Biomedical Engineering Reference
In-Depth Information
programmed to be 30-60 ms shorter than the PAV. The
hidden delay between the actual cardiac event in the atrium
and the time of its detection by the device is thus compen-
sated for. The highest cardiac rate at which all sensed atrial
events are tracked to ventricular pacing is referred to as the
MTR or upper tracking rate parameter.
9.2
Timing in Dual-Chamber Modes
Dual-chamber modes are much more complex than the
above-described single-chamber modes. Many more timing
parameters are added, among which numerous interactions
exist [ 94 ] .
The atrioventricular (AV) delay parameter (AV delay; AV
interval [AVI]) is a programmable time interval from the
occurrence of a paced or sensed event in the atrium until the
occurrence of a paced event in the ventricle. AV delay sus-
tains AV synchronization of the heart. The AV delay param-
eter is used in all dual-chamber modes. In VDD, DDI, and
DDD modes the pacemaker delivers ventricular pacing at the
end of the AVI if the pacing is not inhibited by spontaneous
ventricular depolarization or if the maximum tracking rate
(MTR) is not exceeded. In the D00 mode the device delivers
ventricular pacing at the end of the AVI regardless of sponta-
neous ventricular depolarization. The delay between a sensed
atrial event and corresponding ventricular pacing represents
the AV delay after a sensed event (interval atrial sensing −
ventricular pacing = sensed AVI [SAV]). The delay between
a paced atrial event and corresponding ventricular pacing
represents the AV delay after a paced event (interval atrial
pacing − ventricular pacing = paced AVI [PAV]) (Fig.
9.5
).
If an EI ends sooner than an atrial event is sensed, the
device will pace the atrium and then plan ventricular pacing
to be delivered at the end of the PAV. The SAV can be pro-
grammed to a value lower than or equal to the PAV value.
The influence of the SAV parameter on hemodynamics
depends on how appropriate mutual timing of atrial and ven-
tricular contractions is. Atrial pacing triggers atrial contrac-
tion, while sensing is conducted only during the contraction.
Hence, if the SAV is programmed to the same value as the
PAV, the hemodynamic AVI will differ for paced and sensed
atrial events. If a ventricular event is sensed during the
SAV or PAV, ventricular pacing is inhibited. The SAV is
9.2.1
D00 and 0D0 Modes
In theory, even D00 and 0D0 modes can be classified as dual-
chamber modes. The D00 mode provides AV sequential
asynchronous pacing at a programmed lower frequency and
PAV delay, without the possibility of inhibition by sensed
cardiac activity. Hence, in this mode, sensing is not conducted
in any chamber. In contrast, in the 0D0 mode, rhythm is
sensed in both chambers without any pacing. This mode can
be temporarily programmed for diagnostic purposes; perma-
nent programming must be avoided for safety reasons.
9.2.2
DVI Mode
The DVI mode provides dual-chamber AV sequential pacing
without atrial sensing. Because the intrinsic atrial activity
cannot be tracked to the ventricle, the mode is not regarded
as a tracking mode. The atrial pacing is delivered at the LRL,
with subsequent ventricular pacing after the lapse of PAV if
it is not inhibited by intrinsic ventricular action. Sensed non-
refractory ventricular events during a VA interval trigger a
new VA interval. This mode was commonly used before the
DDD mode is introduced. The mode is not in use any more,
and it has been left as a programmable option for diagnostic
purposes or if loss of sensing occurs in the atrial channel in
the DDD mode [ 94 ] .
9.2.3
DDI Mode
The DDI mode delivers dual-chamber AV sequential pacing
with atrial sensing, but without tracking the atrial activity to
ventricles. So, it is a nontracking mode, like the DVI mode.
Atrial pacing is delivered at the LRL, with subsequent ven-
tricular pacing after PAV if an intrinsic R wave is not sensed.
In DDI(R) modes, sensed atrial events are not tracked. P
waves sensed in the atrium inhibit atrial pacing; the AVI is
not triggered, though. The ventricle is paced at given pacing
rate, and it is not a tracking mode. Ventricular pacing during
sensing of the atrial event is timed according to the LRL.
Sensed nonrefractory ventricular events during the VA inter-
val trigger a new VA interval. If the intrinsic atrial rate
exceeds the LRL, the atrial channel becomes inhibited, and
only the VVI mode is applied in the ventricle.
R
T
P
Q
S
SAV offset
SAV
PAV
Ap
As
Vp
Fig. 9.5
Atrioventricular delay periods
