Biomedical Engineering Reference
In-Depth Information
a programmed threshold (nominally 70 %) are identified as
VT. If at least three of the past eight QRS complexes com-
ply with the stored template, the algorithm suspends the
therapy [ 77 ] .
Another system makes use of analysis of timing and dif-
ference of the resulting electrogram vector for various
rhythms. The ICD first analyzes whether the ventricular rate
is higher than the atrial rate (only in dual-chamber ICDs and
cardiac resynchronization therapy defibrillators). If so, a
therapy is initiated. If the ventricular rate is not higher, an
evaluation is started. At the normal sinus rhythm used as a
template, electrical activity passes from the atrium to the
ventricles. Even supraventricular tachycardia has a similar
electrogram. In contrast, VT originates in the ventricular
myocardium, and thus the resulting vector of electrical activ-
ity differs. The system senses electrograms both by lead pac-
ing electrodes for the near-field and lead shock electrodes in
the far-field. The electrogram from the pacing electrodes is
used for time comparison with the far-field electrogram and
a correlation analysis of stored and currently sensed rhythms.
Amplitudes of eight prescribed instants of time are com-
pared. The correlation threshold (nominally 94 %) may also
be adjusted. If a correlation value is lower than or equal to
the threshold value, the cardiac cycle is regarded as VT. The
template of a patient's intrinsic sinus rhythm may be recorded
both in a doctor's office via a programmer and automatically
at appropriate instants [ 70 ] .
a lower ventricular zone. In most contemporary ICDs, a pro-
cess referred to as ATP is implemented before or during
capacitor charging; even in the highest VF detection zone,
the application of an ATP burst is tried before a shock is
delivered in attempt to terminate tachycardia painlessly.
Based on initial ventricular detection criteria, an ICD
chooses the first prescribed therapy in the zone in which tachy-
cardia is detected. Upon its delivery, redetection of arrhythmia
conversion commences. If the arrhythmia is not converted but
is redetected in the same ventricular zone, the following pro-
grammed therapy in the respective zone is selected and deliv-
ered, and redetection is carried out. If arrhythmia is redetected
after the application of therapy in a higher or lower detection
zone, a therapy with an intensity equal to or higher than the
intensity of the previously applied therapy is selected and
applied from the new zone prescription.
Upon application of a shock, heart contractions must be
sometimes restored by the right ventricular pacing. The pac-
ing delay interval after a shock specifies the possible begin-
ning of pacing upon application of a ventricular shock. The
timing of the initial pacing pulse in the interval depends on
the intrinsic cardiac activity during the delay. Provided that
R waves are sensed during such a delay, the device applies
pacing only if the action sensed is slower than the lower rate
limit after therapy. Further pacing pulses are applied as
needed according to the pacing prescription.
10.3.1 Antitachycardia Pacing
10.3
Tachycardia Therapies
ATP represents the possibility of a VT episode therapy and
restoration of the patient's normal sinus rhythm primarily in
the case of monomorphic VT (Fig.
10.2
). It is assumed, based
on clinical studies, that ATP is efficient for VT up to the rate
of 250 beats/min. ATP consumes less battery energy than a
shock and is better tolerated by patients. During ATP, a
sequence of precisely timed pacing pulses is applied.
Burst, ramp, scan, and their combinations (ramp
+
burst/
scan) are basic ATP schemes. Burst may refer either to a
basic ATP sequence or one cycle of each sequence including
a certain number of pulses. ATP prescription can be pro-
grammed individually by setting the number of bursts to be
applied, the number of pulses in each burst, a coupling inter-
val, burst length, and a minimum pacing interval. Amplitude
and width of the ATP pulse are common for all schemes and
are programmable independent of normal pacing settings.
The coupling interval controls the timing of the first burst
pulse. It defines the time between the last sensed event that
meets the detection criteria and delivery of the first burst
pulse. If an adaptive coupling interval is programmed, the
parameter is adjusted according to the patient's rhythm based
on the average value calculated from the last several cardiac
cycles.
An ICD may deliver two different types of therapies for the
purpose of terminating VT or VF: either ATP or defibrillation
electric shocks. ATP therapies are bursts applied between
defibrillation lead pacing electrodes. Shocks are high-voltage
biphasic or monophasic pulses delivered via defibrillation
shock electrodes.
The type of therapy to be applied in a ventricular detec-
tion zone is subject to ventricular therapy prescription. A
zone prescription, usually with the exception of the highest
VF, may comprise only ATPs, only shocks, or their combina-
tion. Each ventricular zone may be programmed according
to a separate ventricular therapy prescription. The therapy
intensity must, however, always have a nondescending ten-
dency within each zone. In the lowest zone of the multiple-
zone configuration, some or all therapies may be turned off,
and tachycardia in this zone are only recorded in the ICD's
memory. ATP therapies are regarded as therapies with the
same intensity; it is, however, lower than the intensity of any
shock. Shock energy is determined by its programmed value.
In the multiple-zone configuration, therapies may have
lower, higher, or equal intensity in a higher ventricular zone,
depending on the type of device, with respect to therapies in
