grammer, or detection conditions being met without a pro-
grammed therapy. Redetection makes use of the same
detection window and programmed detection zone process
that is used for tachycardia identification during the initial
detection. Differences between the initial detection and rede-
tection lie in varying duration parameters and available
detection enhancing functions.
Upon delivery of ATP, the ICD monitors the heart rhythm
after each burst and makes use of the ventricular redetection
window and redetection duration parameter to identify the
end of arrhythmia. The ATP will continue delivering bursts
in programmed order until the redetection recognizes that
the therapy has been successful, a set number of ATP bursts
in the sequence has been applied, or a transition into another
zone has occurred. Canceling an ATP burst terminates the
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