Biomedical Engineering Reference
In-Depth Information
9
Pacemaker Timing
Coordinating the timing of individual electrical processes in
the heart is required for proper electrical activation.
Pacemakers used for bradycardia treatment must respect this
timing. The more perfectly they try to approximate natural
cardiac activation, the more complex is the timing of indi-
vidual processes. To comprehend how pacemakers work, it is
absolutely essential to become familiar with the timing of
individual events.
The behavior of pacemakers in time is, in certain con-
texts, expressed in the frequency domain or by the number of
a certain type of event per minute (in units [Hertz, per min-
ute, minute −1 ]); other times it is expressed in time intervals
using seconds or milliseconds and minute −1 ) units can be eas-
ily converted using a constant of 60,000, corresponding to
the number of milliseconds in 1 min. Than the equation is
quantity (ms) = 60,000/quantity (/min, min −1 ) . Both of these
methods of describing an event are often interchanged, so at
a pacing rate of 60/min, the interval between stimuli is 1 s, or
1,000 ms. In practice, both approaches are combined when
programming the systems. In the time domain, refractory
and blanking periods and other rather technical parameters
are set. In the frequency domain, clinical parameters, such as
pacing rate, intrinsic heart activity, and limits of tachycardia
detection zones, are set.
For good orientation in this chapter, knowledge of North
American Society of Pacing and Electrophysiology/British
Pacing and Electrophysiology Group code (see Table 5.1 )
and single-chamber and dual-chamber pacing modes
(described in Chap. 5 ) is required. Time relationships between
sensed and paced events in the most frequently used single-
and dual-chamber modes is described here. The manner of a
pacemaker's response to a sensed or paced event in a particu-
lar mode depends on the manufacturer; various types of
devices from the same manufacturer may also differ.
Timing of a cardiac cycle is based on the pacing lower
rate interval (LRI). Sometimes, it is referred to as an auto-
matic interval. In terms of rate, the LRI parameter corre-
sponds to the lower rate limit (LRL). It is a minimum atrial
and/or ventricular pacing rate. Asynchronous modes pace at
the LRL permanently; inhibited modes occur only in the case
of an absence of sensed intrinsic cardiac activity or without
sensor-driven pacing. In the case of inhibited modes, that is,
when intrinsic cardiac activity is sensed, the timing of single-
and dual-chamber pacing systems is supplemented with an
escape interval (EI). The EI is the period of time between a
sensed or paced event and a possible following pacing pulse
in the same heart chamber, unless intrinsic heart activity is
sensed (Fig. 9.1 ). The origin of the term is not clear. It refers
to the interval in which one activation center may escape
from the dominance (control) of another center. So, for
example, in a pacemaker in the VVI mode with an
EI of 1,000 ms, the intrinsic cardiac activity may escape from
the rhythm dominance (control) when it occurs in a period
not longer than 1,000 ms since the last ventricular event.
Technically, it is the period of time after the lapse of a timed
event, that is, atrial or ventricular pacing. It is the time inter-
val after which a running counter of the electronic system,
set to the initial value and counting with certain clock fre-
quency, is reset. The counter reset to the initial value by a
sensed event is the basic principle of timing of inhibited pac-
ing modes.
The EI measured by the pacemaker from the moment an
event is sensed equals the LRI. Nevertheless, the EI measured
on a surface electrocardiogram (ECG) from the beginning of
the QRS complex cannot be determined accurately. That is
why the EI seems to be a little longer than the LRI when
measured manually - the device senses the intrinsic cardiac
activity later than it appears on the surface ECG [69].
9.1
Timing in Single-Chamber Modes
Single-chamber pacing modes are used in patients with
paroxysmal asystoles or without asystoles and in patients
with chronic supraventricular arrhythmias for whom dual-
chamber pacing cannot be utilized. The single-chamber
pacing modes serve for pacing only in the atrium or only in
the ventricle [ 69 ] .
 
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