Biomedical Engineering Reference
In-Depth Information
retrograde P waves, or noise. The PVARP must be pro-
grammed to a value longer than the time of retrograde VA
conduction, so that retrograde atrial depolarization is not
sensed in the atrial channel, which avoids possible occur-
rence of pacemaker-mediated tachycardia (PMT). If the
PVARP value was shorter than the retrograde VA conduc-
tion, the probability of PMT occurring might increase.
Certain devices allow the PVARP to be set automatically
based on the changes in a patient's spontaneous or paced car-
diac action. A sensed atrial event occurring during the PVARP
is classified as refractory, does not inhibit atrial pacing, and
is not tracked [32, 63]. A long PVARP shortens the atrial
detection window. Programming a long PVARP in combina-
tion with a short AVI may cause a 2:1 block, occurring inter-
mittently at a programmed MTR.
Certain systems allow dynamic PVARP and dynamic AV
delay to be programmed, which optimizes the sensing win-
dow at higher pacing or sensing rates. This decreases the
occurrence of 2:1 tracking or the Wenckebach behavior in
DDD(R) and VDD(R) modes. A dynamic PVARP also
decreases the probability of PMT at lower rates and the risk
of competitive atrial pacing. The pacemaker then calculates
the value of the dynamic PVARP using the weighted average
of the preceding cardiac cycles. This results in linear short-
ening of the PVARP with increasing pacing or sensing rate.
When the average cardiac rate is between the LRL and MTR,
the pacemaker calculates the dynamic PVARP based on the
linear dependence. If the average rate is equal to or lower
than the LRL, the value of maximum PVARP is used. If the
average rate is equal to or higher than the MTR interval, the
value of minimum PVARP is utilized.
A ventricular refractory period (VRP) is defined as an
interval after a ventricular event, be it paced or sensed, dur-
ing which electrical activity sensed in the ventricle does not
inhibit the pacemaker. This period is available in any mode
in which ventricular sensing is activated. The use of a long
VRP, however, shortens the ventricular detection window,
and its programming to a value exceeding the PVARP may
lead to competitive pacing because an atrial event may be
sensed after the PVARP, and upon spontaneous tracking to a
ventricle, it would fall into this refractory period. That being
so, the pacemaker would not sense ventricular depolarization
and would pace at the end of the AVI, which would result in
competitive pacing.
9.5
DDD Timing Intervals
In dual-chamber modes, timing is gradually supplemented
with several intervals [69]. In the ventricular channel, it is:
LRI as the longest interval between a sensed or paced
•
ventricular event and following ventricular pacing not
inhibited by sensing.
VRP as an interval triggered by a paced or sensed
•
ventricular event, during which the LRI cannot be
triggered again.
AVI as an interval between a sensed (SAV) or paced
•
(PAV) ventricular event and planned ventricular pacing. It
is a substitution for the intrinsic PR interval. During this
interval, the atrial channel is refractory, and a new AVI
cannot be triggered.
For the atrial channel, an atrial EI (AEI) can be calculated
as AEI = LRI −AVI. It is an interval between a sensed or
paced ventricular event and following atrial pacing not inhib-
ited by sensing.
The above-mentioned PVARP is a separate interval in the
atrial channel; it occurs after a sensed or paced ventricular
event during which an AVI cannot be retriggered by a sensed
atrial event. This interval avoids inappropriate sensing of ven-
tricular events in the atrial channel and retrograde P waves.
Total ARP (TARP) can be calculated as TARP =AVI +
PVARP (Fig.
9.9
).
As
Vs
As
Vp
Ap
Vs
Ap
Vp
EKG
Atrial channel
PVARP
AEI
TARP
Ventricular channel
VRP
AVI
reset
reset
LRI
reset
