Biomedical Engineering Reference
In-Depth Information
Ap
BiV
BiV
As
Ap
Vs
As
Vs
EKG
Atrial channel
PVARP
AEI
TARP
Ventricular channels
AVI
reset
reset
LRI
reset
post sensing RVRP
post pacing RVRP
LVRP
Fig. 11.2
Biventricular pacing timing periods [82] (© 2012 Boston Scientific Corporation or its affiliates. All rights reserved. Used with permis-
sion of Boston Scienti fi c Corporation)
the delivery of CRT and simultaneously lowers the risk of
acceleration of the patient's rhythm to a ventricular tachycar-
dia. Even though CRT should be delivered permanently, there
are certain circumstances in which suppression of therapy is
recommended. LVRP is a period after a sensed or paced LV
event or after the first paced ventricular event (if a nonzero VV
delay is set) during which sensed LV events do not affect pac-
ing timing. The use of a long LVRP shortens the LV detection
window. To suppress inappropriate LV pacing, a sufficiently
long LVRP is required so that a T wave is included.
The left ventricular protection period (LVPP) is another
possible parameter. It is a period after a sensed or paced LV
event when the device does not pace the left ventricle. The
LVPP prevents the device from delivering pacing during an
vulnerable LV phase, for example, if an LV extrasystole
occurs. The use of a long LVPP decreases the maximum LV
pacing rate and the CRT can be suppressed at higher pacing
rates. In patients with heart failure with normal AV conduc-
tion, a long intrinsic intracardial AV interval and a long pro-
grammed postventricular atrial refractory period (PVARP)
may cause the loss of atrial rhythm tracking below the MTR,
resulting in inefficient CRT. A PVARP after an extrasystole
should be programmed as long as possible because of the
risk of pacemaker-mediated tachycardia.
A left ventricle blanking period after atrial pace sup-
presses LV sensing after atrial pacing. Nevertheless, it is not
available and programmable in all manufacturers' devices.
Sensed ventricular events in this period are most often caused
by far- fi eld sensing from the atrium [ 77 ] .
LV offset function or VV interval allows difference between
the time of application of LV and RV pacing pulses to be set.
Thus, it provides flexibility in timing, the advantage of which
may be seen when minimizing the width of a paced QRS
complex after CRT implantation. Provided that biventricular
pacing is close to the MPR, the device adapts the LV offset
according to the lowest programmed value of the tachycardia
detection zone. The AV delay programmed value is based on
RV timing, and it is thus not affected by the LV offset param-
eter. Depending on the manufacturer, the LV pacing offset and
pre-excitation can be set [ 82 ] .
For the support of the intrinsic atrial rhythm, the LRL
parameter should be programmed to a value lower than the
patient's resting rhythm; the MPR, however, should be pro-
grammed to the highest rate that can be tolerated by a patient
to sustain CRT at fast atrial activity. If the patient's atrial
activity exceeds the MPR, CRT can be disturbed by the loss
of AV synchronization. Nevertheless, the devices usually sup-
port the possibility of CRT application during atrial tachycar-
dia. Events sensed in the atrium may come during the PVARP
period, and the device thus classifies them as refractory and
fails to track them to ventricles. Tracking interruptions can
thus restrict CRT application, which is why some algorithms
temporarily shorten the PVARP, so that fast atrial events can
be tracked to ventricles and the CRT can be sustained. CRT
delivery and programmed AV synchronization with the sense
AV delay parameter is restored after the restoration of normal
sinus rhythm. Naturally, atrial rhythm tracking is suppressed
during tachycardia therapies, electrophysiologic testing via
the device, and similar system operations.
11.4
Diagnostic Features of CRT Systems
In patients with an implanted CRT system, progression of
heart failure or its further development should be assumed
after implantation. For this reason, diagnostic functions in the
devices for CRT are much more important than in conven-
tional pacemakers and implanted cardioverter-defibrillators.
